Simultaneous determination of Young's modulus and Poisson's ratio in metals from a single surface using laser-generated Rayleigh and leaky surface acoustic waves

Abstract

Material elastic moduli are used to assess stiffness, elastic response, strength, and residual life. Ultrasound (US) measurements of propagation wave speeds (for longitudinal and shear waves) are now primary tools for non-destructive evaluation (NDE) of elastic moduli. Most US techniques measure the time-of-flight of through-transmission signals or reflected signals from the back wall. In both cases, an independently determined sample thickness is required. However, US methods are difficult for complex (non-flat) samples. When the local thickness is unknown, the propagation speed cannot be determined. On the other hand, the propagation speed of Rayleigh waves can be calculated without knowledge of sample thickness, but another independent measurement is still required to compute both Young's modulus and Poisson's ratio. We present a comprehensive theoretical background, numerical simulations, and experimental results that clearly show that when the material density is assumed known, both elastic constants of an isotropic metal sample can be determined with laser-ultrasound by tracking two types of surface propagating waves without any sample contact (both signal excitation and detection are performed optically). In addition to a conventional surface, or Rayleigh, acoustic wave, a leaky surface wave can also be launched with nanosecond laser pulses in the thermoelastic regime of excitation (i.e., without material ablation) close to the source that propagates along the sample surface with speed close to that of bulk longitudinal waves. Samples can be of arbitrary shape and their thickness need not be measured.