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- CCS (7)
- Corrosion (7)
- Steel (7)
- Concrete (6)
- Strain sensor (6)
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- Structural health monitoring (5)
- CO2-storage (4)
- Emission (4)
- Fibre optic sensors (4)
Structural health monitoring of wind turbine blades by strain measurement and vibration analysis
(2011)
Wind turbine blades have to withstand a high number of load cycles in mostly hard weather conditions over 20 years. In a research project BAM and several partners have designed, evaluated and tested a condition monitoring system for all parts of a wind turbine. At a rotorblade with a length of 58 m fibre bragg grating sensors were applied for in-service strain measurement. Additionally a complex test rig was designed to enable real biaxial loading conditions at a representative test rotor blade to simulate the mechanical loading conditions in the lab. In this test blade with a length of 8 m also fibre bragg grating sensors were implemented to determine their influences for the structure and the condition monitoring system. Vibration measurements were carried out at different test phases. The results were compared with finite element strain and modal analysis.
Neutrons are absorbed particularly by hydrogen containing materials so
they can be used as a tool for visualising heterogeneous density distributions of organic materials. Penetration limits are set by the layer thicknesses and the neutron energies applied. In case of specimens with a flat shape the situation may be encountered that the object only could be penetrated in selected directions. In addition, the overall size may exceed the beam diameter and thus the viewing window if a certain region of interest should be studied by tomography without destroying the integrity of the specimen.
This study shows the capabilities and limits of thermal and fast neutrons to investigate flat wooden specimens such as boards and girders by neutron tomography under such circumstances. Taking projections was impaired either by the limits of penetrability or by the total size of the object. As a consequence, projections were included for reconstruction only from a limited angular range of 90°. It could be shown that an approach based on the slice theorem was capable to visualise structural features along the beam directions while simply omitting the perpendicular ones without causing additional artefacts.
Small samples with a thickness of up to 2 cm but several times broader could be studied with the ANTARES facility of the FRM II neutron source in Garching providing thermal neutrons while larger objects required a beam of higher energy as available in the NECTAR facility of the same institution. The fast (fission) neutrons (1.5 – 2 MeV) of this site allowed investigating an area of interest inside a girder with a cross section of 23.5 x 49 cm². Internal features such as inclusions could be detected as well as a heterogeneous density distribution in glue layers.
Fatigue testing in the VHCF ränge requires testing machines with increased frequencies. However, application of energy-saving resonance testing leads to a linkage between test frequency and specimen Parameters. The frequency ränge and the available forces of the testing machine are limiting possible specimen dimensions.
Electrodynamic shakers combine test frequencies up to 4000 Hz with forces in the kN range. Thus, a relatively wide range of tests is possible which allows studying the influence of specimen size on VHCF behaviour.
Electrodynamic shakers as well as the respective Computer control Systems and Vibration measurement techniques are well established in scientific and technical applications. We present results of a first study of VHCF testing on electrodynamic shakers conducted at BAM. Test frequency, specimen size and stress levels have been varied. One goal is to balance the needs of materials testing with the Standard test Systems of environmental Simulation to provide a powerful and flexible tool for very high cycle fatigue testing. Further investigation to optimize the technique is required.
The material parameters melt flow rate MFR, density D, notched impact strength N.I.S. at -30 °C, stress crack resistance FNCT and resistance to oxidative degradation Ox (determined by the increase of MFR) have been selected for a comparison of polyethylene grades for one design type in the “European Standard EN 15507 - Transport packaging for dangerous goods - Comparative material testing of polyethylene grades”. The parameters have a systematic relationship with design type tests. The notched impact strength at -30 °C and the resistance to oxidative degradation of polyethylene grades determined with test specimens of compression moulded sheets are directly comparable with drop tests at -18 °C of design types produced of these grades after pre-storage with 55 % nitric acid for 21 days at 40 °C. The impact of the pre-storage time of jerricans with 55 % nitric acid at 40 °C and 23 °C on the marginal drop heights in drop tests at -18 °C, on the MFR and on the tensile properties tensile strength, breaking elongation and the elastic modulus were investigated. Packaging design types (jerricans) made of three polyethylene grades from different producers were pre-stored with 55 % nitric acid for 21,42 and 84 days at 40 °C as well as for six months at 23 °C. The MFR and tensile properties of test specimens taken from the design type side walls as well as the marginal drop heights of the jerricans in drop tests at -18 °C were determined.
