Filtern
Erscheinungsjahr
Dokumenttyp
Sprache
- Englisch (209) (entfernen)
Schlagworte
- Non-destructive testing (209) (entfernen)
Organisationseinheit der BAM
- 8 Zerstörungsfreie Prüfung (74)
- 8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen (37)
- 8.0 Abteilungsleitung und andere (20)
- 8.4 Akustische und elektromagnetische Verfahren (13)
- 8.5 Röntgenbildgebung (11)
- 4 Material und Umwelt (8)
- 4.5 Kunst- und Kulturgutanalyse (8)
- 7 Bauwerkssicherheit (6)
- 5 Werkstofftechnik (5)
- 5.3 Polymere Verbundwerkstoffe (4)
Eingeladener Vortrag
- nein (39)
Locating and sizing delaminations is a common inspection task in the maintenance and quality control of construction and rehabilitation. Their detection is an important area of application of nondestructive testing in civil engineering (NDT-CE). To improve this application, NDT test systems and test solutions must be compared, for which specimens containing well-defined delaminations are needed to serve as a reference. Currently, there are no widely accepted procedures available for creating such flaws locally and reproducibly. This study presents procedures for creating artificial delaminations repeatably and as close as possible to natural delaminations. To produce the discontinuities only substances were used which can occur in concrete components and do not affect the application of NDT-CE methods. Ultrasonic pulse-echo (UPE) was used to test the flaws in the specimens. The delaminations were created by applying expansive mortar in prepared through holes. Three specimens with two delaminations each were built and tested using UPE.
AbstractAir‐coupled ultrasonic testing (ACU) is a pioneering technique in non‐destructive testing (NDT). While contact testing and fluid immersion testing are standard methods in many applications, the adoption of ACU is progressing slowly, especially in the low ultrasonic frequency range. A main reason for this development is the difficulty of generating high amplitude ultrasonic bursts with equipment that is robust enough to be applied outside a laboratory environment. This paper presents the fluidic ultrasonic transducer as a solution to this challenge. This novel aeroacoustic source uses the flow instability of a sonic jet in a bistable fluidic switch to generate ultrasonic bursts up to 60 kHz with a mean peak pressure of 320 Pa. The robust design allows operation in adverse environments, independent of the operating fluid. Non‐contact through‐transmission experiments are conducted on four materials and compared with the results of conventional transducers. For the first time, it is shown that the novel fluidic ultrasonic transducer provides a suitable acoustic signal for NDT tasks and has potential of furthering the implementation of ACU in industrial applications.This article is protected by copyright. All rights reserved
Chapter 6: Case studies
(2023)
The through-life management of our constantly ageing infrastructure is a basic requirement in order to ensure their structural safety and serviceability. Each structure experiences deterioration processes with time leading to a decrease of structural safety and serviceability. The design of new structures considers the expected deterioration for a defined period, the design service life. However, a frequent survey of structural safety controlling structural condition should be mandatory and a maintenance plan should be an integral part of the design. In addition, many structures have exceeded their design service life already or are very close to it leading to an increasing demand for condition assessment. On the one hand, assumptions made during design are not valid any more due to change of the loads, e.g., increasing traffic loads in terms of number and weights. On the other hand, design codes evolved over time in such a way that existing structures do not comply with today’s standards. In all these cases, the through-life management is an important tool to maintain the accessibility of existing structures with known reliability.
In line with the new Model Code for Concrete Structures, which includes guidance for both – design of new structures and assessment of existing structures, the Task Group 3.3 focused on the compilation of a state-of-the-art guideline for the through-life management of existing concrete structures, including:
Data acquisition by testing and monitoring techniques;
Condition assessment for the evaluation of existing structures;
Performance prediction using advanced methods;
Decision-making procedures to perform a complete assessment of existing structure.
The overall objective of the through-life management is the assessment of the current condition and the estimation of the remaining service life under consideration of all boundary conditions.
This paper presents an automatic damage imaging technique by employing a signal processing approach based on applying hierarchically clustered filters across different domains. The technique involves time-frequency-wavenumber filter banks which are applied sequentially to ultrasonic guided wave (UGW) data. The study is conducted for a single lap joint composite specimen with a special focus on small voids which were formed due to manual adhesive component mixing. UGW data are acquired with a 3D Scanning Laser Doppler Vibrometer (LDV) over the scan area of the bonded plate. UGWs are excited at the central frequency of 100 kHz by a single piezoelectric transducer mounted on the surface of the single plate. Within each domain of time, frequency, and wavenumber, four filters are designed which results in 64 distinct filtered wavefields. From each filtered wavefield, an image is obtained by using root-mean-square (RMS) calculation of the signals. The obtained results are then combined to create a final, improved-resolution image of the scan area. The final image is compared to the image obtained through RMS calculation of full wavefield with interpolation through Delaunay triangulation and the image obtained by X-ray radiography. The results show that the smallest void that could be detected has a diameter of 2.14 mm.
The European Green Deal and the global fight against climate change call for more and larger wind turbines in Europe and around the world. To meet the increasing demand for maintenance and inspection, partly autonomous methods of remote inspection are increasingly being developed in addition to industrial climbers performing the inspection.
Rotor blades are exposed to extreme weather conditions throughout their lifetime of 20 years, and the leading edge erodes over time. These erosion damages change the aerodynamic features of blades and can cause structural damages. The estimated annual energy production (AEP) losses caused by erosion damages are between 0.5% and 2% per year. The classification of the severity of a rain erosion damage and the quantification of the resulting AEP losses for cost efficient repair and maintenance efforts and improved power production of wind turbines are subject of scientific research.
For the inspection of wind turbine rotor blades, passive thermography is an option that has been used to detect both internal damage [3, 4] as well as erosion on the surface [5, 6]. The advantage is that, given suitable boundary conditions, not only the rain erosion damage itself but also temperature differences caused by the resulting turbulences can be observed on the surface of the blade. Turbulences reduce the efficiency of the rotor blades and result in AEP losses. Optimised thermography inspections can contribute to detect and to evaluate rain erosion damages. The thermal inspection lasts 10 minutes per turbine and is performed while the turbine is in full operation, avoiding downtime and lost opportunities for the turbine owner which are usually caused by conventional blade inspections. The timely inspection procedure is complemented by an automatic data evaluation and results in a considerable number of inspected wind turbines in a certain period of time. A fully convolutional network (FCN) is implemented for the automated evaluation of thermal images.
In the presented study, more than 1000 thermographic images of blades were annotated, augmented and used to train and test the FCN. The aim is the precise marking of thermal signatures caused by erosion damage at the leading edge. The area size of the detected temperature difference caused by turbulences was used to identify and categorise damages. Certain strategies were adopted to group small individual indications as one large damage, in order to develop simplification rules based on realistic thermal imaging resolution.
Information about an existing structure can be collected at certain costs to evaluate the reliability and condition as realistically as necessary. This information can be relevant or irrelevant, true or biased, precise or imprecise. The incorporation of relevant and quality-assessed measured information into reliability reassessment offers the chance to extend remaining lifetimes and support decision making about optimal actions or maintenance strategies. This paper shows recent developments in a national research project that aims to produce a guideline on the NDT-based, structure-specific modification of partial safety factors. The general methodology, results from recalculations according to the Eurocodes and metrologically solvable testing tasks relevant in the recalculation of the concrete bridges are shown and compared with the non-destructive testing methods applicable to concrete bridges. A case study is used to demonstrate that as-built drawings, in this case of the positions of tendons and shear reinforcement, can be verified using the radar method.
Reliability assessment of existing concrete bridges with geometrical NDT results - Case studies
(2023)
The results of and the validity in reliability assessment of existing bridges essentially depend on the information available about the considered system. Information about the actual condition as well as structural and material characteristics can be observed on-site to refine the computation models used in assessment. Non-destructive testing (NDT) methods for concrete structures are capable of reconstructing missing, questioned, or inconsistent as-built plans. This contribution summarizes recent developments within the scope of the national pre-standardization project “ZfPStatik”, which aims to prepare a guideline about NDT-supported structural analyses. The focus is on the purposeful and explicit utilization of geometrical tendon and reinforcement bar positions (measured on-site using the ultrasound echo and ground penetrating radar (GPR) techniques) in probabilistic reliability analyses — shown by means of real case studies. The well-established first order reliability method is applied to different concrete bridges, which are typical for the German road bridge stock, to demonstrate the utility of incorporating quality-evaluated NDT-results in terms of changes in structural reliability.
Wind turbine rotor blades are produced as aerodynamic, three-dimensionally formed sandwich shell structures using mainly glass fiber reinforced plastics (GFRP). The GFRP used today have the potential to withstand the harsh operating conditions of rotor blades over the projected service life of 20 years. Premature damages that occur can be traced primarily back to design and/or manufacturing imperfections, as well as unusually high load scenarios (for instance lightning strikes, or strong winds combined with system control errors). Therefore, rotor blades have to be repaired several times (on average 2-3 times) during their projected service life. Approximately 70% of these are repairs of small- to medium-sized sandwich shells, carried out by technicians directly accessing the blade by suspended roping at the wind turbine location. The goal is to execute repairs that will last over the remaining service life, i.e. be sustainable.
Within the framework of a research project carried out at BAM, Division 5.3 Polymer Matrix Composites, Department 5 for Materials Engineering, not only was the geometric shape (round vs. square in relation to the top view) varied, but also the layup of the scarf repair structure, variables that have not been studied systematically to-date. Sub-component scale sandwich shell specimens with representative repairs were used in this work in addition to coupon-scale specimens that are more commonly seen in literature. These sub-component shell specimens were tested using a unique shell test bench under loading conditions representative of wind turbine blade shell operating conditions with respect to fatigue strength. Using non-destructive testing methods (field strain measurement and thermography), damage development and distribution was monitored and analyzed in-situ. As a result, a concept was developed in which the repaired areas showed at least equivalent if not higher fatigue strength than the reference shells that were not repaired.
Quantification of Deep Discharge Mechanism in a Li-Ion Battery by operando X-ray Computed Tomography
(2023)
Lithium-ion batteries connected in series are prone to an electrical abuse called over discharge. We present a comprehensive investigation of the over discharge abuse mechanism in lithium-ion batteries using operando non-destructive imaging. The study focuses on understanding the behavior of copper dissolution and deposition during over discharge, which can lead to irreversible capacity loss and internal short circuits. By utilizing synchrotron X-ray computed tomography (SXCT), the concentration of dissolved and deposited copper per surface area is quantified as a function of depth of discharge (DoD).
