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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.
This PhD project is part of the Marie Skłodowska-Curie International Training Network MUMMERING. The overarching goal of MUMMERING is the creation of a research tool that exploits the wealth of 3D imaging modalities applied in materials engineering.
The focus of this work is advanced reconstruction algorithms. The primary objective is the development and implementation of the DIRECTT algorithm. Due to restrictions introduced by the sample or the imaging setup during a measurement, it is not always possible to acquire data that fulfil the Nyquist sampling criterion. The DIRECTT algorithm is intended to produce reconstructions of superior quality for such cases of limited data sets, compared to those of other available algorithms, such as algebraic iterative ones. Although DIRECTT has been developed primarily for the reconstruction of volumes from data acquired by an X-ray Computed Tomography scanner, it can also be applied to the different modalities, such as laminography.
The talk describes the reconstruction principle of DIRECTT. Furthermore, reconstructions of measured data are presented and compared to reconstructions produced by other established algorithms.
CFRP materials are progressively used in areas where the weight as well as the strength of materials matter. Since they are used in safety relevant environments, like aircrafts and cars, an assessment of the components for defects is mandatory. Flash thermography has proven to be a valuable tool to resolve defects. However, a quantitative evaluation of the defect extent and depth within the specimen is extremely challenging, since the commonly used analytical models do not incorporate lateral heat flow around the defect areas. Hence a reconstruction of the samples by numerical simulations is a path to improve the results. This finally enables pass and fail tests for specimens based on quantitative results.
The properties of the anisotropic CFRP materials vary dependent on the matrix and fibre chosen. In addition the fabrication processes are distinct, hence it is strongly desirable to measure the properties of a sound sample in a fast and reliable way. The knowledge of diffusivity and heat capacity in three dimensions finally enables the simulation to improve modelling the specimen geometry.
The in-plane diffusivity is determined by heating the sample with a laser line (a flat top profiled focused in one dimension) and monitoring the transient temperature at the front surface of the sample. This method is compared to measurements of the diffusivity using a Gaussian laser spot. The results are compared to results obtained by flash thermography in transmission.
In addition the heating process of the sample on its surface as well the heat conduction is monitored using an IR-microscopic objective. Hence the transparency of the sample as well as the temperature conduction of the two components in microscopic scale can be studied. Finally the results are applied to provide precise parameters in the simulation of defects in a CFRP sample.
It is gratefully acknowledged that the present work is funded by the DFG within the framework of the project WE 2937/6-1.
Carbon-fiber reinforced composites are becoming more and more important in the production of light-weight structures, e.g. in the automotive and aerospace industry. Thermography is often used for non-destructive testing of these products, especially to detect delaminations between different layers of the composite. In this presentation, we aim at methods for defect reconstruction from thermographic measurements of such carbon-fiber reinforced composites. The reconstruction results shall not only allow locating defects, but also give a quantitative characterization of the geometric properties of the defect. We discuss the simulation of the measurement process using finite element methods, as well as the experimental validation. In order to take anisotropic heat transport due to the fibers into account, we compare describing layers separately by individual diffusion tensors with using an averaged diffusion tensor. Especially in pulse thermography, thin boundary layers with steep temperature gradients occurring at the heated surface need to be resolved. Here we consider the combination of a 1D analytical solution combined with numerical solution of the remaining defect equation. Moreover, we discuss the mathematical modelling of various defects like delaminations and undulations, as well as the description of inhomogeneous heating by geometric optics. Finally, we will describe PDE-based methods for the solution of the inverse problem as well as ideas for fast heuristic methods to avoid expensive computations.
A strategy to mitigate typical reconstruction artefacts in missing wedge computed tomography is presented. These artefacts appear as elongations of reconstructed details along the mean direction (i.e. the symmetry centre of the projections). Although absent in standard computed tomography applications, they are most prominent in advanced electron tomography and also in special topics of X-ray and Neutron tomography under restricted geometric boundary conditions. We investigate the Performance of the DIRECTT (Direct Iterative Reconstruction of Computed Tomography Trajectories) algorithm to reduce the directional artefacts in standard procedures. In order to be sensitive to the anisotropic nature of missing wedge artefacts, we investigate isotropic substructures of metal foam as well as circular disc models. Comparison is drawn to filtered backprojection and algebraic techniques. Reference is made to reconstructions of complete data sets. For the purpose of assessing the reconstruction Quality, Fourier transforms are employed to visualize the missing wedge directly. Deficient reconstructions of disc models are evaluated by a length-weighted boundary orientations. The DIRECTT results are assessd at different signal-to-noise ratios by means of local and integral evaluation Parameters.
The paper presents a numerical method to detect and characterise defects and inhomogeneities by means of active thermography. The objective was to determine the wall thickness of structure elements with an inaccessible back wall, e.g., elements of pipes or containers. As test specimens we used PVC samples with the thickness of about 2 cm that had spatial variations in the back wall geometry. Flash lamps provided the heating. To measure the thickness of the wall, we employed the LevenbergMarquardt method, which we applied here to experimental thermographic data for non-destructive testing. We started the inversion procedure by making a rough first estimation of the back wall geometry following the echo defect shape method, and then we calculated the thickness of the back wall. We found reasonable reconstruction results which differed from the real value significantly below 1 mm at the defect centre, whereas the error wais increased at the edge of the defect, depending on its shape and depth.
High-precision magnetic field sensors are of increasing interest in non destructive testing (NDT). In particular GMR-sensors (giant magneto resistance) are qualified because of their high sensitivity, high signal-to-noise ratio and high spatial resolution. We performed magnetic flux leakage measurements of artificial cracks with a GMR-gradiometer and a 3-axes GMR-magnetometer. Cracks of a depth of 44 μm still could be detected with a sufficient high signal-to-noise ratio. A semi-analytic magnetic dipole model was used for swiftly predicting magnetic stray fields. The reliable reconstruction based on measurements of artificial rectangular-shaped defects is demonstrated.