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Laser-based active thermography is a contactless non-destructive testing method to detect material defects by heating the object and measuring its temperature increase with an infrared camera. Systematic deviations from predicted behavior provide insight into the inner structure of the object. However, its resolution in resolving internal structures is limited due to the diffusive nature of heat diffusion. Thermographic super resolution (SR) methods aim to overcome this limitation by combining multiple thermographic measurements and mathematical optimization algorithms to improve the defect reconstruction.
Thermographic SR reconstruction methods involve measuring the temperature change in an object under test (OuT) heated with multiple different spatially structured illuminations. Subsequently, these measurements are inputted into a severely ill-posed and heavily regularized inverse problem, producing a sparse map of the OuT’s internal defect structure. Solving this inverse problem relies on limited priors, such as defect-sparsity, and iterative numerical minimization techniques. Previously mostly experimentally limited to one-dimensional regions of interest (ROIs), this thesis aims to extend the method to the reconstruction of two-dimensionalROIs with arbitrary defect distributions while maintaining reasonable experimental complexity. Ultimately, the goal of this thesis is to make the method suitable for a technology transfer to industrial applications by advancing its technology readiness level (TRL).
In order to achieve the aforementioned goal, this thesis discusses the numerical expansion of a thermographic SR reconstruction method and introduces two novel algorithms to invert the underlying inverse problem. Furthermore, a forward solution to the inverse problem in terms of the applied SR reconstruction model is set up. In conjunction with an additionally proposed algorithm for the automated determination of a set of (optimal) regularization parameters, both create the possibility to conduct analytical simulations to characterize the influence of the experimental parameters on the achievable reconstruction quality. On the experimental side, the method is upgraded to deal with two-dimensional ROIs, and multiple measurement campaigns are performed to validate the proposed inversion algorithms, forward solution and two exemplary analytical studies. For the experimental implementation of the method, the use of a laser-coupled DLP-projector is introduced, which allows projecting binary pixel
patterns that cover the whole ROI, reducing the number of necessary measurements per ROI significantly (up to 20x).
Finally, the achieved reconstruction of the internal defect structure of a purpose-made OuT is qualitatively and qualitatively benchmarked against well-established thermographic testing methods based on homogeneous illumination of the ROI. Here, the background-noise-free two-dimensional photothermal SR reconstruction results show to outclass all defect reconstructions by the considered reference methods.
Validation of artificial defects for Non-destructive testing measurements on a reference structure
(2018)
Non-destructive testing was established over the last decades as an important tool for assessing damages, material characterization and quality assurance in civil engineering. For example, Ground Penetrating Radar (GPR) can be used to scan large areas of concrete structures to determine the spatial position of the reinforcement. With the ultrasonic echo method, the thickness of concrete structures can be easily determined even if a high density of reinforcement is given. Various methods and processes have been developed for the validation of NDT procedures aiming at ensuring the quality of measurements in practical use. The Probability of Detection (POD) for example, is an available method to compare different technical devices with each other quantitatively regarding their performance. With this method, the best suited testing device for a specific inspection task under defined boundary conditions can be selected. By using the Guide to the Expression of Uncertainty in Measurement (GUM), it is possible to quantify the measurement uncertainty of an inspection procedure for a specific task. Another important aspect to improve the acceptance of non-destructive testing methods is the development of reference specimens.
Reference specimens serve for the calibration and further development of NDT methods under realistic conditions in different laboratories under the same conditions. This is an essential prerequisite for round robin tests. A particular challenge here is the most realistic representation of a damage that can occur at building sites. Possible damages include for example horizontal and vertical cracks or honeycombs in concrete. Such a reference structure was built for the development of a new design of power plant constructions. Comparative studies on the manufacturing of realistic honeycombs and delaminations were carried out in advance on a test specimen. The results of this study are presented here.
The aim of this presentation is to highlight how psychology can be used to prevent human error. It starts with examples of accidents, incidents and events that have happened due to too little attention being dedicated to human factors. It continues arguing that human factors are one of the main factors influencing the reliability of non-destructive testing and gives definitions of the main terms. Furthermore, it presents a method used to identify risks in mechanized NDT to be used for the purposes of the final disposal of spent nuclear fuel and presents a study, in which human-centred design and eye tracking have been used to optimism the inspection procedure. The conclusion is that human factors methods can be used to identify problems during the inspection process and generate mitigation strategies that can be used to decrease human error and enhance safety.
