The images of subsurface defects inside soft materials are easy to interpret using a simple ray optical approach. We monitored degradation in epoxy coated samples using the scanning acoustic microscope. Figures below illustrates the imaging of defects located at the interface between aluminum and a 15 µm epoxy layer. Epoxy and polymer coatings are widely used by modern industry for protection purposes and as an adhesive. The surface image of the epoxy layer (left image) shows several defects as dark and bright spots. A comparison of images of the epoxy surface (left image) and of the epoxy-aluminum interface (right image) shows that most of the defects (voids) are located near the interface. The contrast of the defects gets brighter as the SAM focus moves closer to the interface and more defects become visible. Degradation has been seen at discrete sites, rather than uniformly over the surface. Degradation has only been detected when it has resulted in a small void/delamination.
Recently, it has been found that combining the time-of-flight technique with acoustic microscopy is a powerful tool for investigating adhesion problems (Zinin et al., 1999).

Acoustical micrograph (OXSAM) 15 mm epoxy/1 mm OAA/Al: frequency was 300 MHz.