Multiaxial fatigue life assessment for reinforced polymers

The multiaxial fatigue behaviour of PBT-PET GF30 (polybutylene terephthalate-polyethylene terephthalate with 30% mass short glass fibres) and PA66 GF35 (Polyamide 66 with 35% mass short glass fibres) is studied under constant amplitude force loading. Standard flat specimens are used to establish the influence of microstructure on the tension fatigue behaviour. Tubular specimens are used to study the influence of loading path on the tension-torsion fatigue behaviour. Finally an experimental device is developed to test dedicated flat samples in order to study the influence of microstructure on the pure shear fatigue behaviour. All fatigue tests are performed at two load ratios: −1 and 0, 1. Experimental results are analysed using a Through Process Modelling (TPM) based on three major steps. The first one is based on the simulation of the whole process in order to get the fibre orientation tensor at each point of the part. The second one uses the orientation tensor to perform a two-step homogenization procedure in order to estimate local effective properties and compute the stress-strain response as a function of local anisotropy. Mechanical fields thus obtained are used as input data for the application of a fatigue criterion in the third step. Results show that energetic fatigue criterion is an excellent compromise to get good fatigue life assessment for only one experimental input fatigue data.