Evaluation of dynamic tensile characteristics of polypropylene with temperature variation

Abstract

This paper deals with dynamic tensile characteristics for the polypropylene used in an IP (Instrument Panel). The polypropylene is adopted in the dash board of a car, especially PAB (Passenger Air Bag) module. Its dynamic tensile characteristics are important because the PAB module undergoes high speed deformation during the airbag expansion. Since the operating temperature of a car varies from -40 degrees C to 90 degrees C according to the specification, the dynamic tensile tests are performed at a low temperature (-30 degrees C), the room temperature (21 degrees C) and a high temperature (85 degrees C). The tensile tests are carried out at strain rates of six intervals ranged from 0.001/sec to 100/sec in order to obtain the strain rate sensitivity. The flow stress decreases at the high temperature while the strain rate sensitivity increases. Tensile tests of polymers are rather tricky since polymer does not elongate uniformly right after the onset of yielding unlike the conventional steel. A new method is suggested to obtain the stress-strain curve accurately. A true stress-strain curve was estimated from modification of the nominal stress-strain curves obtained from the experiment. The modification was carried out with the help of an optimization scheme accompanied with finite element analysis of the tensile test with a special specimen. The optimization method provided excellent true stress-strain curves by enforcing the load response coincident with the experimental result. The material properties obtained from this paper will be useful to simulate the airbag expansion at the normal and harsh operating conditions.