A 17 µm diameter UHMWPE fiber consists of over 100,000 fibrils with diameters ranging from 10 to 100 nm. These fibrils can exhibit various relative rotations around the axial direction, forming interphases between distinct crystal planes. Fiber failure often occurs through defibrillation, determined by the adhesion between fibrils. In this study, we quantify adhesion via cohesive traction laws that describe the strength, progressive damage, and energy absorption during fibril separation. Using molecular dynamics (MD) simulations, we predict mixed-mode cohesive traction laws for PE interphases between crystals of various orientations and compare these results to the stress-displacement response of perfect bulk crystals of similar thickness.
Surface effects are primarily observed in the outermost layer of PE chains, where molecular structure deviates from the bulk crystal, resulting in higher surface energy. This defines an interphase thickness approximately equal to two PE chain layers (~1 nm). The disrupted crystal structure at these interfaces leads to a 32% reduction in Mode I peak traction and a 46% reduction in Mode I energy absorption compared to the perfect bulk crystal, with corresponding reductions in Mode II peak traction and energy absorption of 75% and 77%, respectively. Additionally, our results show that strain rate does not affect the traction laws within the range of 108 s-1 to 1010 s-1. We also derive characteristic parameters for a cohesive zone model by fitting its empirical parameters to the MD traction-separation responses.
Our approach uniquely uses MD simulations to predict interphase thickness and generate the traction-separation law directly from MD data, avoiding assumptions of an empirical form. The interphase traction laws predicted in this study provide interface properties that can be utilized to bridge length scales in multiscale simulations of defibrillation.
