Mixed-frequency surface acoustic wave generation and applications via non-uniformly spaced grating masks

To address the limitations of traditional high-order harmonic-based surface acoustic wave detection methods, such as low signal-to-noise ratio and interference from instrumentation nonlinearity, this study proposes a novel non-contact excitation technique for mixed-frequency surface acoustic waves (MSAWs) using a non-uniformly spaced grating mask. A line-source excitation model based on the superposition principle is developed, and the grating mask structure was optimized using the differential evolution algorithm to achieve narrowband excitation at fundamental frequencies of 8 MHz and 12 MHz. The excitation performance was validated through finite element simulations. A laser ultrasonic experimental platform was constructed, employing PVDF piezoelectric sensors to detect the generated sum-frequency signals. The experimental results further confirm the effectiveness of the MSAW generation scheme. The ratio of the sum-frequency amplitude to the product of the two fundamental frequency amplitudes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mrow> <mml:mi>a</mml:mi> <mml:mo>+</mml:mo> <mml:mi>b</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:msub> <mml:mi>A</mml:mi> <mml:mi>a</mml:mi> </mml:msub> <mml:msub> <mml:mi>A</mml:mi> <mml:mi>b</mml:mi> </mml:msub> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> increases monotonically with the propagation distance, showing an obvious linear cumulative effect, which is consistent with nonlinear acoustic theory that predicts continuous accumulation of nonlinear components during wave propagation. The proposed technique was then applied to 7075-T6 aluminum alloy specimens with varying degrees of plastic deformation. The results indicate that the nonlinear acoustic parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>β</mml:mi> </mml:mrow> </mml:math> exhibits a strong positive correlation with the degree of plastic deformation. This work successfully demonstrates the feasibility of the proposed MSAW-based method for micro-damage evaluation in metallic materials, offering a high-precision, non-contact alternative for surface integrity assessment and early-stage damage detection.