All-optical, non-contact, local measurement of both Young’s modulus and Poisson’s ratio in metals using a combination of Rayleigh and leaky surface acoustic waves

In non-destructive evaluation (NDE), measuring ultrasound (US) longitudinal and shear wave speeds is the main method to determine two independent mechanical moduli (Young’s modulus and Poisson’s ratio). Most US techniques use time-of-flight measurements. However, when the local sample thickness is unknown or the sample geometry is complex, bulk-wave propagation speeds cannot be accurately defined. Here we show that a properly shaped beam of nanosecond laser pulses can be used to efficiently excite (without material ablation) two types of surface acoustic waves. In addition to a conventional surface, or Rayleigh, wave, a leaky surface wave (LSAW) can be launched in the near field of the excitation region. We present the theoretical background, numerical simulations, and experimental results clearly showing that both elastic constants can be reconstructed locally by tracking Rayleigh and LSAW waves. Spatially resolved elastic properties can be obtained using local values of wave speeds obtained by sample scanning. Non-contact optical detection of propagating waves at the sample surface, in which a fiber-optic Sagnac interferometer was used, is a key piece of the method. It does not require acoustic coupling and allows remote measurements in the near field of the laser source with micron-scale resolution.