Ultrasonic Characterization of Defects and Damage in Thick Composites

Abstract

The fabrication and use of composites in thick sections create special needs for flaw detection and characterization which cannot be met by conventional nondestruc- tive evaluation (NDE). Thick composite sections are susceptible to a variety of fab- rication defects and in-service damage. Typical fabrication defects include matrix cracking, porosity, delaminations, fiber misalignment and waviness, fiber fractures, and nonuniform matrix distribution (fiber volume ratio). A consequence of fabri- cation defects is the variation of mechanical properties through the thickness and the buildup of residual stresses. In-service defects result from environmental factors, such as thermal gradients, and mechanical loading, such as impact. A variety of NDE techniques are used for evaluating the integrity of composite materials [1]. Although ultrasonic methods have some limitations related to attenu- ation and penetration, they still offer the most promising approach to NDE of thick composites. Techniques for characterization of some manufacturing defects in thick composites were discussed before [2]. Previously developed techniques were extended to three-dimensional imaging and to detection of transverse matrix cracks. The material investie;ated here was AS4/3501-6 graphite/epoxy (Hercules, Inc.). Three 10.16 X 15.24 em (4 X 6 in.) plates with different types of defects were fabri- cated and studied.