The values of the notched impact strength at -30 °C of polyethylene grades are not comparable with the marginal drop heights determined in drop tests at -18 °C. The oxidative damaging impact of nitric acid has a greater effect on the grade with lower resistance to oxidative degradation by reducing more significantly the marginal drop heights with increasing pre-storage time.
Pre-damage with 55 % nitric acid for 21 days at 40 °C, as recommended in ISO 16101 and EN 15507 caused an increase in the marginal drop heights of most design types in drop tests when compared with design types without pre-damage. The test results demonstrated clearly that pre-storage of the jerricans for six months at 23 °C caused a higher increase in the MFR and lower marginal drop heights of the jerricans in drop tests at -18 °C compared with pre-storage of the jerricans for 21 days at40 °C.
The mechanical properties change as a result of diffusion of nitric acid into the polyethylene grades and the reaction of the nitric acid with the polymers. The percentage decrease and increase in the tensile strength, breaking elongation and elasticity modulus of test specimens taken from the design type side walls after different pre-storage times were only partly comparable with the marginal drop heights in drop tests at -18 °C.
A novel quasi-distributed long-gauge fiber optic strain sensor system for dynamic measurement
(2011)
We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.
Thin layers of textile reinforced concrete (TRC) can be used as strengthening for reinforced concrete (RC) slabs or beams to increase their foad-bearing capacity. The concrete matrix consists of a fine-grained concrete with a thickness of 8 to 15 mm. Information on the behaviour of strengthened structures subjected to fire is of particular inlerest for practical applications of this innovative construction method. A total of 7 RC slabs with Steel reinforcement were produced: 2 slabs were used as reference and 5 slabs were strengthened with three layers of textile reinforcement. Ultimate capacity of reference slabs and particular TRC strengthened slabs was determined from statie load results fom 4-point-bending tests.
Fire tests on these slabs were carried out with utilization fectors using a Standard temperature curve (ISO-834-1) for fire testing. Test loads varied between 0 % and 65 % ofuMmate load.
To detect temperatures during fire exposure thermocouples were arranged throughout the cross-section of the slabs. Depending on the utilization the slabs were exposed to fire for 30 minutes or for 60 minutes. In some cases bending fäilure occurred, in other cases the slabs survived. None of the specimens exhibited any visual damage or concrete spalling during fire exposure. Results of deformation measurements and residual strength measurements are presented. Even though mechanisms are not fifily understood, it is important to discuss the presumable reasons for the surprisingiy positive test results. It is presumed tbat the key mechanisms contributing to the outstanding fire resistance capability presented are: superior crack control of the TRC and load redistribution between textile and Steel reinforcement as well as the primary load transfer directly into the slabs.
Recent advances in the fabrication of diffractive X-ray optics have boosted hard X-ray microscopy into spatial resolutions of 30 nm and below. Here, we demonstrate the fabrication of zone-doubled Fresnel zone plates for multi-keV photon energies (4-12 keV) with outermost zone widths down to 20 nm. However, the characterization of such elements is not straightforward using conventional methods such as knife edge scans on well-characterized test objects. To overcome this limitation, we have used ptychographic coherent diffractive imaging to characterize a 20 nm-wide X-ray focus produced by a zone-doubled Fresnel zone plate at a photon energy of 6.2 keV. An ordinary scanning transmission X-ray microscope was modified to acquire the ptychographic data from a strongly scattering test object. The ptychographic algorithms allowed for the reconstruction of the image of the test object as well as for the reconstruction of the focused hard X-ray beam waist, with high spatial resolution and dynamic range. This method yields a full description of the focusing performance of the Fresnel zone plate and we demonstrate the usefulness ptychographic coherent diffractive imaging for metrology and alignment of nanofocusing diffractive X-ray lenses.