The dataset contains raw data gathered with the ultrasonic pulse-echo method on concrete specimens. The surfaces of the measuring objects were automatically scanned point by point. Pulse-echo measurements were performed at each of these measuring points. The test specimens represent two typical testing tasks in construction industry: the detection of objects and the determination of dimensions to describe the geometry of components. By automating the measurement process, the different test scenarios are examined with a high repeatability, precision and measuring point density. Longitu- dinal and transversal waves were used and the geometrical aperture of the testing system was varied. The low-frequency probes operate in a range of up to approximately 150 kHz. In addition to the specification of the geometrical dimensions of the individual probes, the directivity pattern and the sound field characteristics are provided. The raw data are stored in a universally readable format. The length of each time signal (A-scan) is two milliseconds and the sampling rate is two mega-samples per second. The provided data can be used for comparative studies in signal analysis, imag- ing and interpretation as well as for evaluation pur- poses in different, practically relevant testing scenarios.
Adopting X-ray computed tomography (XCT) for ex-situ characterization of battery materials has gained interest in the past decade. The main goal of this paper is to demonstrate the effectiveness of several X-ray computer tomography techniques to study commercial batteries. General guidelines are provided to select the most suitable imaging equipment and parameters for investigations of lithium-ion batteries, spanning the length scales from cell to electrode, down to particle level. Relevantly, such parameters would also be suitable for operando experiments.
Safety mechanisms and manufacturing inconsistencies at cell level as well as defects and inhomogeneity in cathode and anode is illustrated and quantified. Furthermore, relation of beam energy and sample-detector-distance on contrast retrieved from attenuation and phase shift is inspected using Synchrotron XCT.
This data set contains three different data types obtained from concrete specimens. For each specimen, the rebound numbers, ultrasonic data (ultrasonic velocity, time of flight), and destructive concrete strength are given. Two kind of specimen geometries were tested: cubes and drilled cores. The files are labeled according to the specimen geometry as "cube" or "core" and the type of measurement data as "compressive_strength", "rn_R" and "rn_Q" for rebound numbers as well as "us" for ultrasonic data. The ultrasonic data were generated by six independent laboratories, the rebound numbers by five independent laboratories and the destructive tests by one laboratory. The designation of each specimen establishes the relationship between the different data types.
The rails of modern railways face an enormous wear and tear from ever increasing train speeds and loads. This necessitates diligent non-destructive testing for defects of the entire railway system.
Non-destructive testing of rail tracks is carried out by rail inspection trains equipped with ultrasonic and eddy current test devices. However, the evaluation of the gathered data is mainly done manually with a strong focus on ultrasonic data, and defects are checked on-site using hand-held testing equipment. Maintenance measures are derived based on these on-site findings.
The aim of the AIFRI project (Artificial Intelligence For Rail Inspection) is to
- increase the degree of automation of the inspection process, from the evaluation of the data to the planning of maintenance measures,
- increase the accuracy of defect detection,
- automatically classify detected indications into risk classes.
These aims will be achieved by training a neural network for defect detection and classification. Since the current testing data is unbalanced, insufficiently labeled and largely unverified we will supplement fused, simulated eddy current and ultrasonic testing data in form of a configurable digital twin.
Decades of non-destructive evaluation (NDE) for the quality assurance of concrete bridges revealed the necessity of developing procedures for building reference specimens for different defects. External objects have been widely used in specimen construction to represent defects. However, embedded materials can alter the actual defect characteristics, interrupting the NDE responses. This study proposes new approaches to fabricating reference specimens for reinforced concrete that feature substantial defects, including cracks, delamination, honeycombing and rebar corrosion, without external objects. As a result, realistic responses can be acquired to identify promising NDE methods for characterising actual deteriorations. The procedures were produced with the intent of being reproducible in any laboratory. Destructive and non-destructive testing methods were performed to verify the effectiveness of the processes in creating defects. This study provides research laboratories with techniques for
fabricating reference specimens for characterising defects and assessing the performance of NDE technologies.
This study investigates the non-destructive detection of delaminations in concrete plates using non-contact laser ablation, instead of the conventional hammer excitation, as part of the impact echo method. We performed tests on five concrete specimens of different sizes, two of which contained artificial delaminations. A range of steel ball hammers was used as reference impulse sources, the responses of which were compared with wave excitation generated by a 7 ns pulsed 1064 nm Nd:YAG laser with 150 mJ pulse energy. Signals were recorded by surface-mounted accelerometers and two contactless methods: microphones and a laser Doppler vibrometer. The laser generates frequencies across a broad range of frequencies (0 to 150 kHz) but with much less energy than the hammers' narrower frequency spectra; the laser pulse energy transferred into the specimen is 0.07 mJ, corresponding to about 0.5 ‰ of the impulse source energy. Because of this, the thick intact plates' characteristic thickness stretch resonance frequency can be reliably detected by the hammer excitations but not when using laser excitation. However, the laser can excite low-frequency flexural vibration modes over a shallow delamination at 3 cm depth. The low-frequency flexural vibration results are verified by numerical natural frequency analysis.
In order to detect small and hidden material defects or micro-cracks, weak and localized magnetic stray fields need to be detected and characterized on the magnetic surface. Current flux leakage sensor elements are rigid and rely on air gaps to detect stray fields several hundred micrometers above the surface. Furthermore, the increasing complexity of critical components, enabled by novel fabrication technologies, primarily additive manufacturing, poses a significant challenge to the available flux leakage testing technologies.
The emergence of flexible magneto-electronics paves the way to a compliant form of magnetic sensors that, on one hand, are able to adapt to highly curved surfaces, allowing these measurements to be conducted on irregularly formed components, which are inaccessible for nowadays electromagnetic NDT equipment and, on the other hand, could enhance the detection limit for small magnetic stray fields, by allowing the magneto-sensitive elements to approximate the object’s surface until only a few µm, mediated by thin and flexible packaging.
In non-destructive testing and structural health monitoring with ultrasonic waves, the quantification of damage in components is one of the main tasks. In many shell-like structures, such as plates, pipes, or laminate components, ultrasonic waves propagate as guided waves. Although guided waves enable the testing of large areas, their multimodal and dispersive properties make it challenging to analyze signals. So, there is a need for more advanced algorithms to handle these properties, especially when reconstructing damage position and geometry.
The reconstruction can be formulated as an inverse problem where the measured signals are fitted with a simulative forward model. Due to the small wavelength of ultrasonic waves, classic forward models based on, e.g., the Finite Element Method are computationally intensive. In contrast, the authors use the semi-analytical Scaled Boundary Finite Element Method (SBFEM) to reduce the computational effort. The SBFEM approximates arbitrary long, undamaged parts of the structure with only a few degrees of freedom.
This contribution summarizes a general inverse procedure based on algorithmic differentiation in combination with the SBFEM. Results are presented for damaged 2D cross-sectional models of waveguides. These results include an analysis of the robustness of the proposed algorithms against noise.
The majority of ultrasonic devices used for non-
destructive testing in civil engineering require contact with the
surface of the concrete (specimen), which significantly increases
the time required for the measurement. This makes it impractical
for extensive investigation of large-scale structures such as bridge
decks, foundations, or tunnels.
In a pioneering approach, fluidic oscillators are used as contact
free ultrasonic sources to overcome the aforementioned limi-
tations. These robust and cost-effective actuators require only
pressurised air and are ideally suited for harsh environments. At
a constant supply pressure, they generate a continuous mono-
frequent actuation signal. Further, varying the supply pressure
via a fast pressure regulator was found to generate a frequency
modulated signal which enabled time-of-flight measurement with
an added advantage of increased signal to noise ratio. To
demonstrate the feasibility of this novel idea of non-contact
ultrasound, the results of the initial tests are presented.
The majority of ultrasonic devices used for non-destructive testing in civil engineering require contact with the surface of the concrete (specimen), which significantly increases the time required for the measurement. This makes it impractical for extensive investigation of large-scale structures such as bridge decks, foundations, or tunnels.
In a pioneering approach, fluidic oscillators are used as contact
free ultrasonic sources to overcome the aforementioned limitations. These robust and cost-effective actuators require only pressurised air and are ideally suited for harsh environments. At a constant supply pressure, they generate a continuous monofrequent actuation signal. Further, varying the supply pressure via a fast pressure regulator was found to generate a frequency modulated signal which enabled time-of-flight measurement with an added advantage of increased signal to noise ratio. To demonstrate the feasibility of this novel idea of non-contact ultrasound, the results of the initial tests are presented.
The majority of ultrasonic devices used for non-destructive testing in civil engineering require contact with the surface of the concrete (specimen), which significantly increases the time required for the measurement. This makes it impractical for extensive investigation of large-scale structures such as bridge decks, foundations, or tunnels.
In a pioneering approach, fluidic oscillators are used as contact free ultrasonic sources to overcome the aforementioned limitations. These robust and cost-effective actuators require only pressurised air and are ideally suited for harsh environments. At a constant supply pressure, they generate a continuous mono-frequent actuation signal. Further, varying the supply pressure via a fast pressure regulator was found to generate a frequency modulated signal which enabled time-of-flight measurement with an added advantage of increased signal to noise ratio. To demonstrate the feasibility of this novel idea of non-contact ultrasound, the results of the initial tests are presented.
Direct Laser Writing (DLW) and X-ray computed tomography (XCT) both offer unique possibilities in their respective fields. DLW produces full three-dimensional (3D) polymer structures on the microscale with resolutions below 100 nm. The fabricated structures can be analysed by XCT or X-ray microscopy (XRM), which incorporates additional X-ray lenses, in three dimensions down to a minimal basic spatial resolution of about 500 nm or 50 nm, respectively. In this work, two different DLW structures are analysed via XCT. Internal defects are detected and analysed for the purpose of quality control. Defects and structures with sizes down to 1.5 µm are successfully analysed. A 3D reconstruction and internal, hidden features of the fabricated structures are shown and discussed. In a first-of-its-kind study, we demonstrate the detectability of a single-voxel line inside a fabricated structure that would not be detectable with SEM or light microscopy. Furthermore, the direct fabrication on a PET substrate is shown to overcome the high X-ray absorbance of commonly used glass substrates. Attenuation spectra of SZ2080 and glass substrates are compared to a fabrication route direct on a 170 µm PET foil. The practical aspects of XCT measurements for DLW structures on different substrates will be discussed.
