Mechanical features of the rubbery amorphous phase in tension and torsion in a semi-crystalline polymer

Mechanical tension and torsion tests are performed to characterize the mechanical behaviour of a semi-crystalline thermoplastic (Polyamide 11) far above its glassy transition temperature. The aim is to investigate the relaxed state of its amorphous phase. A strain-rate independent loading curve (called “asymptotic curve”) is observed below a critical strain-rate depending on temperature. Comparison between tension and torsion indicate that this “asymptotic curve” does not depend on the hydrostatic stress. Such observations are consistent with a relaxed state of the rubbery amorphous phase. Nevertheless, the unloading curve significantly differs from the loading one. Besides that, creep or stress-relaxation responses are observed when loading is interrupted along the “asymptotic curve”. An additional micro-mechanism involving crystalline lamellae is suggested to explain the high-temperature behaviour. Nevertheless, numerical simulations show that a non-linear elasto-viscoplastic law capturing most of the usual features and the strain-rate sensitivity of polymers behaviour is insufficient to model this particular behaviour. Both the viscoelastic and viscoplastic contributions must be considered to build a suitable model.