Tensile yield in polypropylene

Untaxial tension tests to the yield point were performed on polypropylene as a function of temperature from 22 to 143°C at a strain rate of 2 min −1 . At 22, 42, and 71°C, measurements were also made at strain rates from 0.02 to 8 min −1 . Yield energy was found to be a linear function of temperature extrapolating to zero at the melting point (164°C). The ratio of thermal to mechanical energy to produce yielding is about three times smaller than for glassy polymers. The ratio of yield stress to (initial) Young's modulus is about 0.024 at room temperature and increases to 0.043 at 143°C. Yield stress is a linear function of unstrained volume.