Visually detecting irregularities on components is a complex process, influenced by physical, technical, and human factors. Current standards mainly focus on illuminance, demanding highest accuracy for it, without providing scientific justification. In this research, 63 inspectors visually inspected forgings, with and without crack-like indications. The influence of illuminance on probability of detection was investigated. Therefore, illuminance was varied in three steps (200 lx, 350 lx, 500 lx). Human factors (e.g., experience, motivation) were recorded by using questionnaires. The results show no significant difference in probability of detection between the three illuminance levels. Equal probability of detection rates could be achieved at illuminances below and at the required threshold (500 lx). A difference in illuminance was found between the measurement point and the test area of the respective participant. This leads to higher assumed illuminance than present on the component. Compared with the measurement accuracy of the used illuminance meter, none of these deviations could be used to explain the similar results at different illuminance. The fact that illuminance has a significant influence in detecting indications has been disproven for this case. Whether human factors can provide explanation for the results remains unclear. This will be investigated in further research.
Existing concrete structures were usually designed for lifetimes of several decades. The current and urgently required efforts to increase sustainability and protect the environment will likely result in extended service lives up to 100 years. To achieve such objectives, it is required to assess structures over their entire lifecycles. Non-destructive testing (NDT) methods can reliably support the assessment of existing structures during the construction, operational, and decommissioning phases. One of the most important and safety-relevant components of a prestressed concrete structure are the tendons. NDT methods such as the ultrasonic echo method are suitable for both the detection and the localization of the tendons, i.e., the measurement of their geometrical position inside the component. The uniqueness of structures, concrete heterogeneity, and varying amounts of secondary components such as the reinforcement represent obstacles in the application of these methods in practice. The aim of this contribution is to demonstrate a practicable procedure, that can be used in the field to determine the parameters required for the measuring data analysis without extensive knowledge about the investigated components. For this purpose, a polyamide reference specimen is used to show which steps are required to obtain reliable imaging information on the position of tendons from the measurement data. The procedure is then demonstrated on a concrete test specimen that covers various relevant and practice-oriented test scenarios, such as varying tendon depths and component thicknesses.
In addition to already established structural monitoring methods such as deformation, inclination or strain gauges or acoustic emission sensors, sonic or ultrasonic monitoring might provide valuable information about the condition or alteration of a structure. Sensors such as geophones, recording ambient noise in the sonic and subsonic frequency range can provide information beyond modal analysis by using interferometric methods. Wave velocities determined by this method are related to the elastic properties and stiffness of material and structure and can be converted into damage indicators. Embedded active ultrasonic transducer networks can provide more detailed insight about deterioration or damages again, using interferometric technologies. This approach is extremely sensible, detecting relative change in velocity on down to 10-5. These methods, including benefits and remaining challenges, are demonstrated using data from a test structure at BAM’s test site demonstrating the case of prestress loss, and data from an actual bridge still under traffic.
Existing concrete structures were usually designed for lifetimes of several decades. The current and urgently required efforts to increase sustainability and protect the environment will likely result in extended service lives up to 100 years. To achieve such objectives, it is required to assess structures over their entire lifecycles. Non-destructive testing (NDT) methods can reliably support the assessment of existing structures during the construction, operational, and decommissioning phases. One of the most important and safety-relevant components of a prestressed concrete structure are the tendons. NDT methods such as the ultrasonic echo method are suitable for both the detection and the localization of the tendons, i.e., the measurement of their geometrical position inside the component. The uniqueness of structures, concrete heterogeneity, and varying amounts of secondary components such as the reinforcement represent obstacles in the application of these methods in practice. The aim of this contribution is to demonstrate a practicable procedure, that can be used in the field to determine the parameters required for the measuring data analysis without extensive knowledge about the investigated components. For this purpose, a polyamide reference specimen is used to show which steps are required to obtain reliable imaging information on the position of tendons from the measurement data. The procedure is then demonstrated on a concrete test specimen that covers various relevant and practice-oriented test scenarios, such as varying tendon depths and component thicknesses.
Introduction of a monitoring system for Bingham fluids in additive manufacturing with concrete
(2022)
Freeform additive manufacturing of concrete structures is a rising technology in civil engineering with several fascinating advantages. Nonetheless, to ensure reliability and structural integrity, standards and quality control are required in the future to bring this technology into the market. As the concrete is manufactured continuously, continuous quality control of the printing process is also required, i.e. comprehensive process monitoring. At BAM, a test rig will be installed, enabling the printing of concrete structures with a maximum size of 2 m x 1 m x 1 m (l x w x h). Here, process monitoring is the focus of the test rig. In this study, we show the results of the first pump tests, including the measurement of several parameters such as temperature and pressure along the supply system, i.e. from the concrete pump to the printer head.
Introduction of a monitoring system for Bingham fluids in additive manufacturing with concrete
(2022)
Freeform additive manufacturing of concrete structures is a rising technology in civil engineering with several fascinating advantages. Nonetheless, to ensure reliability and structural integrity, standards and quality control are required in the future to bring this technology into the market. As the concrete is manufactured continuously, continuous quality control of the printing process is also required, i.e. comprehensive process monitoring. At BAM, a test rig will be installed, enabling the printing of concrete structures with a maximum size of 2 m x 1 m x 1 m (l x w x h). Here, process monitoring is the focus of the test rig. In this study, we show the results of the first pump tests, including the measurement of several parameters such as temperature and pressure along the supply system, i.e. from the concrete pump to the printer head.
Learned block iterative shrinkage thresholding algorithm for photothermal super resolution imaging
(2022)
Block-sparse regularization is already well known in active thermal imaging and is used for multiple-measurement-based inverse problems. The main bottleneck of this method is the choice of regularization parameters which differs for each experiment. We show the benefits of using a learned block iterative shrinkage thresholding algorithm (LBISTA) that is able to learn the choice of regularization parameters, without the need to manually select them. In addition, LBISTA enables the determination of a suitable weight matrix to solve the underlying inverse problem. Therefore, in this paper we present LBISTA and compare it with state-of-the-art block iterative shrinkage thresholding using synthetically generated and experimental test data from active thermography for defect reconstruction. Our results show that the use of the learned block-sparse optimization approach provides smaller normalized mean square errors for a small fixed number of iterations. Thus, this allows us to improve the convergence speed and only needs a few iterations to generate accurate defect reconstruction in photothermal super-resolution imaging.
Alkali-silica reaction (ASR) is a chemical reaction within concrete which can lead over time to cracking and spalling. Due to the complexity of the problem, it still causes damage to concrete constructions worldwide.
The publication aims to illustrate the interdisciplinary research of the German Federal Institute for Materials Research and Testing (BAM) within the last 20 years, considering all aspects of ASR topics from the macro to the micro level. First, methods for characterization and assessment of ASR risks and reaction products used at BAM are explained and classified in the international context. Subsequently the added value of the research approach by combining different, preferably nondestructive, methods across all scales is explained using specific examples from a variety of research projects. Aspects covered range from the development of new test-setups to assess aggregate reactivity, to analysis of microstructure and reaction products using microscopical, spectroscopical and X-ray methods, to the development of a testing methodology for existing concrete pavements including in-depth analysis of the visual damage indicator and the de-icing salt input using innovative testing techniques. Finally, research regarding a novel avoidance strategy that makes use of internal hydrophobization of the concrete mix is presented.
The dataset presented contains ultrasonic data recorded in pulse echo mode. The investigated specimen is made of the isotropic homogeneous material polyamide and has a drill hole of constant diameter running parallel to the surface, which was scanned in a point grid using an automatic scanner system. At each measuring position, a pitch-catch measurement was performed using a sampling rate of 2 MHz. The probes used are arrays consisting of a spatially separated receiving and in-phase transmitting unit. The transmitting and receiving sides each consist of 12 point-shaped single probes. These dry-point contact (DPC) probes operate according to the piezoelectric principle at nominal frequencies of 55 kHz (shear waves) and 100 kHz (longitudinal waves), respectively, and do not require a coupling medium. The measurements are performed with longitudinal (100 kHz) and transverse (55 kHz) waves with different geometric orientations of the probe on the measurement surface. The data presented in the article provide a valid source for evaluating reconstruction algorithms for imaging in the low-frequency ultrasound range.
This paper describes a new ultrasonic measuring device called “W-Box”. It was developed based on the requirements of the DFG Forschergruppe (research unit) CoDA for a portable device for monitoring of concrete specimens, models and actual structures using embedded ultrasonic transducers as well as temperature and humidity sensors. The W-Box can send ultrasonic pulses with a variable frequency of 50–100 kHz to one selectable transducer and records signals from up to 75 multiplexed channels with a sample rate of 1 MHz and a resolution of 14 bits. In addition, it measures temperature and humidity with high accuracy, adjustable amplification, restarts automatically after a power failure and can be fully controlled remotely. The measured data are automatically stored locally on-site data quality checks and transferred to remote servers. The comparison of the W-Box with a laboratory setup using commercial devices proves that it is equally reliable and precise, at much lower cost. The W-Box also shows that their measurement capacities, with the used embedded ultrasonic transducers, can reach above 6 m in concrete.
Development of an Accurate and Robust Air-Coupled Ultrasonic Time-of-Flight Measurement Technique
(2022)
Ultrasonic time-of-flight (ToF) measurements enable the non-destructive characterization of material parameters as well as the reconstruction of scatterers inside a specimen. The time-consuming and potentially damaging procedure of applying a liquid couplant between specimen and transducer can be avoided by using air-coupled ultrasound. However, to obtain accurate ToF results, the waveform and travel time of the acoustic signal through the air, which are influenced by the ambient conditions, need to be considered. The placement of microphones as signal receivers is restricted to locations where they do not affect the sound field. This study presents a novel method for in-air ranging and ToF determination that is non-invasive and robust to changing ambient conditions or waveform variations. The in-air travel time was determined by utilizing the azimuthal directivity of a laser Doppler vibrometer operated in refracto-vibrometry (RV) mode. The time of entry of the acoustic signal was determined using the autocorrelation of the RV signal. The same signal was further used as a reference for determining the ToF through the specimen in transmission mode via cross-correlation. The derived signal processing procedure was verified in experiments on a polyamide specimen. Here, a ranging accuracy of <0.1 mm and a transmission ToF accuracy of 0.3μs were achieved. Thus, the proposed method enables fast and accurate non-invasive ToF measurements that do not require knowledge about transducer characteristics or ambient conditions.
