Puncture deformation and fracture mechanism of oriented polymers

In this study, we investigated the effect of orientation by solid-state cross-rolling on the morphology, puncture deformation, and fracture mechanism of an amorphous TROGAMID material and three semicrystalline polymers: high-density polyethylene (HDPE), polypropylene (PP), and nylon 6/6. In amorphous TROGAMID, it was found that orientation preferentially aligned polymer chains along the rolling deformation direction and reduced the plastic deformation of TROGAMID in a low-temperature puncture test. The decrease of ductility with orientation changed the fracture mechanism of TROGAMID from ductile hole enlargement failure in the unoriented control to a more brittle delamination failure in TROGAMID cross-rolled to a 75% thickness reduction. For semicrystalline polymers HDPE, PP, and nylon 6/6, the randomly oriented crystalline lamellae in the controls were first oriented into an oblique angle to the rolling direction (RD) before the lamellae became fragmented and preferentially oriented with the chain axis parallel to the RD. The morphological change resulted in the decrease of ductility in HDPE in the low-temperature puncture test. In PP and nylon 6/6, the brittle fracture of unoriented controls was changed into ductile failure when they were cross-rolled to a 50% thickness reduction. This was attributed to the tilted crystal lamellae morphology, which permitted chain slip deformation of crystals with the chain axis parallel to the maximum shear stress direction. With further orientation of PP and nylon 6/6 to a 75% thickness reduction, the failure mechanism changed back to brittle fracture as the morphology transformed into a layered discoid structure with the chain axis of the fragmented crystal blocks parallel to the RD; this prevented chain slip deformation of the crystals.