In the field of active ultrasonic testing, research activities focus on the description of the interaction of incident waves with complex defect geometries. For this purpose, classical ultrasonic testing methods with (partially) automated scanners as well as single oscillators are considered. In addition, an ultrasonic microscope for frequencies up to 150MHz is available for high-resolution imaging of the acoustic properties. In contrast to conventional ultrasonic methods, special transducers with confocal sapphire lenses are used, which allow a lateral resolution of up to 5 µm. The penetration depth of the ultrasonic wave depends essentially on the transducer frequency used and the material properties. This is typically in the range between 400 µm and 2000 µm. A 2D image of the elastic properties of the sample can thus be generated by scanning the surface of the sample. In addition, the arrival time of the pulse-echo signal can be used to take a look inside the sample as a C-scan or 3D image. Furthermore, systems are available for testing with guided waves (Lamb waves, Rayleigh waves, ...).
In the field of radiography, the focus is on the microstructure of fiber-reinforced materials. The resolution of the existing devices reaches up to 400nm voxel size. This is used to make internal defects visible and to track their growth in-situ or before and after exposure. Specially developed components for the mechanical and thermal loading of test specimens in the X-ray computer tomograph are available for this purpose. In addition, algorithms are being developed for the automated evaluation of volume images (e.g. for damage detection and quantification).
Acoustic emission analysis
Acoustic emission analysis is based on ultrasonic waves that are released as a result of microdeformations in a material. These microdeformations are often associated with an irreversible change in the material (e.g. crack growth) and provide early information about impending material failure. The surface oscillation caused by sound waves is converted into a voltage signal by piezoelectric sensors and recorded. By using several sensors at different positions, it is possible to locate the origin of the wave using the arrival time differences. In addition, the frequency composition and radiation direction of the individual waveforms can be used to identify the underlying damage mechanism. The focus of the work is on automated signal classification using multi-variant data analysis and localization methods.
As another highly dynamic phenomenon during the fracture process of a solid, the analysis of the electromagnetic emission can provide further information about the processes during the fracture. Based on the resulting charge imbalance, electromagnetic fields can be detected during fracture. Currently, the focus of research activities is on the improvement of the necessary sensor technology and the clarification of fundamental questions regarding the cause of electromagnetic emissions.
In the field of thermography, our work focuses on the use of early damage detection in fibre-reinforced materials. In particular, the transient thermal energy release during fracture and the local heating caused by friction during load cycling are analyzed.