Integration of fiber reinforcement in high-performance cementitious materials has become widely applied in many fields of construction. One of the most investigated advantages of steel Fiber reinforced concrete (SFRC) is the deceleration of crack growth and hence its improved sustainability.
Additional benefits are associated with its structural properties, as fibers can significantly increase the ductility and the tensile strength of concrete. In some applications it is even possible to entirely replace the conventional reinforcement, leading to significant logistical and environmental benefits.
Fiber reinforcement can, however, have critical disadvantages and even hinder the Performance of concrete, since it can induce an anisotropic material behavior of the mixture if the fibers are not appropriately oriented. For a safe use of SFRC in the future, reliable non-destructive testing (NDT) methods need to be identified to assess the fibers’ orientation in hardened concrete. In this study,
ultrasonic material testing, electrical impedance testing, and X-ray computed tomography have been investigated for this purpose using specially produced samples with biased or random Fiber orientations. We demonstrate the capabilities of each of these NDT techniques for fiber orientation measurements and draw conclusions based on these results about the most promising areas for future research and development.
Our standard protocol for the characterisation of writing materials within advanced manuscript studies has been successfully used to investigate manuscripts written with a pure ink on a homogeneous writing surface. However, this protocol is inadequate for analysing documents penned in mixed inks. We present here the advantages and limitations of the improved version of the protocol, which now includes imaging further into the infrared region (1100−1700 nm).
Ultrasonic testing is a widely applied measurement method in materials research and medicine. Commonly, a transducer is coupled to the specimen directly or via a liquid coupling agent. While reducing acoustic transmission losses significantly, this procedure is time-consuming and cannot be used for sensitive specimens. Air-coupled ultrasound is a viable alternative in such cases, although suffering from very high acoustic transmission losses between transducer, air and specimen. The recently introduced fluidic transducer (FT) generates ultrasound by utilizing the instability of a supersonic air jet switched inside a fluidic amplifier. Since only air is used as the working medium and no vibrating surfaces are used for ultrasound generation, the transducer is able to efficiently generate large acoustic pressure amplitudes. The resulting acoustic field shares its directivity with the ejected high-velocity air jet. Thus, the acoustic energy needs to be redirected from the jet axis in order to make the fluidic transducer applicable to sensitive specimens. In this study, the effectivity of using sonic crystals (SCs) for this redirection is investigated using acoustic and flow measurements. SCs are air-permeable while being reflective to large acoustic frequency bands. It was shown that both a defect waveguide and a mirroring strategy successfully redirected the acoustic field from the air jet. Furthermore, the interaction of flow and SC showed strong acoustic quenching if the SC was placed too close to the FT outlet. Blockage of the jet entrainment due to the SC may result in slightly higher off-axis flow velocities locally, which should be considered in sensitive applications.
ML has been successfully applied to solve many NDT-CE tasks. This is usually demonstrated with performance metrics that evaluate the model as a whole based on a given set of data. However, since in most cases the creation of reference data is extremely expensive, the data used is generally much sparser than in other areas, such as e-commerce. As a result, performance indicators often do not reflect the practical applicability of the ML model. Estimates that quantify transferability from one case to another are necessary to meet this challenge and pave the way for real world applications.
In this contribution we invetigate the uncertainty of ML in new NDT-CE scenarios. For this purpose, we have extended an existing training data set for the classification of corrosion damage by a new case study. Our data set includes half-cell potential mapping and ground-penetrating radar measurements. The measurements were performed on large-area concrete samples with built-in chloride-induced corrosion of reinforcement. The experiment simulated the entire life cycle of chloride induced exposed concrete components in the laboratory. The unique ability to monitor deterioration and initiate targeted corrosion initiation allowed the data to be labelled - which is crucial to ML. To investigate transferability, we extend our data by including new design features of the test specimen and environmental conditions. This allows to express the change of these features in new scenarios as uncertainties using statistical methods. We compare different sampling and statistical distribution-based approaches and show how these methods can be used to close knowledge gaps of ML models in NDT.
Fibre-Matrix-Debonding in glass fibre reinforced dental posts can play an important role for their mechanical performance and long term stability… We show that synchrotron X-ray refraction radiography (SXRR) allows analysis of large samples (up to several millimetres) without compromising the detectability of sub micrometer defects.
An approach to adhesive bond characterisation using guided acoustic waves in multi-layered plates
(2021)
An approach for the non-destructive characterisation of adhesive bonds using guided ultrasonic waves is presented. Pulsed laser radiation is used to thermoacoustically excite broadband ultrasonic waves in a multi-layered sample, consisting of a metal plate adhesively joined to a polymeric layer using synthetic resin. The resulting signals are received by a purpose-built piezoelectric transducer. Varying the distance between excitation and detection yields spatio-temporal measurement data, from which the dispersive properties of the propagating waves can be inferred using a two-dimensional Fourier transform, assuming the plates to act as coupled waveguides. Coupled multi-layered waveguides show an effect referred to as mode repulsion, where the distance between certain modes in the frequency-wavenumber domain is assumed to be a measure of coupling strength. Measurements at different stages of curing of the adhesive layer are performed and evaluated. A comparison of the results shows changes in the dispersive properties, namely an increased modal bandwidth for the fully cured sample as well as an increased modal distance.
While contact and immersion ultrasonic testing are established methods in non-destructive testing (NDT), generating high power air-coupled ultrasound remains a challenging task. Solutions often involve setups that are restricted to lab environments. When field measurements are required, such as in NDT for civil engineering, a handy, robust and safe transducer is needed. For this purpose, an ultrasonic transducer based on a fluidic switch has been developed. A sonic air flow inside the device is switched rapidly so that an ultrasonic signal is generated. Both theory and previous flow simulations suggest that the control flow pressure ramp has only little influence on the switching time of the device. This publication gives an overview over the operating principle of the fluidic ultrasonic transducer and investigates the influence of control tube length and pulsing repetition rate on the ultrasonic pressure amplitude. High repetition rates are found to reduce the signal amplitude, whereas long tubing has only little negative influence on the amplitude while improving signal quality.
Motivation:
• Air-coupled ultrasound (AC-US) enables faster measurements in non-destructive testing for civil engineering
• Current state of the art: piezo and capacitive transducers are mostly used for AC US
• Currently >99.9% sound intensity loss due to impedance mismatches
Innovation:
• Fluidic transducers generate US (30-60 kHz) by rapid switching of a supersonic jet
• Fluidic transducers rely on external components such as solenoid valves and tubing
--> How do repetition time and tube length influence the signal?
To date, the destructive extraction and analysis of drilling cores is the main possibility to obtain depth information about damaging water ingress in building floors. The time- and costintensive procedure constitutes an additional burden for building insurances that already list piped water damage as their largest item. With its high sensitivity for water, a ground-penetrating radar (GPR) could provide important support to approach this problem in a non-destructive way. In this research, we study the influence of moisture damage on GPR signals at different floor constructions. For this purpose, a modular specimen with interchangeable layers is developed to vary the screed and insulation material, as well as the respective layer thickness. The obtained data set is then used to investigate suitable signal features to classify three scenarios: dry, damaged insulation, and damaged screed. It was found that analyzing statistical distributions of A-scan features inside one B-scan allows for accurate classification on unknown floor constructions. Combining the features with multivariate data analysis and machine learning was the key to achieve satisfying results. The developed method provides a basis for upcoming validations on real damage cases.
Ultrasonic measurement technology has become indispensable in NDT. In order to reduce measurement time and extend the application to other materials, contactless ultrasound is the subject of many different research groups. Department 8 has been researching successfully in this field for years. A novel approach is based on so-called fluidic devices. These devices can be used to perform binary logic operations with the help of natural flow instabilities. Hence the abbreviated name, Fluidic (FLUID+LogIC). Only a pressure reservoir of the used fluid is required as energy supply. This enables the production of very robust actuators that generate ultrasonic signals in an extremely energy efficient way.
The presentation includes the research results of the ZIM innovation project OsciCheck. The original idea will be presented and its application on different building materials is validated. Beyond this, the possible application areas are much larger and a detailed outlook is given to discuss the future potential of fluidic ultrasonic actuators.
Pulse-compression thermography is an emerging technique that has shown versatility by combination of pulsed and lock-in thermography. Accordingly, several aspects of this technique are still unexplored, and some others not fully developed yet. Barker codes were widely used in radar applications due to their simplicity and their optimum autocorrelation function. Nevertheless, applications were limited by the amplitude of the sidelobes present in the autocorrelation function and therefore, several filters have been developed which aim to reduce the sidelobes. However, the filters usually depend on empirical parameters which must be determined for each application. A better alternative would improve the applicability of the Barker codes. In this work, we further develop the pulse-compression thermography technique by introducing a 13-bit modified Barker code (mBC): This allows to drastically reduce the sidelobes characteristic of the 13-bit Barker code (BC). Consequently, the thermographic impulse response, obtained by cross-correlation, is almost free of such sidelobes. Deeper defects become easier to detect in comparison with using a 13-bit Barker code. Numerical simulations using the finite element method are used for comparison and experimental measurements are performed in a sample of steel grade St 37 with machined notches of three different depths: 2 mm, 4 mm and 6 mm.
Pulse-compression thermography is an emerging technique that has shown versatility by combination of pulsed and lock-in thermography. Accordingly, several aspects of this technique are still unexplored, and some others not fully developed yet. Barker codes were widely used in radar applications due to their simplicity and their optimum autocorrelation function. Nevertheless, applications were limited by the amplitude of the sidelobes present in the autocorrelation function and therefore, several filters have been developed which aim to reduce the sidelobes. However, the filters usually depend on empirical parameters which must be determined for each application. A better alternative would improve the applicability of the Barker codes. In this work, we further develop the pulse-compression thermography technique by introducing a 13-bit modified Barker code (mBC): This allows to drastically reduce the sidelobes characteristic of the 13-bit Barker code (BC). Consequently, the thermographic impulse response, obtained by cross-correlation, is almost free of such sidelobes. Deeper defects become easier to detect in comparison with using a 13-bit Barker code. Numerical simulations using the finite element method are used for comparison and experimental measurements are performed in a sample of steel grade St 37 with machined notches of three different depths: 2 mm, 4 mm and 6 mm.
