Determining the constitutive response of polymeric materials as a function of temperature and strain rate

Abstract
Compression stress-strain measurements have been conducted on two commercial high temperature thermoplastic polymers (polyamideimide (PAI) and partially crystalline polyetheretherketone (PEEK)) as a function of temperature (-55 °C to 300 °C) and strain rate (0.001 s ^-1 to 2500 s ^-1). A split-Hopkinson pressure bar (SHPB) was used to achieve strain rates of about 2500 s ^-1 and conventional testing machines were used for strain rates from 0.001 s ^-1 to 1 s ^-1. A large variation in the mechanical response was observed over the range of temperatures tested for both polymers showing "yield" and "plastic" deformation below the glass transition temperature (T ). Above T both thermoplastic polymers exhibited increasing loading moduli with increasing strain rate or decreasing temperature. T appears to shift to higher temperatures as the strain rate increases. Below T both thermoplastic polymers exhibited a yield-type behavior followed by anelastic deformation. PEEK was seen to be more sensitive to strain rate and temperature than PAI. A range of different flow stress versus strain behaviors was observed at different temperatures and strain rates.