Exploring 3D elastic-wave scattering at interfaces using high-resolution phased-array system

Abstract

The elastic-wave scattering at interfaces, such as cracks, is essential for nondestructive inspections, and hence, understanding the phenomenon is crucial. However, the elastic-wave scattering at cracks is very complex in three dimensions since microscopic asperities of crack faces can be multiple scattering sources. We propose a method for exploring 3D elastic-wave scattering based on our previously developed high-resolution 3D phased-array system, the piezoelectric and laser ultrasonic system (PLUS). We describe the principle of PLUS, which combines a piezoelectric transmitter and a 2D mechanical scan of a laser Doppler vibrometer, enabling us to resolve a crack into a collection of scattring sources. Subsequently, we show how the 3D elastic-wave scattering in the vicinity of each response can be extracted. Here, we experimentally applied PLUS to a fatigue-crack specimen. We found that diverse 3D elastic-wave scattering occurred in a manner depending on the responses within the fatigue crack. This is significant because access to such information will be useful for optimizing inspection conditions, designing ultrasonic measurement systems, and characterizing cracks. More importantly, the described methodology is very general and can be applied to not only metals but also other materials such as composites, concrete, and rocks, leading to progress in many fields.

Multi-mode 3D ultrasonic phased array imaging method using piezoelectric and laser ultrasonic system (PLUS)

“…The experimental configuration for multi-mode PLUS is essentially the same as the single-mode PLUS, [39][40][41] as schematically illustrated in Fig. 1.…”

“…Furthermore, the intrinsically low sensitivity of the LDV can be compensated by a large-amplitude ultrasonic incidence with a piezoelectric transmitter. [39][40][41] We formulate the 3D imaging algorithm for the multimode PLUS. In this study, the multi-mode PLUS utilizes longitudinal (LL) and mode-converted shear (LS) waves for a longitudinal-wave incidence.…”

“…Hence, the propagation time differences Δt LL (r, nx, ny) and Δt LS (r, nx, ny) between r 00 and r nx, ny for the LL and LS scattered waves are given by which correspond to delay laws. As a 3D imaging algorithm, delay-and-sum (DAS) processing [39][40][41] is used here.…”

“…However, such simulation requires a high computation cost because of tremendous number of meshes due to 3D. In contrast, the simple simulation 40) for creating received waves based on theoretical propagation time can be performed without high computation costs. In this study, to investigate the fundamental difference between the 3D imaging results based on the LL and LS scattered waves in ideal cases, we simulated the received waves under the following assumptions.…”

“…39) Thus far, we have demonstrated the importance of a large receiving aperture composed of ultra-multiple receiving points in the PLUS for high-resolution 3D imaging. [39][40][41] However, the ultrasonic mode used has been limited to a single mode. That is, the mode conversion 34,[42][43][44][45] due to ultrasonic scattering at defects has yet to be considered.…”