This data article presents characteristic acoustic and flow data of a fluidic ultrasonic transducer as well as acoustic data of a commercial piezoelectric ultrasonic transducer used in non-destructive testing for civil engineering. The flow data has been acquired using hot-wire anemometry and a Pitot tube. The three-dimensional acoustic data of both devices has been acquired using a calibrated microphone. The distribution of characteristic acoustic properties of both transducers are extracted and given in addition to the raw data. The data presented in the article will be a valuable source for reference and validation, both for developing fluidic and alternate ultrasound generation technologies. Furthermore, they will give additional insight into the acoustic-flow interaction phenomena of high speed switching devices. This article is accompanying the paper Experimental Analysis of the Acoustic Field of an Ultrasonic Pulse Induced by a Fluidic Switch (Bühling et al., 2021) published in The Journal of the Acoustical Society of America, where the data is interpreted in detail and the rationale for characteristic sound properties of the fluidic transducer are given.
While X-ray based darkfield imaging with grating interferometers is already widely used, darkfield imaging with neutrons has still a relatively small user community focused mostly on magnetic materials. Here, we demonstrate the application of neutron darkfield imaging byTalbot-Lau type grating interferometry to fiber reinforced plastics. Common carbon and glass fiber composites have been investigated including characteristic damage structures. The darkfield images show a strong signal response caused by fiber delamination, suitable fiber direction, particles, pores and cracks. The basic principles of neutron darkfield imaging applied to fiber composites are highlighted.
The aim of this work is to improve the current structural health monitoring (SHM) methods for civil structures. A field experiment was carried out on a two-span bridge with a built-in un-bonded prestressing system. The bridge is a 24-metre long concrete beam resting on three bear-ings. Cracks were formed subsequently when a prestressing force of 350 kN was changed to 200 kN, so that different structural states could be demonstrated. The structural assessment of this reference bridge was accomplished by the non-destructive testing using ultrasonic devices and vibration measurements. The ultrasonic velocity variations were investigated by using the coda wave interferometry method. The seismic interferometry technique was applied to the vi-bration recordings to reconstruct the wave propagation field in the bridge. This investigation shows that the wave velocity is sensitive to the current structural state and can be considered as the damage indicator. Overall, the implementation of coda cave interferometry and seismic interferometry technique facilitates structural health monitoring (SHM) in civil engineering.
Pressure vessels for the transport of hazardous gases are subjected to a hydraulic internal pressure test. Basis for the design of metallic pressure vessels is the fatigue strength according to Wöhler, whereby the number of load changes can be equated with the number of filling / emptying cycles of the pressure vessel. Metallic pressure vessels are increasingly replaced by composite pressure vessels due to the weight savings. However, at composite preasure vessels it is not possible to conclude from the number of load cycles on the fatigue strength, because composite pressure vessels are ageing even at constant internal pressure. Non-destructive conventional and high frequency eddy current testing of the metallic liner and load bearing CFRP layer are presented as an alternative to hydraulic internal pressure test.
Air-coupled ultrasonic testing is widely used in the industry for the non-destructive testing of compound materials. It provides a fast and efficient way to inspect large concrete civil infrastructures for damage that might lead to catastrophic failure. Due to the large penetration depths required for concrete structures, the use of traditional piezoelectric transducer requires high power electric systems. In this study, a novel fluidic transducer based on a bistable fluidic amplifier is investigated. Previous experiments have shown that the switching action of the device produces a high-power broadband ultrasonic signal. This study will provide further insight into the switching behaviour of the fluidic switch. Therefore, parametric CFD simulations based on compressible supersonic RANS simulations were performed, varying the inlet pressure and velocity profiles for the control flow. Switching times are analyzed with different methods, and it was found that These are mostly independent of the slope of the velocity profile at the control port. Furthermore, it was found that an inversely proportional relationship exists between flow velocity in the throat and the switching time. The results agree with the theoretical background established by experimental studies that can be found in the literature.
Experimental analysis of the acoustic field of an ultrasonic pulse induced by a fluidic switch
(2021)
Ultrasonic inspection is a common tool for non-destructive testing in civil engineering (NDT-CE). Currently, transducers are coupled directly to the specimen surface, which makes the inspection time-consuming. Air-coupled ultrasound (ACU) transducers are more time-efficient but need a high pressure amplitude as the impedance mismatch between the air and the concrete is high and large penetration depth is needed for the inspection. Current approaches aim at eliminating the impedance mismatch between the transducer and the air to gain amplitude; however, they hardly fulfill the NDT-CE requirements. In this study, an alternative approach for ultrasound generation is presented: the signal is generated by a fluidic switch that rapidly injects a mass flow into the ambience. The acoustic field, the flow field, and their interaction are investigated. It is shown that the signal has dominant frequencies in the range of 35–60 kHz, and the amplitude is comparable to that of a commercial ACU transducer.
High quality tubular products are essential to the oil and gas industry. Quality control during their production focuses on the non-destructive detection of surface defects. The structured light technique is a candidate for the challenge to detect, monitor and evaluate such defects in real-time.
In the present study the automatic processing of structured light measurements is performed and validated. The algorithm for the automatic Analysis of inspection data has an advantage over current data evaluation methods based on individual assessments of operators.
Objectives. There is concern that the integrity of fiberglass dental posts may be affected by chairside trimming during treatment. We hypothesize that hard X-ray methods of phase contrast-enhanced micro-CT (PCE-CT) and synchrotron based X-ray refraction (SXRR) can reliably identify and help characterize the extent of damage.
Methods. Fiberglass posts were imaged both as manufactured and following trimming with a diamond bur. Each of the posts was imaged by SXRR and by PCE-CT. Datasets from PCE-CT were used to visualize and quantify 2D and 3D morphological characteristics of intact and of damage-affected regions caused by trimming. Results. The SXRR images revealed fiber inhomogeneities from manufacturing with a significant increase in internal surfaces in sample regions corresponding to damage from trimming. PCE-CT volumes unveiled the micromorphology of single fibers in the posts and some damage in the trimmed area (e.g. fractures, splinters and cracks). Area, perimeter, circularity, roundness, volume and thickness of the glass fibers in the trimmed area were statistically different from the control (p < 0.01).
Significance. The integrity of single fibers in the post is critical for bending resistance and for long-term adhesion to the cement in the root canals. Damage to the fibers causes substantial structural weakening across the post diameter. Glass fragments produced due to contact with the dental bur may separate from the post and may significantly reduce bond capacity. The above mentioned synchrotron-based imaging techniques can further facilitate assessment of the structural integrity and the appearance of defects in posts (e.g. after mechanical load).
News from NDT@BAM
(2021)
The presentation summarizes some recent research at BAM in the field of NDT in civil engineering. the firste xample decribes lab experiments on the degardation of concrete during tunnel fires. Radar measuremenst were sucessfully used to detect interior damage. The second example decribes the first ever experiment on using muon imaging to evaluate concrete constructions.
The manuscripts – which have been inventoried as Manuscripta Americana in the State Library of Berlin and the Biblioteka Jagellońska in Krakow with the signatures Ms. Americ. 1-15 – are codices mainly from the colonial times of the 16th to 18th centuries. They uniquely reflect the relationships between indigenous culture and Spanish colonial masters. This study focusses on the scientific investigation of three manuscripts (Manuscripta Americana 3, 8, and 10) that are kept today in Krakow. It is assumed that these three parts belong together in a certain way, so various material analyses were carried out to prove this assumption. The scientific investigation focusses on the different black and brown writing inks.
Crack detection in steel bars at high surface temperatures is a critical problem in any manufacturing industry. Surface breaking cracks are the major problems during the billet casting. Many NDT techniques are proven its capability in crack detection at room temperature. Here, we are demonstrating the possibility of exposure of cracks using laser line thermography at higher surface temperatures (up to 600 °C). A continuous-wave (CW) laser is used to excite the sample kept at higher surface temperatures. The temperature distribution over the sample due to the laser line scanning is captured using a temperature calibrated infrared (IR) thermal camera. The response of the sample temperature in crack detection is investigated using a validated FE model. The impact of the oxide layer in crack detection is investigated by using two types of samples; one without any oxide layer and the second is with the oxide layer. The influence of laser power in the detection of defects at high temperatures is studied. 3D numerical models were developed for the cases; when the sample is with oxide layer and without any oxide layer for a better understanding of physics. The surface temperature rise due to laser heating is higher for the scaled sample compared to the no-scale sample. The presence of the oxide layer above the parent metal will reduce the reflectivity of the surface. Lower reflectivity will lead to increased absorption of incident energy so that the surface temperature rise will be higher than the surface with no scale. Thermal contrast linearly depends on laser power, which means higher laser power will increase the defect detectability even at a higher surface temperature.
Nuclear Power Plants have been in operation for ca. 50 years. Based on this experience, non-destructive testing tasks specific to thick and highly reinforced nuclear containment structures have materialized. The performance based Service life extension of existing NPPs also needs a measurement based decision to support continuing the service of the concrete part of the installations.
By nature, concrete is a very durable material and any natural Deterioration processes may take a long time to become critical to the structure. The experience of more than 50 years of service limits the testing tasks to a few ones which are not yet solved.
Research in NDT of concrete structures is performed by many research institutions all over the world with different technical and systematic approaches. Results are mainly obtained on laboratory specimens, sometimes additional field studies are reported. This research takes place independently without coordination and as a result, the outcomes mostly lack full comparability.
Software for data analysis has become indispensable and very powerful. This part of testing needs more attention when it comes to evaluate test results. Validation of NDE solutions is becoming a critical part in concrete structure testing.
Validation is by definition the proof that a customer´s requirements in the test are being met by the testing solution. This includes equipment, personnel qualification and data analysis. In the following tables, the research recommendation Validation is used in the sense, that proof of the performance of existing solutions needs to be adressed. In general, a validation methodology for NDE solutions for concrete testing in itself needs to be researched and established.
Comparability of research also needs an accepted and easily accessible reference. From experience, it is almost impossible to manufacture exact copies of test specimens at different locations. Round Robin tests are therefore needed to evaluate the performance of a test.
