Cyclic behavior of short glass fiber reinforced polyamide for fatigue life prediction of automotive components

Fatigue life prediction of polymer matrix composites requires the investigation of the cyclic behavior. This paper deals with the experimental study conducted on a polyamide 66 reinforced with 35 wt% of short glass fibers (PA66 GF35), at room temperature. The material was tested dry-as-molded or at the equilibrium with an air containing 50% of relative humidity. The dynamic mechanical analysis leads us to focus on the material conditioned at RH=50%, which appears to exhibit the most complex mechanical effects, and is yet representative of the actual service life. An exhaustive experimental campaign in tensile mode has been carried out, including various strain or stress rates, complex mechanical histories and local thermo-mechanical recording. The material has thus been tested in static tension at stress rates ranging over four decades (from 2.5 to 2500 MPas), and also in cyclic tension with loading histories combining creep, stress relaxation or strain recovery steps at different strain/stress levels. Such an extended database allowed us to highlight several mechanical phenomena: at least two characteristic viscous times co-exist, and are independent of any irreversible mechanism. Residual strain appears above a stress threshold, and a kinematic hardening law is suggested to explain tension-relaxation-recovery tests. Eventually, hints of a non-linear viscous flow law and of a cyclic damage law are pointed out.