The tensile yield stress is compared with the uniaxial compression yield stress for specimens of polycarbonate tested at constant strain rate from −120 to + 120°C. The strain rate dependence of the tensile yield stress is also studied from − 140 to + 20°C.
The yield behaviour is described by a modification of the Eyring theory of non-Newtonian viscosity where the deformation is supposed to involve two different flow processes.
The validity of a yield criterion previously established by one of us, is checked throughout the range of temperatures explored; the proposed equation gives a good fit to the data provided it is applied separately to each flow process.
Then the present investigation, combining the yield condition with the modified Eyring theory, provides a formalism which agrees with the data and accurately renders the temperature, strain rate and pressure dependence of the yield stress of polycarbonate.