Data evaluation is generally done using dedicated software, sometimes Hardware dependant and not interchangeable between systems. Software is ever more increasingly becoming more powerful and sophisticated. An in-depth evaluation needs to address the comparability and validation of Software used for data analysis and evaluation. The vision of a unified software pool for NDE investigations would undoubtedly support research tremendously.
Quantitative NDE is mostly recommended to assess the condition of a structure. However, qualitative data can be very useful, especially for processes which change the material properties or deteriorate the structure (e.g. corrosion of reinforcement). The need for reliable baseline data is a key factor for such monitoring tasks.
Monitoring of concrete structures is of utmost importance in maintenance and preservation of infrastructure. As a part of the DFG research group CoDA (Concrete Damage Assessment by Coda Waves), the works within this PhD project aim for identification of damage sensitive parameters extracted from ultrasonic measurements obtained with embedded sensors, the determination of environmental (reversible) influences on the signal and the localisation of damaged areas. The main technique used for the detection of changes in the monitored material is the so-called Coda Wave Interferometry (CWI). It uses the later part of the ultrasonic recording for the detection of small changes in the sensed area and the calculation of a relative velocity change. Using this technique we can show that we are able to detect changes in concrete temperature with ultrasound measurements. This enables temperature correction for ultrasound monitoring with embedded sensors. The crucial point for imaging and localisation with coda waves is the calculation of the so-called sensitivity kernels. We propose the application of numerical wave simulations for the kernel calculations instead of the commonly used diffusivity equation. Using finite-difference wave modelling code from our DFG project partners we are able to calculate those Kernels and first results are shown in this presentation. The major task for the second year will be the development and implementation of the inversion algorithm as well as the preparation of validation experiments.
X-ray back scatter imaging is rarely applied compared to classical X-ray projection imaging. More than 20 years ago the company Philips developed “COMSCAN”, a first application case for aircraft industry, which allowed even a depth resolution using back scatter imaging. The company AS&E in Boston offers back scatter imaging solutions for the security market. The principle is to scan the object with a highly collimated X-ray needle beam from one side only and to detect the backscattered radiation by a large area detector side by side with the collimation wheel. A new prototype is investigated at BAM for application and optimization in non-destructive testing. As modern industrial application field in-situ inspection in additive manufacturing is targeted. The accessibility of the printed part during the production process is very limited. This prevent the application of a two sided X-ray inspection or Computed Tomography, were an rotation of the object is required to acquire projections from 360 degrees. An important advantage for the X-ray back scatter technique are also the materials used in additive manufacturing (polymers, ceramics, light metals like Aluminum or Titanium). These materials with lower density and lower Z values give better scatter signals than metals with higher densities and Z values. The back scatter intensity decreases with increasing density and Z value of the material. But the requirements on spatial resolution and contrast sensitivity are more stringent for non-destructive testing of additive manufactured parts compared to the security area. In NDT sizes of indications smaller than 1 mm have to be detected clearly. The investigation of these limits on a state-of-the-art prototype for X-ray back scattering using rotating collimated X-ray needle beams is a part of the BAM project “ProMoAM”. The contribution shows first results of the optimization for NDT and the achieved application limits for several example cases.
The aim of the lecture is to present the productive interplay of connoisseurs and material analysis using the example of selected drawings from the Rembrandt collection of the Klassik Stiftung Weimar. We try to determine the point at which the question of the materiality of the drawings makes sense in the traditional discourse of connoisseurship. On the one hand, this involves the question of authorship – Rembrandt – or not? – and the locating of attributed drawings in the context of the work. On the other hand, it is about making drawing processes visible, characterizing ways of drawing and workshop practices as well as distinguishing different forms of reworking and copying within the Rembrandt workshop. The material analyses culminate in the question of whether Rembrandt’s inks can be distinguished at all.
It becomes apparent that the newly gained knowledge about the materiality of the drawing no longer reveals itself to the “mere” view of the connoisseur. Rather, it is only generated by the digital image. The rhetoric of connoisseurs must therefore be contrasted with the necessity of a hermeneutics of the digital image. The envisaged “material turn” of drawing science is thus simultaneously able to take up the critique of the question of authorship, which has been growing ever more recently, and to transfer it to a more comprehensive perspective of aesthetics of production.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
Using magnetic materials for energy conversion as an example, this lecture shows how X-ray tomography investigations can contribute to structure elucidation in composites and solid samples. The components are tested non-destructively in order to characterize cracks, pores and other defects and their influence on the functional properties three-dimensionally and in good time in the life cycle of the material. If you combine microtomography with other methods of magnetic material characterization, you can make unique statements about the structure and the functional properties.
To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. For thermomagnetic materials, we used a commercial magnetocaloric alloy with a transition temperature of 300 K.
Commercially available air-coupled transducers applied in non-destructive testing consist of a piezocomposite material and matching layers to reduce the impedance mismatch between the transducer and air. This contribution is an overview of innovative approaches using new piezoelectric materials and other physical principles to transmit and receive an ultrasonic pulse in air. Capacitive and piezoelectric micromachined ultrasonic transducers (CMUTs and PMUTs) produce high pressure levels, but they exhibit a very narrow bandwidth. Optical laser-based methods for transmitting and receiving ultrasound promise a higher bandwidth, but do not achieve the same sensitivity as conventional air-coupled transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties, having a very small acoustic impedance well matched to air. Ferroelectret transducers achieve about the same bandwidth as the most broadband conventional air-coupled transducers, having a higher sensitivity. Thermoacoustic transducers use heat to initiate an acoustic wave, acting as transmitters in ultrasonic range. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing. The newest member of the family of air-coupled ultrasonic transmitters is the plasma-based transducer, using both the thermoacoustic effect and the movements of the ions (so called ionic wind) to create acoustic waves.
Statistically the mean time between damage events on rotor blades is 6 years (Deutscher Windenergie Report 2006). Due to imperfection in the production the shell structures get cracks after a few years fare before the designed life time. A shell test rig was built at BAM for efficient research on the effects of defects in production.
In-situ and ex-situ NDT give a better understanding from degradation processes in composite materials.
With advanced methods in the research on the fatigue behaviour of FRP it was found a load level of infinite life for GFRP and CFRP. This is in the range of typical strain values of airliners and rotor blades in normal operation.
Due to the fibre-composite nature NDT techniques have to be suitable to a wide length scale to image micro cracking as well as bigger defects. Therefore different techniques have to be applied and developed.
A shell test bench was developed at BAM 5.3 which allows for static and fatigue testing of curved fiber-reinforced plastic (FRP) structures, during which in-situ the damage state can be non-destructively inspected by thermography and strain-field measurement techniques. Sandwich shell specimens with typical wind turbine blade manufacturing defects were designed and tested. The tested imperfections show a fairly significant reduction (up to 90%) of the shell test specimens‘ lifetime, depending on the type of imperfection. Using the in-situ NDT methods incorporated in the shell test bench, the location and cycle time of the initial defects and the damage evolution was investigated.
An overview of the BAM funed Focus Area Materials Project "AGIL" will be presented. AGIL focussed on the stdiy of the ageing characteristics of additively manufactured austenitic stainless steel with a "powder to mechanical failure" Approach. Recent Highlights are presented and a perspective for future studies.
Conventional ultrasonic testing involves the application of a fluid couplant for impedance matching. Increasing use of lightweight structures, composite materials and adhesive joints mostly in aerospace and automotive industry created an increasing interest in air-coupled ultrasonic testing methods, to protect sensitive surfaces and simplify maintenance. Commercially available air-coupled transducers consist of a piezocomposite material and matching layers to reduce the impedance mismatch between the transducer and air. This contribution is an overview of innovative approaches using new piezoelectric materials and other physical principles to transmit and receive an ultrasonic pulse in air.
Capacitive and piezoelectric micromachined ultrasonic transducers (CMUTs and PMUTs) produce high pressure levels, but they exhibit a very narrow bandwidth. Optical laser-based methods for transmitting and receiving ultrasound promise a higher bandwidth, but do not achieve the same sensitivity as conventional air-coupled transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties, having a very small acoustic impedance well matched to air. Ferroelectret transducers achieve about the same bandwidth as the most broadband conventional air-coupled transducers, having a higher sensitivity. Thermoacoustic transducers use heat to initiate an acoustic wave, acting as transmitters in ultrasonic range. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing. The newest member of the family of air-coupled ultrasonic transmitters is the plasma-based transducer, using both the thermoacoustic effect and the movements of the ions (so called ionic wind) to create acoustic waves.
The investigation of physical properties and chemical composition generates data important for answering cultural-historical questions that cannot be solved by historical and philological methods alone. Due to technological developments, technical diagnostics in art and culture are in ever-greater demand in such fields of transdisciplinary research. Natural sciences play auxiliary role in the studies of manuscripts. The success of their contribution depends strongly on the formulation of the question, the choice of the methods to obtain the requested answer, and appropriate reference databases. First, report was given on the measurements performed on local raw materials, such as, plants, minerals and animals, which constitute the reference session. Then, the first results of the scientific analysis of several fragments from the Malian manuscript collections were present.
A novel approach for rating fatigue-initiating inclusions in highly demanding steel (INCAFAT)
(2018)
INCAFAT project aimed to improve existing fatigue damage models by establishing the most suitable combination of measurement techniques to characterise harmful inclusion populations in highly demanding steels. The different inclusion assessments carried out confirm that, chemical composition, secondary metallurgy and manufacturing route affect content, nature, size and shape of inclusions. According to the FEM model, inclusions produce an alteration of the stress field in their surrounding region, which can promote a fatigue failure. Experimental work on fatigue testing has demonstrated that depending on the stressed direction fatigue failures in highly demanding steels could be produced by different types of inclusions. Fractography analyses confirmed that meso-inclusions harmful in fatigue cannot be rated by standard methods, nor 10 MHz ultrasonic testing (macro) or micro-cleanness assessments. The necessity of rating these meso-inclusions has led to critical evaluation of Extreme Value Analysis according to ASTM E2283-08 and the development of high frequency immersion ultrasonic testing. EVA methodology based on inclusion width can be applied reliably when principal stress is parallel to the rolling direction. On the contrary, if inclusions are testing in the elongated directions its fails. On the other hand, the guidelines and recommendations for high frequency ultrasonic testing have been compiled in a new European standard draft. This method based on focal beam probes and high-resolution devices is able to provide information on meso-inclusion distribution.
Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to fatigue phenomena that could lead to premature failure of certain structural elements. To date, fatigue of reinforced concrete has been given little focus. Knowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate.
Meanwhile, this topic has got much more attention as time-variant loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, and for wind turbines for renewable energy production, e.g. for offshore wind turbine support structures affected by wind and waves.
The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing concrete structures.
First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.
The worldwide spread of windfarms brings new challenges, especially for concrete structures as a part of towers, connecting joints and foundations of wind turbines. High-cyclic loadings in such structures lead to a high relevance of the subject of fatigue. A proper assessment of the fatigue strength of concrete demands therefore a basis of reliable experimental data and the development of standardized testing methods. This article presents first results of an ongoing research program of BAM (Bundesanstalt für Materialforschung und -prüfung) which is a part of a joint project (WinConFat) funded by the German Federal Ministry for Economic Affairs and Energy. The subproject investigates the effects of size and slenderness of the specimens on the fatigue behaviour of high strength concrete at different stress levels. Not only the fatigue strength, but also the fatigue process itself is monitored by means of several measurement methods. Strain measurements are used to calculate the load dependent elastic modulus in the fatigue hysteresis as indicators for fatigue development. Furthermore, the application of non-destructive methods like acoustic emission analysis and ultrasonic measurement in laboratory tests gives a deeper insight into damage processes under cyclic loading. The results shall be used to improve design rules for concrete members under fatigue load and to develop or improve non-destructive techniques for in-service structural health monitoring.
The worldwide spread of windfarms brings new challenges, especially for concrete structures as a part of towers, connecting joints and foundations of wind turbines. High-cyclic loadings in such structures lead to a high relevance of the subject of fatigue. A proper assessment of the fatigue strength of concrete demands therefore a basis of reliable experimental data and the development of standardized testing methods. This article presents first results of an ongoing research program of BAM (Bundesanstalt für Materialforschung und -prüfung) which is a part of a joint project (WinConFat) funded by the German Federal Ministry for Economic Affairs and Energy. The subproject investigates the effects of size and slenderness of the specimens on the fatigue behaviour of high strength concrete at different stress levels. Not only the fatigue strength, but also the fatigue process itself is monitored by means of several measurement methods. Strain measurements are used to calculate the load dependent elastic modulus in the fatigue hysteresis as indicators for fatigue development. Furthermore, the application of non-destructive methods like acoustic emission analysis and ultrasonic measurement in laboratory tests gives a deeper insight into damage processes under cyclic loading. The results shall be used to improve design rules for concrete members under fatigue load and to develop or improve non-destructive techniques for in-service structural health monitoring.
Improvement of effectiveness of in-service inspection in nuclear power plants (IAEA-TECDOC-1853)
(2018)
This publication represents a consensus among experts drawn from the IAEA and global practitioners to compile a set of common or good individual practices for use at nuclear power plants to improve ISI effectiveness. It sets forth a number of strategies and practices for improving the effectiveness of ISI and investigates the role of ISI in maintaining or improving safety at plants and the relationship of ISI improvement to cost. Strategies for improving ISI effectiveness are discussed, with consideration given to the entire framework of ISI, including effective selection of the proper inspection scope, inspection interval and NDE efficiency.
This publication is an update of the publication IAEA-TECDOC-1400, Improvement of In-service Inspection in Nuclear Power Plants, which was published in 2004. It takes into account new techniques and technologies for qualification processes for methodologies, equipment, procedures and personnel as well human factors in NDE inspections.
More than 80 representatives of SMEs, industrial companies and research institutes met on September 12 at the workshop "Challenges in Additive Manufacturing: Innovative Materials and Quality Control" at BAM in Adlershof to discuss the latest developments in materials and quality control in additive manufacturing.
In special lectures, researchers, users and equipment manufacturers reported on the latest and future developments in additive manufacturing. Furthermore, funding opportunities for projects between SMEs and research institutions on a national and European level were presented.
Fatigue is one of the most prevalent issues, which directly influences the service life expectancy of concrete structures. Fatigue has been investigated for years for steel structures. However, recent findings suggest that concrete structures may also be significantly subjected to the fatigue phenomena that could lead to premature failure of certainfatigue prone structural elements. To date, fatigue of reinforced concrete has been given little focus. and Kknowledge on the influence factors and durability/capacity effects on this material should be improved. Current technological means to measure fatigue in civil structures like bridges and wind turbines (both onshore and offshore) are outdated, imprecise and inappropriate.
Meanwhile, this topic has got much more attention as dynamic loading on concrete structures plays an increasing role, e.g. in bridges with increasing traffic and heavier trucks, in wind energy production due to inherent vibrations e.g. in offshore wind turbine support structures affected by wind and waves.
The European Innovative Training Networks (ITN) Marie Skłodowska-Curie Actions project INFRASTAR (Innovation and Networking for Fatigue and Reliability Analysis of Structures - Training for Assessment of Risk) provides research training for 12 PhD students. The project aims to improve knowledge for optimizing the design of new structures as well as for more realistic verification of structural safety and more accurate prediction of the remaining fatigue lifetime of existing structures.
First, the INFRASTAR research framework is detailed. Then it will be exemplified through the presentation of the major results of the four PhD students involved in the work package dealing with auscultation and monitoring. This includes the development and improvement of Fiber Optics (FO) and Coda Wave Interferometry (CWI) for crack sizing and imagery, new sensor technologies and integration, information management, monitoring strategy for fatigue damage investigation and lifetime prediction.
The manuscript Codex germanicus 1 (Cod. germ. 1) of the Staats- und Universitätsbibliothek Hamburg is a fifteenth-century German-language manuscript. It comprises two codicological units and has an especially complex developmental history. To trace this developmental history, neglected until now in the research literature, the manuscript was investigated, for the first time not solely with classical codicological and palaeographical methods, but also with the aid of X-ray fluorescence spectroscopy, in order to determine the composition of the writing materials. These methods made it possible, first, to support and check palaeographic findings and, second, to gain information about the stratigraphy of the manuscript where palaeographic methods find their limits – in regard to short entries, rubrications, and non-alphabetical signs.
It is well documented that throughout Antiquity, ancient Egyptians used mostly carbon inks as a writing material. In Late Antiquity, some metals started to be added to carbon based inks. We have records of five manuscripts from the Dead Sea Scrolls collection whose carbon inks were found to contain copper Also, lead was recently found as an additive in carbon inks on a charred fragment from Herculaneum. Furthermore, the earliest evidence of iron-gall ink was found in the Book of Proverbs (Codex Ms. Berol. orient. oct. 987) dating to the third fourth centuries ce. It has been suggested that along with carbon and iron-gall inks, there is no reason to think that purely tannin inks were not also in use in Egypt. However, so far, we just have evidence of a copper-tannin ink identified in a number of documents from Egypt in the first third centuries bce. In an attempt to fill this gap in this extremely fragmented scenario during our studies of the socio-geographic history of inks, we arrived at the conclusion that the continuous production of Coptic manuscripts from Late Antiquity to the Middle Ages offers a unique opportunity for the historical study of inks across a large geographic area.
Advanced codicological studies of Cod. germ. 6 (Hamburg, Staats- und Universitätsbibliothek): Part 2
(2018)
The work presented here follows the article Combining Codicology and X-Ray Spectrometry to Unveil the History of Production of Codex germanicus 6 (Staats- und Universitätsbibliothek Hamburg), published in 2014.1 It confirms the main result of the previous article: the Artusnotiz, the fourth text in the bound manuscript, must have been introduced as the last one. This paper offers further details of the codex production, based on the composition of the black and red inks collected in four measurement campaigns. Furthermore, using imaging μ-XRF, we succeeded in understanding the strong variation of the composition of the red inks in the initials of all the texts except for Parzival and Jeanne d’Arc.
The quest for the mixed inks
(2018)
In this article, we would like to share our observations concerning the inks produced by intentionally mixing soot or charcoal with tannin extracts or iron-gall ink. Aside from Zerdoun’s mention in her outstanding review of written sources, “Les encres noires au Moyen-Âge”, this ink category has received little if any attention from scholars and scientists. And yet, if analytically attested, the use of such inks could serve as an additional category to classify and distinguish the writing inks on the historical socio-geographic map of the writing inks we are trying to build.
In order to test their suitability different non-destructive methods were performed to inspect a GFRP plate with artificial defects. These defects were manufactured by means of thin PTFE sheets inserted between two plies in three different depth. The inspection methods were microwave reflection, flash thermography and phased array ultrasonics, all applied to the same specimen. Selected results are shown for all methods demonstrating opportunities and limits of the particular inspection methods. The achieved detection limits and further application aspects are compared directly to provide a useful information for the planning of inspection tasks.
A composite material is a combination of two or more materials with very different mechanical, thermal and electrical properties. The various forms of composite materials, due to their high material properties, are widely used as structural materials in the aviation, space, marine, automobile, and sports industries. However, some defects like voids, delamination, or inhomogeneous fiber distribution that form during the fabricating processes of composites can seriously affect the mechanical properties of the composite material. In this study, several imaging NDE techniques such as: thermography, high frequency eddy current, ultrasonic, x-ray radiography, x-ray laminography, and high resolution x-ray CT were conducted to characterize the microstructure of carbon fiber composites. Then, a 3D analysis was implemented by the destructive technique of serial sectioning for the same sample tested by the NDE methods. To better analyze the results of this work and extract a clear volume image for all features and defects contained in the composite material, an intensive comparison was conducted among hundreds of 3D-NDE and multi serial sections’ scan images showing the microstructure variation.
While additive manufacturing (AM) is blossoming in nearly every industrial field, and the most different process are being used to produce components and materials, little attention is paid on the safety concerns around AM materials and processes.
Leveraging on our leading expertise in non-destructive testing (NDT) and materials characterization, we approach AM at BAM under two important viewpoints: first the on-line monitoring of the process and of the product, second the evolution of the (unstable) microstructure of AM materials under external loads.
These two subjects are the core of the two new-born internal projects ProMoAM and AGIL, respectively.
A detailed view of the goals and the organization of these two projects will be given, together with the expected output, and some preliminary results.
Magneto resistance sensors like GMR (giant magneto resistance) or TMR (tunnel magneto resistance) are widely used in industrial applications, examples are position measurement and read heads of hard disk drives. However, in case of non-destructive testing (NDT) applications these sensors, although their properties are outstanding like high spatial resolution, high field sensitivity, low cost and low energy consumption, never reached a technical transfer to an application beyond sci-entific scope.
