Time Resolved Line Focus Acoustic Microscopy of Composites
Acoustic microscopy has been used to measure material properties since the 1980s [1–4]. The velocity of the leaky surface wave can be accurately determined from the V(z) curve which is formed by the interference between the leaky surface wave and specular reflection. By fitting the leaky wave velocity or the V(z) curve itself, Kim et al.  reconstructed the material properties (elastic constants and mass density). Another approach is time-resolved acoustic microscopy [5–8]. In this method, the leaky surface wave and the specular reflection are separated in the time domain and the velocity is determined from the time of flight. For a graphite/epoxy composite, due to the complexity of the reflected signal and the absence of Rayleigh wave excitation, it is impractical to determine material properties from the V(z) curve. In time-resolved acoustic microscopy, the different reflection signals are separated in the time domain and the velocity measurement is simplified. For graphite epoxy composite materials, due to their low density and significant fluid loading, the acoustic microscopy response is significantly different from that for higher density materials. To model the time domain acoustic microscopy response for a mutilayered composite, we applied the global matrix method in the form similar to that of Mal , thus avoiding the numerical instability at high frequency.