Filtern
Erscheinungsjahr
- 2022 (87) (entfernen)
Dokumenttyp
- Vortrag (31)
- Beitrag zu einem Tagungsband (22)
- Zeitschriftenartikel (19)
- Posterpräsentation (11)
- Buchkapitel (2)
- Forschungsdatensatz (2)
Sprache
- Englisch (87) (entfernen)
Schlagworte
- Concrete (20)
- Ultrasound (20)
- Non-destructive testing (12)
- Monitoring (11)
- Civil engineering (8)
- LIBS (8)
- Air-coupled ultrasound (7)
- Nondestructive testing (7)
- Reliability (7)
- Coda wave interferometry (6)
Organisationseinheit der BAM
- 8.2 Zerstörungsfreie Prüfmethoden für das Bauwesen (87) (entfernen)
Eingeladener Vortrag
- nein (31)
In general, wind turbines transform the kinetic energy of the wind into electric power. Thereby, the wind turbine blades are facing unsteady loads which are transferred to the hub to generate a rotation of the turbine’s axis. This brief introduction focuses on the aerodynamics of the blades and the corresponding loads. Starting with the basic flow field and loads of an airfoil, terms like stagnation point, boundary layer, Reynolds number, transition, and separation are introduced. For different geometries, lift and drag coefficient curves are discussed. Then, full wings will be considered, including their three-dimensional flow field due to wing tip vortices and crossflows. As a main source of increased loads, unsteady effects are explained in more detail such as gusts, tower passing, earth boundary layer crossing, free stream turbulences, yaw misalignment, etc. At the end, extra loads due to an oscillating free stream are introduced.
Attenuation of ultrasonic signals in concrete has the potential to carry much information about the microstructure of the material. In this work a series of concrete specimens of varying porosities and pore size distributions were internally imaged with x-ray computed tomography (CT), and then subsequently examined with throughtransmission ultrasound. The CT images were used to quantify both capillary porosity of cement paste as well as internal interfaces that are likely to produce elastic wave scattering. Ultrasound signals were represented as a diffusion process such that absorption and scattering attenuation could be isolated. As implemented, the diffusion model was a poor predictor of capillary porosity, while diffusivity was a reasonable predictor of scattering interfaces. Diffusivity was found to scale extremely well with small scale porosity, which made it a good predictor of compressive strength.
Damage detection of reinforced concrete (RC) structures is becoming a more attractive domain due to the safety issues arising in the last few decades. The damage in concrete can be caused by excessive exploitation of the structure or environmental effects. The cracks in concrete can be detected by different nondestructive testing methods. However, the available methods used for this purpose have numerous limitations. The technologies available in the market nowadays have difficulties detecting slowly progressive, locally limited damage. In addition, some of These methods cannot be applied, especially in hard-to-reach areas in the superstructures. In order to avoid these deficiencies, an embedded ultrasonic methodology can be used to detect cracks in RC structures. In this study, the methodology of crack detection supported with the advanced Signal processing algorithm was proposed and verified on RC structures of various types, and cracks occurring between embedded sensors can be detected. Moreover, different pairs of ultrasonic sensors located in the considered structures are used for the analysis of the sensitivity of distance between them. It is shown that the ultrasonic sensors placed in the range of 1.5–2 m can detect cracks, even when the other methods failed to detect changes in the structure. The obtained results confirmed that diffuse ultrasonic sensor methodology is able to monitor real structures more effectively than traditional techniques.
To improve ultrasonic imaging of concrete structures, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing. A 2D elastic RTM algorithm was tested on synthetic ultrasonic echo data. Compared to the typically used synthetic aperture focusing technique (SAFT) as well as our acoustic RTM algorithm, the presented elastic RTM results show an enhancement in imaging vertical reflectors and complex features inside the 2D numerical concrete model.
The ultrasonic echo technique is widely used in non-destructive testing for investigation and damage analysis of concrete constructions. Important applications include thickness measurements, geometry determination, the localization and characterization of built-in components as well as the detection of quality issues (cracks, honeycombing, low concrete strength).
To improve ultrasonic data imaging of complicated structures in concrete, we transferred a seismic migration technique, the Reverse Time Migration (RTM), to non-destructive testing in civil engineering. In a preliminary study, we tested a 2D acoustic RTM algorithm on measured ultrasonic echo data acquired at a concrete foundation slab. Compared to the conventional used synthetic aperture focusing technique algorithms (SAFT) for ultrasonic data reconstruction, our acoustic RTM results showed a significant improvement in imaging the interior structure of the concrete slab. Vertical reflectors were reconstructed which was not possible by traditional imaging.
In contrast to SAFT, RTM is a wavefield-continuation method in time and uses the full wave equation. RTM is, thus, able to include multiple reflections and to handle multi-pathing as well as many other complex situations. As a drawback RTM requires extensive computing power and memory capacity. Nevertheless, due to progresses in parallel processing and other computational technologies RTM has become appealing for the application in the field of non-destructive testing.
