Surface Acoustic Wave Suppression for Near-Surface Defect Imaging Using Laser Induced Phased Arrays

Abstract

Laser induced phased arrays (LIPAs) offer fast and efficient remote ultrasonic imaging for processes operating in extreme environments and restricted access such as additive manufacturing and welding. In this work, LIPAs are synthesized in the non-destructive thermoelastic regime using an 8 ns pulsed 1064 nm generation laser and a 532 nm continuous wave detection laser. The acquired Full Matrix data is post-processed using the Total Focusing Method (TFM) to image near-surface side-drilled holes inside an Aluminium sample. The images generated, however, contain contribution from the surface acoustic wave (SAW). In laser ultrasonics, SAW is the strongest wave mode generated, and consequently, a region of the image generated is saturated by the SAW arrival (SAW cross-talk). The SAW cross-talk region extends into the sample starting at the scan surface and hence masks any features/defects within this region. This study explores and compares various signal processing techniques such as frequency-wavenumber filtering, phase coherence imaging and amplitude thresholding of ultrasonic signals in order to suppress/remove the SAW cross-talk from the ultrasonic data captured using LIPA for successful imaging of near-surface defects. The mode suppression is achieved by targeting the characteristics of the SAW: its velocity, amplitude and phase. The different methods of wave suppression are compared, and relative merits of each technique are discussed.