Anisotropic deformation and failure behaviors of the necked HDPE materials induced by oligo-cyclic loading

Oligo-cyclic loading tests are performed between a fixed nominal macroscopic strain 1.5 and zero force on high density polyethylene (HDPE) with different microstructural properties (i.e. crystallinity, lamellar dimensions, density of stress transmitters). Based on the results of simultaneous Digital Image Correlation (DIC), the local strain is significantly localized when the macroscopic strain exceeds the elastic limit, confirming the necking propagation during the first loading path. Upon the consecutive cycles, the accumulation of longitudinal residual strain (along tensile direction) mainly occurs in the necked region, whereas the transverse reduction remains limited. The anisotropic deformation and failure behaviors of the necked region is systematically investigated using combined tensile tests and synchrotron small angle X-ray scattering. Along the longitudinal direction, the deformation of the necked sample is mainly ascribed to the inter-fibrillar region, where the chains can be deformed more easily than the ones located in the intra-fibrillar region. Along the transversal direction, a clear reorientation of the microfibrils can be observed and interpreted as the rotation of crystal blocks. Due to the similar microstructural parameters of lamellar stacks in the two directions, the anisotropic failure behaviors of the pre-loaded sample may be induced by the different density of chain between the intra-and inter-fibrillar regions or the easier growth of the oriented cavities due to the transversal stretching.