The effects of tensile preloads on the impact response of carbon/epoxy laminates
Low velocity drop weight impact tests were conducted on carbon/epoxy laminates under various magnitudes of uniform tensile stress. The composite plates were 8 ply (+45,0,- 45,90)s laminates supported in a clamped-clamped/free-free configuration. Tensile preloads from near zero to approximately 60% of ultimate breaking strength were applied to specimens which were impacted at energies of 3.4, 4.5 and 6 Joules (2.5, 3.3 and 4.4 ft- Ibs). The amount of damage induced into the specimen was evaluated using instrumented impact techniques, x-ray inspection and cross-sectional photomicroscopy. Some static indentation tests were performed to examine if the impact events utilized in this study were of a quasi-static nature and also to gain insight into the shape of the deflected surface at various preload/transverse load combinations. Load-displacement curves from these tests were compared to those of the impact tests as was damage determined from x-ray inspection. The finite element technique was used to model the impact event and determine the stress field within the laminae.
Results showed that for a given impact energy level, more damage was induced into the specimen as the tensile preload was increased. The majority of damage observed consisted of back face splitting of the matrix parallel to the fibers in that ply, associated with delaminations emanating from these splits. Tensile preloads tended to increase the length of these splits. The analysis showed qualitatively the results of tensile preloads on maximum load of impact, maximum transverse deflection and first failure mode and location.