Strain measurement in structures witli the purpose of long-term structurai health inonitoring must provide reliable inforination about the structure’s behavior over the whole period of use. The user must be sure that installed sensors are validated and work to the utmost satisfactiou. For this purpose, sensor Systems are tested using special facilities. Bccause it is not easy to characterize the strain transfer quality from the host structure into surface-applied strain sensors, a unique testing facility has been developed. Originally developed for fiber Bragg grating based sensors, the KALFOS facility (= calibration of fiber optic sensors) can also be used for electrical strain sensors.
Calibration ineasurements are referenced by unbinsed Digital Image Correlation (D1C) and Electronic Speckle Pattern Interferometer (ESPI) methods. The strain transfer behavior can experimentally be analyzed and investigated under combined thermal and mechanical loading conditions and allows revealing wealuiesses in couimonly used attachment methodologies. The deformation of all members (particularly the coating/substrate - adhesive combination) in the sensing area is physically independently gained and recorded. Results achievcd allow precise description of the strain transfer function, Validation of the longterm strain sensor characteristics, matching of specific measurement requirements with environmental conditions, and, moreover, the verification of Standards for use of strain sensors.
How do application-related issues influence the reliability of fiber optic strain measurements?
(2011)
Fibre optic strain sensors are increasingly used and sensor systems are provided with specifications. Even if the performance is well specified, the strain characteristics of the sensor, strain transfer factor, mechanical stability under thermal influences, the performance of applied strain sensors can seriously differ from virgin sensor's the performance. The contribution will focus on issues that can deteriorate the sensor function or reduce the reliability of measurement results. Aspects are considered how to come to reliable strain measurements and how to validate strain measurements of applied sensors. Related to this topic, European activities like the recently started European COST TD1001 action, called (OfSeSa) will be presented.
Ultra high molecular weight polyethylene (UHMW-PE) - due to its high hydrogen content - is used as neutron shielding material in casks for storage and transport of radioactive materials. Besides this, UHMW-PE - as a high-performance polymer - has been used for several years in medical technology due to its excellent slip and wear properties. Due to the special properties of UHMW-PE which result basically from its extreme chain lengths and its high degree of crystallinity, it is predestinated for the aforementioned applications.
In both cases, irradiation and its impact on the molecular structure of polyethylene play an important role. In the first case, irradiation exists as a by-product of inserting the radioactive material in the cask. Hence PE has to withstand any type of degradation affecting safety relevant aspects to be applicable for long term radiation shielding purposes for instance over a period of 40 years. In the second case irradiation is applied deliberately for purposes such as sterilization and crosslinking, leading to partial improvement of the mechanical properties (e.g. fracture toughness, crack propagation resistance, wear resistance) and better Chemical stability.
Specifically concerning their use in the field of medical technology, different types of UHMW-PE have been objects of numerous publications. It is generally accepted that two parallel and competitive processes, based on chain scission and reactions of Ccentered radicals and molecular fragments, occur in PE as a consequence of radiation: radical recombination accounts for crosslinking, together with some disproportionation, formation of low molecular weight fragments, and recrystallization. Furthermore, formation of oxygenated structures in the presence of traces of oxygen is an antagonist of the C-centered radicals. Radiation induced scission preferentially takes place in the amorphous phase and noncrystalline surface of the crystals. It is followed by folding of molecular fragments, crystallinity growth and increased perfection in the crystal lamellae [4, 5]. Chain scission results in shorter polymer molecules, fewer entanglements, and consequently, increased molecular mobility.
The scope of our investigation comprises an estimation of the radiation and thermal impact on the molecular and supra molecular structure of the two types of PE used for neutron shielding cask components. A further point which is worth to explore is to what extent these changes are detectable by thermo-analytical (TA) methods, such as Differential Scanning Calorimetry (DSC), Thermo Mechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA) and Thermogravimetry (TG). Additionally FT-IR and solubility measurements have been performed.
In our studies we focused on the UHMW-PE material GUR 4120 which was y irradiated at RT and compared to the untreated material.
For long-range measurements in geotechnical and industrial applications, distributed optical fiber sensors have become a tool of increasing importance throughout the past decade.
