It has recently been shown in our laboratories that quasi-isotropic, graphite-epoxy, composite laminates develop a typical damage state that eventually leads to final failure. This damage state cannot be represented by a single through crack that propagates in a self-similar manner in the fashion ordained by fracture mechanics. To the contrary, the damage state is a complex one which begins by transverse cracking in the weakest lamina, continues by an increase in transverse crack density until a stable equilibrium spacing is achieved, proceeds by growth into the adjacent laminae.and ends by final, catastrophic failure. In certain stacking sequences, the damage state is further complicated by delamination. Several NDE methods are being developed in our laboratories specifically to identify and quantitatively describe this damage state. The vibrothermography technique uses low amplitude vibrations as a steady state energy source in the composite laminate. The mechanical energy is preferentially absorbed in the region of damage and converted to heat, which can then be detected by thermography. This technique is especially applicable to detecting delamination. An ultrasonic pulse-echo method utilizing a straightforward diffraction analysis is. being developed to detect the transverse cracks which, as they approach and attain an equilibrium spacing, present the appearance of a changing diffraction grating to the ultrasonic beam