Influence of the real energy input on the sensitivity of thermographic testing in case of GFRP
(2018)
Thermographic testing (TT) is an upcoming nondestructive method, which requires no contact at all to the specimen and can be applied on larger areas simultaneously. The measurement concept is based on the production of a thermal imbalance at the surface of the object under test. When the surface of this object is heated by an external source for a certain time, the surface temperature drops subsequently, influenced by inner defects of the sample. This leads to thermal contrasts at the surface. It is crucial that those contrasts are large enough to be detectable above the noise level. In a first approximation, the observed temperature contrast at a defect is proportional to the energy which was really introduced into the specimen during the heating period. However, the real energy input in a TT experiment is almost always unknown due to distinct parameters of the experimental setup or the material investigated. Typically, only the power consumption of the heating sources is reported, sometimes combined with the distance to the specimen surface.
This contribution describes the thermographic inspection of a rear side thickness variation from 1 to 2 cm at GFRP. This could represent a rear side adhesive bond i.e. in a wind turbine rotor blade. The front side heating was realized by usual halogen lamps with variable radiation power. The detected temperature contrast at the front side will be related to the different energy inputs determined by means of a simple analytical model applied to the experimental data. Additionally, the experimental data are compared with results of FEM simulations performed by COMSOL Multiphysics.
The results clearly demonstrate the key role of the real energy input in a real TT setup, if detection limits have to be evaluated.
Laser thermography has its roots in the late-1960s when Kubiak first used a focused light source to raster-scan a specimen containing surface breaking cracks and employed an infrared detector to monitor the blocking effect of the cracks on the heat flow. Nowadays, this technique has evolved into the flying-spot laser thermography. In the mid-1970s, when Rosencwaig and Gersho worked out the theory for the photoacoustic effect, they laid the foundation for the development of the photothermal testing methodology. The decisive difference to the flying-spot technique is that in the photothermal methodology, a high-frequency modulated laser beam is used, which explicitly uses the concept of the thermal wave.
Starting in the 1980s, infrared cameras came into play and enabled full-field thermal imaging, but this time, slow thermal light sources were used. Today, flash lamps (pulsed planar heating) and halogen lamps (modulated planar heating) have been established as the standard light sources for the specific regimes of pulsed and lock-in thermography.
Only recently, the availability of novel technologies – fast and high-resolution infrared cameras, innovative brilliant laser sources and high-performance data acquisition and processing technology – has enabled a paradigm shift from the separated photothermal and thermographic methodologies to a versatile tool for NDE that combines both approaches. In addition, the heating can also be spatially structured using laser arrays. This new degree of freedom allows the development of completely new thermography NDE methods.
In the talk, we present current activities with kilowatt-class high-power laser sources for advanced flying-spot, pulsed, lock-in and laser-projected thermography.
Vortrag über die Aktivitäten des Fachbereichs 8.2 in der zerstörungsfreien Prüfung im Bauwesen.
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.
With the mandatory introduction of the May 2011 directive for reassessment of bridges in Germany, the administrations of the federal and state governments have the duty to prove the stability of their bridge stock. Verification of bridge stability will be realized with consideration of the newly increased traffic loads. Particularly in older bridges, the verification can only be achieved if calculative surplus load capacity of the original structural design is taken into account in the recalculation. One option for considering these reserves is the exact determination of the dead weight of the bridge. Within this case study, it will be demonstrated how the problem can be practically solved.
In order to determine the dead weight of a concrete bridge, its volume has to be calculated. as a first step, a 3D laser scanner is used to record the internal geometry of a hollow box bridge girder. For the determination of the thickness of the concrete member, the non-destructive technique ultrasonic echo is applied. The construction must be segmented in approximately equidistant parts in order to be able to carry out an economic and efficient investigation. The description of the segmentation of the point cloud, carried out in a 2D model, was presented in the first part of the publication. The subject of this presentation is the merging of 2D cross sections into a 3D model, from which the weight of the bridge can be calculated.