Impact Damage Tolerance Assessment in Polymer Composites : A New Approach

The impact behaviour of polymer composites is a complex phenomena affected by material parameters, instrument parameters, testing conditions and process variations. As it has not been possible to quantify the impact behaviour based solely on impact studies, post-impact tests such as compression-after-impact or tension-after-impact have been used to study the degradation in strength properties and quantities impact damage. Since these post impact tests are very expensive and rigorous, this work, gives a simpler approach route for evaluation of impact damage tolerance of advanced composites by the use of repeated drop tests. Low velocity (1.2-2.4 m/s, incident energy range being 3.55 - 159 impact tests have been carried out on three variants of glass and one variant of carbon epoxy composites using an instrumented impact test machine ( DYNATUP 8250). The composites were subjected to repeated impacts until failure for a fixed energy level. Number of drops to failure (Nf) data were obtained for each of the pre-determined incident energies (E in) in the above range. E in, Vs Nf plot for each) composite was obtained and a relationship between them was evolved and verified. Further, &om the I&, Vs Nf plots, a critical incident energy parameter termed Ec was obtained. This critical energy value turns out to be the threshold value, below which the composite takes infinite drops to fail, and above which, it fails after very few drops. Ec, therefore, can be used as an important input in the design of impact damage tolerant composites by composite designers. This E, forms yet another criteria for damage tolerance assessment in advanced composites, apart from the very expensive/ sophisticated compression-After-Impact tests, hitherto being considered