This paper deals with benefits of GMR/TMR sensors in terms of high spatial resolution testing for different NDT applica-tions. The first example demonstrates the preeminent advantages of MR-elements compared with conventional coils used in eddy current testing (ET). The probe comprises one-wire excitation with an array of MR elements. This led to a better spatial resolution in terms of neighboring defects.
The second section concentrates on MFL-testing (magnetic flux leakage) with active field excitation during and before test-ing. The latter illustrated the capability of highly resolved crack detection of a crossed notch. This example is best suited to show the ability of tiny magnetic field sensors for magnetic material characterization of a sample surface. Another example is based on characterization of samples after tensile test. Here, no external field is applied. The magnetization is only changed due to external load and magnetostriction leading to a field signature which GMR sensors can resolve. This gives access to internal changes of the magnetization state of the sample under test.
Photon counting detectors (PCD) offer new possibilities for x-ray micro computed tomography (CT) in the field of non-destructive testing. For large and/or dense objects with high atomic numbers the problem of scattered radiation and beam hardening severely influences the image quality. This work shows that using an energy discriminating PCD based on CdTe allows to address these problems by intrinsically reducing both the influence of scattering and beam hardening.
Based on 2D-radiographic measurements it is shown that by energy thresholding the influence of scattered radiation can be reduced by up to in case of a PCD compared to a conventional energy-integrating detector (EID).
To demonstrate the capabilities of a PCD in reducing beam hardening, cupping artefacts are analyzed quantitatively. The PCD results show that the higher the energy threshold is set, the lower the cupping effect emerges. But since numerous beam hardening correction algorithms exist, the results of the PCD are compared to EID results corrected by common techniques. Nevertheless, the highest energy thresholds yield lower cupping artefacts than any of the applied correction algorithms.
As an example of a potential industrial CT application, a turbine blade is investigated by CT. The inner structure of the turbine blade allows for comparing the image quality between PCD and EID in terms of absolute contrast, as well as normalized signal-to-noise and contrast-to-noise ratio. Where the absolute contrast can be improved by raising the energy thresholds of the PCD, it is found that due to lower statistics the normalized contrast-to-noise-ratio could not be improved compared to the EID. These results might change to the contrary when discarding pre-filtering of the x-ray spectra and thus allowing more low-energy photons to reach the detectors.
Despite still being in the early phase in technological progress, PCDs already allow to improve CT image quality compared to conventional detectors in terms of scatter and beam hardening reduction.
Defect recognition in CFRP components using various NDT methods within a smart manufacturing process
(2018)
The manufacturing process of carbon fiber reinforced polymer (CFRP) components is gaining a more and more significant role when looking at the increasing amount of CFRPs used in industries today. The monitoring of the manufacturing process and hence the reliability of the manufactured products, is one of the major challenges we need to face in the near future. Common defects which arise during manufacturing process are e.g. porosity and voids which may lead to delaminations during operation and under load. To find irregularities and classify them as possible defects in an early stage of the manufacturing process is of high importance for the safety and reliability of the finished products, as well as of significant impact from an economical point of view. In this study we compare various NDT methods which were applied to similar CFRP laminate samples in order to detect and characterize regions of defective volume. Besides ultrasound, thermography and eddy current, different X-ray methods like radiography, laminography and computed tomography are used to investigate the samples. These methods are compared with the intention to evaluate their capability to reliably detect and characterize defective volume. Beyond the detection and evaluation of defects, we also investigate possibilities to combine various NDT methods within a smart manufacturing process in which the decision which method shall be applied is inherent within the process. Is it possible to design an in-line or at-line testing process which can recognize defects reliably and reduce testing time and costs? This study aims to show up opportunities of designing a smart NDT process synchronized to the production based on the concepts of smart production (Industry 4.0). A set of defective CFRP laminate samples and different NDT methods were used to demonstrate how effective defects are recognized and how communication between interconnected NDT sensors and the manufacturing process could be organized.
One of the great strengths of X-ray computed tomography over conventional inspection methods (ultrasound, thermography, radiography) is that it can image damage in 3D. However for curved ordeformed composite panels it can be difficult to automatically ascribe the damage to specific plies or inter-ply interfaces. An X-ray computed tomography (CT) data processing methodology is developed to extract the through-thickness distribution of damage in curved or deformed composite panels. The method is applied to [(0°/90°)2]s carbon fibre reinforced polymer (CFRP) panels subjected low velocity impact damage (5 J up to 20 J) providing 3D ply-by-ply damage visualisation and analysis. Our distance transform approach allows slices to be taken that approximately follow the composite curvature allowing the impact damage to be separated, visualised and quantified in 3D on a ply-by-ply basis. In this way the interply delaminations have been mapped, showing characteristic peanut shaped delaminations with the major axis oriented with the fibres in the ply below the interface. This registry to the profile of the panel constitutes a significant improvement in our ability to characterise impact damage in composite laminates and extract relevant measurements from X-ray CT datasets.
This paper addresses the computation of dispersion curves, mode shapes and propagation of elastic guided waves. It summarizes the approaches based on the Scaled Boundary Finite Element Method.
Descriptions for plates, rods, pipelines and waveguides with an arbitrary cross section are included. The important steps for the approximation of the displacement in bounded and unbounded domains are stated. The grid generation process is explained. It is highlighted that the Scaled Boundary Finite Element Method is very efficient, if large portions of the domain are either straight or with a constant curvature. The computation of dispersion curves for layered structures is presented.
Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular lock-in and flash-thermography.
In vertical cavity surface emitting lasers (VCSELs), laser light is emitted perpendicular to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp.
Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this ansatz, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation.
Electromagnetic methods are widely used in Non-destructive Testing industries (NDT). In order to meet the requirements, safety-relevant products and structures have to be tested reliably during production or within subsequent maintenance cycles. For this purpose industrial users call for adapted testing methods which allow the detection of relevant defects such as cracks and pore in ferromagnetic or conductive materials. Automated testing systems and the application of sensor arrays are of great interest in order to enhance inspection quality and to decrease testing time and costs. Magneto resistance (MR) sensors like giant magneto resistance (GMR) or tunnel magneto resistance sensor (TMR) have proofed their suitability in many NDT-applications due to their extraordinary properties which combine high field sensitivity, a high spatial resolution and low costs. Due to their small size these sensors can be useful – in addition to the pure detection task – for a more detailed defect characterization and defect reconstruction providing defect size, orientation and geometry. This chapter gives a brief introduction of electromagnetic testing methods, i.e., magnetic particle inspection (MPI) and eddy current testing (ET), followed by two different approaches for which MR sensor arrays were successfully adapted.
Defect recognition in CFRP components using various NDT methods within a smart manufacturing process
(2017)
The manufacturing process of carbon fiber reinforced polymer (CFRP) components is playing a more and more significant role when looking at the increasing amount of CFRPs used in industries today. The monitoring of the manufacturing process and hence the reliability of the manufactured products, is one of the major challenges we need to face in the near future. Common defects which arise during manufacturing are e.g. porosity and voids which may lead to delaminations during operation and under load. To find those defects in an early stage of the manufacturing process is of huge importance for the safety and reliability of the finished products, as well as of significant impact from an economical point of view. In this study we present various NDT methods which are applied to similar CFRP laminate samples in order to detect and characterize defective volume. Besides ultrasound, thermography and eddy current, different x-ray methods like radiography, laminography and computed tomography are used to investigate the samples. These methods are compared with the intention to evaluate their capability to reliably detect and characterize defective volume. Beyond the detection of flaws, we also investigate possibilities to combine various NDT methods within a smart manufacturing process in which the decision which method to apply is made by the process itself. Is it possible to design an in-line or at-line testing process which can recognize defects reliably and reduce testing time and costs? This study aims to show up opportunities of designing a smart NDT process synchronized to the production based on the ideas of smart production (Industry 4.0). A set of defective CFRP laminate samples and different NDT methods were used to demonstrate how effective defects are recognized and how communication between interconnected NDT sensors and the manufacturing process could be organized.
Components made of fibre-reinforced plastic are a prominently used material in modern safety-relevant structures. Periodic in-service non-destructive inspection of these structures is necesary. The EMRP-funded project VITCEA aims on the evaluation of the performance of various non-destructive testing methods on the inspection of fibre-reinforced plastics.
The physical layout of fibre layers leads to complex structures of fibre-reinforced plastic parts. Furthermore different material properties of the fibre and the resin cause anisotropic behaviour of the material.This poses a challenge for ultrasonic NDT by causing a complex acoustical response.
The acoustical behaviour of fibre-reinforced plate materials has been simulated. Specimens with artificial flaws for the evaluation of the detection thresholds have been designed and manufactured.
Mechanized ultrasonic testing methods using phased-array sensors, air-coupled transducers, immersion tank testing and contact technique have been evaluated on the specimen.
In this talk results of different ultrasonic testing methods will be compared and discussed.
This paper focuses on the aspect on how damage evolution processes in composite materials and structures including matrix fracture, delamination, fibre-matrix debonding, and fibre fracture can be detected by taking advantage of the material’s inherit mechanical properties. These properties can be described on the basis of non-linear mechanical phenomena measurable as an inert frequency response signal. The approach is proposed as a means for residual life structural assessment specifically in the context of VHCF.
The paper presents an attempt for the calibration of an active
thermography method that is suitable for the non-destructive
evaluation of spot welds. Nowadays, the quality of spot welds is
commonly characterised by the application of random chisel tests,
which are time consuming, expensive and destructive. Recently a
non-destructive testing method by means of active thermography
was proposed that relies on the fact that the mechanical connection
formed by the spot weld also serves as a thermal bridge between
the two steel sheets joined in the welding process. It is shown in this
paper that this thermal bridge can be thermographically characterised
by extracting a measure for the spot weld diameter and hence the
quality of the spot weld. The determination of the absolute value
of the diameter hereby relies on a calibration of the testing system,
which is performed by means of X-ray computed tomography in this
study. The experiments were carried out using different experimental
approaches, namely transmission as well as reflection geometry
wSetup in reflectionith laser illumination. A comprehensive evaluation
of samples produced using different welding currents, hence different
quality, was carried out in order to validate the thermographic results.