Effects of stacking sequence on post-impact thermoplastic composites

The effect of impact damage on advanced composite materials has long been a concern within the aerospace industry. This study seeks to determine the effect of laminae stacking sequence on the compression strength of impacted AS4/APC-2, advanced composite material. Three different 48 ply quasiisotropic laminates were tested. Only the order of the plies was varied between the three laminates. Thus extensional stiffness for each laminate was the same. Specimens from each laminate were impacted at 20, 30, and 60 ft-lbs of energy using the drop tower method. It was found that damage area increased with higher impact energy and with increased stacking of like-angled-plies. Results lend support to the theory that bending stiffness plays a strong role during the impact event in determining the amount of damage. When compared with other thermoset matrix material systems, the thermoplastic matrix exhibited a lower threshold to barely visible damage. This may lead to earlier detection of damage in thermoplastics than in thermosets. Its post-impact compression strength did not appear to be less than that off thermosets. Damage radius or width had a inverse relationship to failure strain. This relationship is similar to that of buckling strain to plate width in classical plate theory. A strong parallel was drawn between laminates with lumped plies and compression strength. It was concluded that sub-laminates of uni-direction tape of more than two plies should be avoided in damage tolerant designs. The test results show that failure strains can be reduced by 50 percent or more in laminates with heterogeneity of order three.