The Correlation Between The Mixed-Mode Oligo-Cyclic Loading Induced Mechanical And Microstructure Changes In Hdpe

Two grades of HDPE (one is for PE-100 pipeline application and one is for blow molding) were processed to vary their crystallinity and molecular topology in order to investigate and understand the microstructural origin of the seismic performance of HDPE materials. For a given grade, the quenched sample exhibits a lower crystallinity, thinner crystallites, and a higher density of entanglement/tie molecules compared to the isothermal one. The HDPE samples are submitted to mixed-mode, oligo (plastic)-cyclic tensile deformation (that simulates the seism) with the applied maximum strain below or at the onset of necking propagation. The changes of numerous macroscopic mechanical metrics with increasing cycles were monitored (e.g. stress at maximum strain, residual strain at zero stress, secant modulus, and hysteresis loop area). These mechanical changes not only depend on the maximum imposed cyclic strain, but also the microstructure of HDPE. A physical scenario is proposed to describe the microstructural changes during the cyclic deformation and explicate the macro-micro correlations. Moreover, the post-cyclic tensile (reloading) curve can return to the original one rapidly for each sample, this phenomenon indicates the neglecting effect of the following cycles on the further loading path. It is also found that the material designed for the PE-100 pipeline shows limited variation between the initial and post-cyclic tensile toughness, indicating that the pre-loaded materials require nearly the same energy to rupture as the pristine ones. This is evidence that the material for PE-100 pipeline has a good resistance against the damage induced by seismic events.