Influence of plying strategies and fabrics type on crashworthiness properties of carbon fiber laminates cured through autoclave processing

The advent of composite materials in aircraft and automotive structures has recently revealed the lack of data about the behavior of these structures under operative loads and in particular in case of impacts. The introduction of high performance composite materials (i.e. carbon fibers reinforced polymer, CFRP), overcame the knowledge collected from experience and tests of impacts on traditional metal structures thus making the data obtained, no longer sufficient or representative. The overall objective of designing for crashworthiness is to eliminate injuries and fatalities in relatively mild impacts, and to minimize them in case of severe collisions. A crashworthy vehicle has to be also able to control the extent and position of crash impact damage for minimizing personnel and material losses. Advantage of composites laminates is the opportunity to realize a tailored design of the structures proprieties by controlling the pre-preg type, orientation or the stacking sequence in the laminate. In the paper a small-scale experimental test able to characterize a material under compression in terms of energy absorption was initially developed by innovatively introducing self-supporting sinusoidal shape specimens thus avoiding the buckling of classical flat specimens. Two pre-preg types were tested: 12 plies of unidirectional tape (ACG T700 24k/MTM57) or 8 plies of plain weave fabric (GG200P/IMP530R) or the production of a laminate of around 1.8 mm thickness in both cases. Both materials were cured in autoclave, according to the supplier indications at 120°C for 1hr for unidirectional tape and ½ hr at 120°C then ½ hr at 150° for plain weave both under 6.2 bar compacting pressure. Three stacking sequence (UD[0/90]3s, PW0/90° and PW±45°) were tested in order to identify the configuration able to maximize the specific energy absorption SEA (i.e. the energy absorbed per unit mass of crushed structure expressed in J/g). The UD[0/90]3s specimen provided the best condition with 100J/g maximum value stable all along the specimen stroke. Regarding the other important feature for a crashworthy structure namely the chance to control the position of initial damage and its propagation, 8 additional specimens were produced with auto-triggering configurations. Indeed fibers continuity was interrupted in selected position and for different extent in order to investigate the SEA of the weakened laminates. It was found that for unidirectional specimens UD [0* ; 90]3s with fiber interrupted on the 2 internal plies in the 0° direction for an extent from 0 up to 2mm the SEA did not modified its maximum value of 100J/g and behavior over the stroke. As conclusion, the auto-triggering configuration was then able to control the position of the initial failure of the specimen without any decrease in safety performance. KEYWORDS Crashworthiness, plying sequence, carbon fiber, autoclave