“…As a result, it will be more viable and reliable to measure the very small changes in ultrasonic wave velocities in the frequency domain caused by the stress field. Since its discovery in the 1980s, the LLW technology has been applied for many NDE applications, such as the measurement of material properties [6], the cohesive properties of bonds [5], and the properties of composite materials [7]. However, the key parameters in the LLW dispersion equation, such as the plate thickness, density, and longitudinal and transverse ultrasonic wave velocities, are coupled in the symmetric and antisymmetric Lamb wave modes, and cannot be decoupled easily in general [8].…”