Influence of loading conditions on the tensile response of degraded polyamide 11

The chemically induced embrittlement of polyamide 11 (PA11) has been the subject of a considerable research effort to understand and predict the degree of embrittlement based on the material's intrinsics. However, the presence of highly complex loading scenarios and stress states in real-world structures raises the question of whether extrinsic factors such as loading conditions and temperature also affect the mechanical response of the degraded material. Motivated by this, this study investigates how the large-strain tensile response of chemically degraded PA11 is influenced by temperature, strain rate and stress triaxiality. Material samples with plasticizer contents of 0% and 6% were chemically degraded by exposure to butanoic acid at elevated temperature. The mechanical properties were determined by tensile testing of round notched samples while measuring local strains using digital image correlation. The mechanical responses span from highly ductile behaviour with necking and subsequent cold-drawing to a brittle response without any apparent plasticity. Reduced ductility was found for increasing degradation, amplified when the temperature, the notch radius or the softener content was reduced. A comparison of local and nominal strains showed that large local strains could be present even on samples which appear brittle when only nominal strains are measured. This study demonstrates that both a brittle and a ductile response can be obtained for the same state of chemical degradation by varying the loading conditions.