Classic deformation monitoring (performed by strain gauges etc.) and temperature monitoring (Pt100 and alike) deliver data from fixed, single spots of a structure; quasidistributed measurements (fiber bragg gratings) provide a Chain of discrete measurement points along a limited sensing length. In contrast, an opticai fiber connected to a device for distributed strain and temperature sensing (DTSS) will provide a continuous profile of strain and temperature - spatially resolved down to less than 1 m - over a ränge of several tens of kilometers.
In DTSS measurements, the nonlinear opticai effect of stimulated Brillouin Scattering (SBS) is employed: two light waves with a stable, tunable frequency offset are injected into opposite ends of the sensing fiber, where they will form a beat pattem, at which parts of the light will be scattered. By matching the frequency offset of the light waves to the propagation of acoustic fluctuation in the opticai fiber, a power transfer from one light wave to the other can be measured; since the acoustic propagation directly shifts with train and temperature of the fiber, these two quantities can be measured by tuning the light waves’ frequency offset.
Strain sensors embedded in or attached to structural components have to measure the real deformation of the structure over the whole period of use. The user must know how reliably installed sensors provide strain measurement results. For this purpose, test facilities or coupon tests are used. In order to characterize the strain transfer quality from the host structure into surface-applied strain sensors, a unique testing facility has been developed. This facility can be used both for fiber optic and resistance strain sensors. Originally developed for fiber Bragg grating based sensors, the KALFOS facility (=calibration of fiber optic sensors) uses Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometer (ESPI) as unbiased referencing methods. It is possible to determine experimentally the strain transfer mechanism under combined thermal and mechanical loading conditions. This experimental characterization method will reveal weaknesses in commonly used strain sensors, and the investigation of the material systems used for fiber optic and other strain sensors (particularly the coating/substrate - adhesive combination). The KALFOS facility allows matching of specific measurement requirements with environmental conditions.
Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin Scattering in silica optical fibers and OTDR in polymer optical fibers. Such “smart” technical textiles can be used for reinforcement of geotechnical and masonry structures and the embedded fiber optic sensors can provide information about the condition of the structures and detect the presence of any damages and destructions in real time.
Thus, structural health monitoring of critical geotechnical and civil infrastructures can be realized. The paper highlights the results achieved in this innovative field in the framework of several German and European projects.
Fiber optic strain sensors are increasingly used and sensor Systems are provided with specifications. Even if the performance is well specified, the strain characteristics of the sensor, strain transfer factor, mechanical stability under thermal influences, the performance of applied strain sensors can seriously differ from virgin sensor’s the performance. Therefore the contribution considers validation issues to come to reliable strain measurements and how to validate strain measurements of applied sensors. A new validation facility will be presented.
Sensitive partial discharge (PD) measurements on HV/EHV cable systems are usually based on electrical or electromagnetic PD detection. Unfortunately, interferences may significantly reduce sensitivity, especially in on-site after-installation testing and, even more, in on-line PD monitoring. This paper deals with optical PD detection, which is absolutely immune to any kind of electromagnetic interference. Optical PD detection necessarily requires transparent or translucent insulation systems to be applicable. Stress cone elements for HV/EHV cable accessories meet the requirements for optical PD detection, if made from transparent silicone elastomers.
Monitoring of corrosion protection in reinforced concrete structures using an integrated pH optode
(2011)
Corrosion in steel-reinforced concrete structures is a critical issue. Corrosion appears if the pH value of the concrete matrix decreases due to deterioration of the Calcium hydroxide layer on the Steel surface. The safest Information about potential danger of Steel corrosion is the knowledge of the pH value of concrete. The paper presents a Tiber optic pH sensor for long-term monitoring of the pH value of reinforced concrete structures at risk from corrosion. The sensor probe is small (about 8 mm in diameter), can be embedded into the concrete matrix and observe changes in pH over years. Several sensor probes can be staggered to detect the progression of pH decrease. The pH values can be resolved with 0.1 to 0.2 pH value in the ränge between 12.0 and 9.5. The structure of the pH measurement System, longterm test results and the design to achieve stable probes will be explained. Experienee from test applications of pH sensors embedded in grouted anchors and in concrete components for a cooling tower will be presented.
