FRACTURE-MECHANICS OF POLYMERS - CRITICAL-EVALUATION FOR LINEAR ELASTIC BEHAVIOR AT HIGH-SPEED TESTING

Over the last few years considerable effort has been made to obtain reliable stress intensity factor and strain energy release rate (K//I//C and G//I//C) data on polymeric materials. Experience has shown that a valuable method to minimize viscoelastic losses and plastic deformation is to work at high speed. However, some problems remain for this experimental method which have to be solved before standard methods can be defined. On the basis of measurements done with 3-point bending geometry on poly(vinylchloride) (PVC) and poly(propylene) (PP) at room temperature and over a range of notch depths, the work demonstrates that the linearity of experimental data both in the load (F) and in the energy (U) against (2BW**2/3LY ROOT a) and (BW phi ) plot is not a critical test for linear elastic behaviour so that K//I//C and G//I//C values can be affected by large errors. Only the knowledge of the experimental curves which can be obtained by means of instrumented pendula in optimized test conditions allows a critical test to be applied for linear elastic behaviour based on comparison between experimental and predicted data for load, displacement and energy. These tests show that the linear elastic fracture mechanics, LEFM, criterion is satisfied for PVC but not for PP. This conclusion is further supported by the morphologies of the fracture surfaces.