Björn Maack

In this work, we present the full-field measurement of guided ultrasonic waves (GUWs) on technical surfaces using lensless digital holography. The setup works with a pulsed coherent laser illumination and a complementary metal-oxide semiconductor (CMOS) sensor without using any imaging optics. It provides highly resolved single-shot height measurements featuring the deformation introduced to the material by ultrasound excitation via piezoelectric actuators. For the evaluation of the resulting height distribution, we propose the multidirectional structure function offering pixel-precise determination of the wavelength. This ability is used to carry out a comparison between two aluminum plates of different thicknesses. Here, the resulting difference in wavelengths reflects the change in the thickness and is in agreement with simulated results for the two specimens, qualifying the proposed method as a detector for GUWs.

Remotely in-line monitored acoustic emission (AE) of interlaminar shear strength tests in a 3-point bending test of fiber-reinforced polymer composites (FRP) conducted under cryogenic conditions in liquid nitrogen (LN2) were studied. The AE sensors were reversibly mounted at the outer components of the testing machine to thermally decouple them from the cryogenic area and use AE signal transmission through the testing machine structure as a waveguide. The damage mechanisms and effect of the cryogenic temperature were studied at 296 K room temperature (RT) and in LN2 at 77 K, considering machine and process noises. The correlation of machine data with acoustics and the AE hit analysis revealed matrix cracking as the most frequent damage mechanism under both conditions but with different failure mechanisms. At RT and applying higher loads, the most damage suddenly occurred, and the specimen failed. In LN2, the damage occurred continuously from the beginning of testing. The amount of fiber failure increased, and the AE feature ranges enlarged. This study presents a method by AE for remote monitoring the mechanical response of FRP in cryogenic fluids such as liquid hydrogen. The method provides a new approach to support the more efficient development of FRP materials for storage vessel structures and structural health monitoring systems.

For the safe operation of vehicles and full utilization of lightweight materials, assurance of structural integrity is a prerequisite at all times. Structural health monitoring with permanently installed transducers offers a great advantage for primary load-bearing structures of all means of transportation and other safety-relevant components such as hydrogen tanks: allowing damage detection during operation. One means to detect internal defects is the method of guided ultrasonic waves (GUWs), which can be generated and recorded by piezoelectric transducers. GUWs propagate along the elongated dimension of a structure, and a transducer network can completely cover and monitor structures. Defects can alter the signal along affected paths and allow for their detection. However, a challenge and obstacle for the application of such a testing technique in the service of means of transportation is the large influence of temperatures. These influences are difficult to distinguish from the effect of defects. One approach to overcome this difficulty is the “continuous baseline update”. Recurrence quantification analysis is tested and compared to established features as a new approach to “continuous baseline update” in this paper. Publicly available GUW data (http://www.openguidedwaves.de/) recorded under varying temperature conditions have been used to show how the methods perform. They reliably separate temperature and damage effects, while the recurrence quantification analysis yields the best results.