In Operational Modal Analysis, the modal parameters (natural frequencies, damping ratios and mode shapes) obtained from Stochastic Subspace Identification (SSI) of a structure, are afflicted with statistical uncertainty. For evaluating the quality of the obtained results it is essential to know the respective confidence intervals of these figures. In this paper we present algorithms that automatically compute the confidence intervals of modal parameters obtained from covarianceand data-driven SSI of a structure based on vibration measurements. They are applied to the monitoring of the modal parameters of a prestressed concrete highway bridge during a progressive damage test that was accomplished within the European research project IRIS. Results of the covariance- and data-driven SSI are compared.
Plasma processes are used in many fields of Science and technology. While a plasma oxygen treatment produces a broad variety of different oxygen-containing functional groups, tailored - surface functionalisation demands preset functional groups in known concentration. Due to the fact that most of plasma experiments need expensive vacuum equipment increased efforts are to observe to apply atmospheric pressure plasma processes. Thus, Dielectric Barrier Discharge (DBD) and corona treatment of polymer foils have a long tradilion in industry to modify them for printing, laminating, adhesion promotion, etc. Thercibre, increasing interest is observed to make these atmospheric pressure processes more effeclive. Newly developed methods like plasma-printing were used for surfaces in biological and medical applications or for printed circuits.
BAM Federal Institute for Materials Research and Testing is the competent authority for mechanical safety assessment of transport packages for radioactive material in Germany. The further development of state-of-the-art technology concerning assessment methods is essential for a qualified work of involved designers and authority experts. The paper gives an example of current development done to improve understanding and modeling capabilities of wood filled impact limiter. In order to reduce the loads applied to the package containment, which result from regulatory drop tests, most packages are protected by energy dissipating impact limiter. Wood, encapsulated by steel sheets, is one of the materials typically used for energy dissipation in these impact limiter. Very often, mechanical safety cases regarding the 9 m drop test are performed computationally, where it is essential to use reliable and verified computational methods and models. In this context, the paper presents an approach for a finite element material model for wood. Thereby, the mechanical behavior of wood under compression loading is the focus of the development work. Additionally, material orientation as well as strain rate, temperature and lateral constraint may vary. A large number of experiments, particularly compression tests, was designed and performed to establish an adequate experimental database for modeling verification. The experimental results enabled the derivation of necessary requirements: The material model has to take into account strain rate and temperature dependencies as well as the anisotropic characteristics of the material, a proper yield criterion, flow rule and hardening law. Such a material model is currently not available in established commercial dynamic finite element codes. Thus it is necessary to create a user-defined material model considering the mentioned requirements. A first step was done by determining a yield surface as well as detecting flow and hardening mechanisms from experimental force-deflection curves. In a next step the LS-DYNA material model MAT_75 was altered according to conclusions of former BAM development work, regarding the modeling of post-peak softening as a function of lateral constraint. Future research will contain the further development, implementation and verification of a material model for wood.
Damage to buildings occurs if a construction component fails. The result is a partial or total collapse which can be dangerous for people for example if it's a bridge or a large hall. The collapse of the terminal building at the Airport Charles de Gaulle in Paris and the damage at the historic City Archives of Cologne are typical examples of such accidents. Another problem is the contradiction of the increasing volume of traffic (particularly heavy traffic) and the great age of bridges. The probability increases that the load-bearing capacity of a bridge decreases. For example the collapses of the Mississippi Bridge and of the Inntal Motorway Bridge can be seen as results. Therefore it is necessary to control endangered structures during their life span.
In order to prevent these kinds of accidents, the Federal Institute for Materials Research and Testing developed a radio-based, self-configuring measuring system in cooperation with the ScatterWeb Company, Berlin (Germany). This measuring system consists of identically designed sensor modules which are self-sustaining, wireless, act as transmitters and receivers and are equipped with a special sensor technology for long-term monitoring of buildings or engineering facilities. The sensor unit uses strain gauges for stress analysis and contains interfaces for additional sensors. The system in particular applies to buildings and structures for transport and traffic and large-scale industrial facilities, where a subsequent wiring installation is difficult or impossible.