Experimental and numerical investigation of deformation and fracture of semicrystalline polymers under varying strain rates and triaxial states of stress

Tensile tests have been carried out on plain and notched ultra high molecular weight polyethylene and polyoxymethylene specimens over a range of quasistatic strain rates and stress triaxiality conditions. Numerical simulations of the experiments have been carried out using the finite element code NIKE2D in order to give accurate predictions of the triaxial state of stress at the fracture initiation site as a function of initial geometry and axial strain. The predicted axial load-time curves obtained from the numerical simulations were in a good agreement with the experimental curves demonstrating that the NIKE2D code has the ability to model the deformation behaviour of these polymers accurately. The experimental results for plain cylindrical specimens show that the materials under investigation are sensitive to changes in strain rate, with plastic flow stress increasing with increasing strain rate. The results from the tests on notched specimens show that the local failure strain decreases with reducing specimen notch profile radii (i.e. increasing stress triaxiality) but this dependence is less clear for ultra high molecular weight polyethylene as a result of its much higher ductility leading to large axial strains and consequent molecular orientation.