An RTM algorithm, which uses the full elastic wave equation instead of the full acoustic one (as applied in our preliminary work) has the potential to optimize the imaging results even further. This is due to the fact, that our ultrasonic measurement data are generated by exciting elastic waves. Thus, in a first step, two 2D elastic RTM algorithms were tested on synthetic ultrasonic echo data generated with a concrete model consisting of several steps and circular shaped air inclusions. In addition, two imaging conditions were evaluated to reduce migration artifacts. Our synthetic elastic RTM results showed an enhancement in imaging the features inside the test model compared to acoustic RTM and SAFT. In a second step, we acquired ultrasonic measurement data at a concrete test specimen consisting of three steps and four air filled tendon ducts. The evaluation of the real data with our elastic RTM code was also successful and the reconstruction of the geometries of the steps and tendon ducts could be improved. With our study we have shown that elastic RTM is a step forward for ultrasonic testing in civil engineering.
The main application of LIBS in civil engineering is the detection of harmful ions in concrete, which can penetrate the component through the porous concrete structure. The advantages of LIBS over standard methods are the possibility of multi-element analysis, measurement speed, spatially resolved measurements, and minimal sample preparation. The spatially resolved measurements of LIBS allow the assessment of the heterogeneity of the concrete by measuring separately the chemical composition of the aggregates and the binder matrix. The latter is particularly relevant because the determined elemental distribution can be directly related to the binder matrix. This is not possible with standard methods, since the material is homogenized to powder during sample preparation stage and the determined concentration is thus related to the total mass. In addition to the use of LIBS for the specific analysis of individual harmful ions, LIBS can also be used to estimate the concrete composition and thus determine, for example, the type of cement used. Corresponding information are relevant for the estimation of the remaining service life and for the preparation of a maintenance concept. In recent years, LIBS has been increasingly used in civil engineering. Currently, however, it is primarily used in research institutions and only occasionally in building materials laboratories. Special commercial devices have also been developed, which greatly simplify the application due to the high degree of automation. Mobile LIBS systems allow on-site application. A central point, which limits the use of LIBS in the commercial sector, is the lack of norms and standards. Therefore, within the framework of a project funded by the German government, work has been carried out on the preparation of a leaflet on quantitative chlorine determination in concrete, which will be published this year. In interlaboratory comparisons the robustness and accuracy for the practical application was demonstrated. LIBS also has great potential in the recycling of construction waste in conjunction with hyperspectral sensors. This issue is currently being addressed in a national project. During the presentation, the state of the art of LIBS in civil engineering will be presented, next steps will be discussed, and future challenges will be outlined.
Responsible treatment of the environment and resources is a key element of sustainability. The building and construction industry is one of the largest consumers of natural resources. Consequently, there is a particular need for regulations and technologies that help to create closed material cycles. From the technological point of view, such efforts are complicated by the growing material diversity and the amount of composites contained in present and future construction and demolition waste (CDW). Nowadays, simple but proven techniques like manual sorting are mainly used. However, this practice not only poses health risks and dangers to the staff performing the work, but also relies on merely obvious, visually striking differences. Automated, sensor-based sorting of these building materials could complement or replace this practice to improve processing speed, recycling rates, sorting quality, and prevailing health conditions. The preliminary results for the identification of a wide variety of building materials with LIBS are presented.
Each engineering decision is based on a number of more or less accurate information. In assessment of existing structures, additional relevant information collected with on-site inspections facilitate better decisions. However, observed data basically represents the physical characteristic of interest with an uncertainty. This uncertainty is a measure of the inspection quality and can be quantified by expressing the measurement uncertainty. The internationally accepted rules for calculating measurement uncertainty are well established and can be applied straightforwardly in many practical cases. Nevertheless, the calculations require the occasionally time-consuming development of an individually suitable measurement model. This contribution attempts to emphasize proposals for modelling the non-destructive depth measurement of tendons in concrete using the ultrasonic echo technique. The proposed model can serve as guideline for the determination of the quality of the measured information in future comparable inspection scenarios.
Several countries, which use or used nuclear power, started to destruct nuclear power plants. Thus, large amounts of radioactive waste have to be stored safely during the next decades. Large underground caverns will
be used a nuclear waste disposal. Once the cavern is full, a sealing construction is required. The sealing should avoid the contact to any fluids or substances of the storage barrels and reduce hazardous radiation.
Within a DFG-funded research group, we investigate the possibility of concrete damage assessment with coda waves. Coda wave interferometry (CWI) can be a valuable asset to structural health monitoring (SHM) as we can detect small signal changes and quantify them with the calculation of relative velocity changes. By equipping an in-service road bridge with more than 24 embedded ultrasonic transducers, we want to analyze how methods developed within the research group perform in an uncontrollable environment. We evaluate individual source-receiver combinations for detection of temporal changes as well as the entire installed sensor array to create maps of material change comparing two distinct states of the bridge.