Burial Depth Effect of Crack on the L(cr) Wave Acoustoelastic Coefficient for Stress Measurement of Laser Cladding Coating

In this paper, the influence of burial depth of crack on stress measurement of laser cladding coating with the critical refracted longitudinal wave (L(cr) wave) was discussed based on the L(cr) wave acoustoelastic effect. The regular rectangular slots with different depths that were used to simulate the burial crack in coating was based on the equivalent theory. The experimental system including an ultrasonic wave generator, digital oscilloscope (2.5 GHz sampling rate), and two L(cr) wave transducers (2.5 MHz center frequency) was used to collect the L(cr) wave under different tensile loads, and the L(cr) wave was denoised by using wavelet analysis technology, then the fracture morphology was observed using SEM. The results show that after the denoising by wavelet analysis technology, the signal-to-noise ratio of L(cr) wave becomes bigger and the mean square deviation of L(cr) wave becomes smaller. When the tensile load is within the turning point load, the difference in time of flight between L(cr) wave increases linearly as the tensile load increases, and the deviation of the experimental results becomes obvious as the tensile load increases. When the tensile load is the same, as the burial depth of the slot increases, the nominal L(cr) wave acoustoelastic coefficient decreases and tends to be stable gradually. At last, the experimental results are discussed based on the L(cr) wave acoustoelastic effect and deformation theory, and it is analyzed that the uneven deformation caused by the interface in coating, anisotropic microstructure, and the burial crack is considered as the main reason.