Effect of Curing Stresses on the First Ply-failure in Composite Laminates

Abstract

This paper describes an experimental technique of detecting the first ply-failure in laminated composites using a combination of long strain gages and acoustic emission measurement. The material system chosen for this study is T300/5208 graphite/epoxy laminates of [0/903] S,[±45/902] S and [0/±45/90] S orientations. Each specimen was instrumented with a strain gage and an acoustic emission transducer. In general, the strain gage technique was good for only those cracks that formed under the gage; the cracks in other areas were barely detected by this technique. The acoustic emission technique, however, was very effec tive regardless of crack locations; matrix cracking always manifested itself in a sudden acoustic emission. The reliability of the above techniques was established by visually observing cracks on photomicrographs. Further more, the experimental data for all three types of laminates agreed fairly well with the analytical predictions with and without residual stresses pres ent. Thus, the effect of residual stresses on the first ply-failure has been delineated.