Essential work of fracture and the phase transformation in beta-iPP

The possibility of phase transformation toughening is demonstrated by the example of the beta-modification isotactic polypropylene (beta-iPP), which undergoes the beta-alpha transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The beta-alpha recrystallization was examined on essential work of fracture (EWF) specimens with two processing conditions. Differential scanning calorimetry and wide-angle x-ray diffraction demonstrate the occurrence of this beta-alpha transformation. Some structural changes can be identified in EWF specimens by using nano-indentation machine. Toughness of the alpha- and beta-iPP is studied and compared with the EWF concept by using static-loaded deeply double-edge-notched tensile specimens. The main effect of the beta-alpha transformation is a large increase in the specific plastic work consumed in the necked zone.