FURTHER DEVELOPMENTS IN THE INTERPRETATION OF SIGNALS FROM INSTRUMENTED FALLING WEIGHT IMPACT (IFWI)

The instrumented falling weight impact (IFWI) technique is widely used for evaluating the impact and fracture toughnesses of polymeric materials. The method involves obtaining a force-time signal which can subsequently be processed to give force-deflection and energy-deflection curves. The development of photographic techniques for observing the impact process and the recognition that the level of filtering of the force-time signal can significantly affect results have together permitted a greater understanding of the test. In particular, the yielding deformation of ductile specimens and crack growth in brittle specimens can be monitored, as can forces and energies associated with significant failure events. Results are given for thermoplastics with and without fibre reinforcement. IFWI techniques are also used with notched specimens to generate fracture mechanics data. As with plate specimens, the conditions of the impact test, e.g. filtering, are shown to be important. In addition, the testing of plate specimens can give data which may be correlated to fracture mechanics data from other sources on a variety of short-fibre reinforced polymeric materials. Consequently, the IFWI test may be used to obtain far more information on a material than was hitherto supposed.