Toughness of polymers under various fracture conditions

Fracture toughness and deformation behaviour of ductile polymers were investigated under various conditions and fracture modes. New methodologies to encourage the fracture modes that are hard to be generated in the past have been developed.

The thesis proposes test methodology to evaluate the fracture toughness of highly ductile polymers such as high-density polyethylene (HDPE). The first method is based on essential work of fracture (EWF) concept to measure toughness in plane-strain condition, which is about one order of magnitude smaller than the plane-stress counterpart. A new work-partitioning principle was developed to generate thickness-independent EWF values.

The thesis also discusses deformation and fracture of polymers involving stable necking. The study shows that crack growth of double-edge-notched tensile (DENT) test on HDPE can be divided into 2 stages. The EWF values for each stage were determined. The study concludes that the EWF value for stable necking varies with the deformation behaviour.

Another new method developed is to evaluate the toughness of polymers in shear fracture. The method was firstly applied to poly(acrylonitrile-butadiene-styrene) (ABS). The measured shear fracture toughness was then compared with that in the tensile mode. The results suggest that the ratio of shear to tensile fracture toughness is about 2.5. Validity of the new shear test was further evaluated using HDPE. For HDPE, shear fracture toughness could be determined by double extrapolation of specific work of fracture to zero ligament length and zero ligament thickness.

The last part of the thesis explores the yielding behaviour of HDPE using FEM. The study shows that the traditional way to determine the yield stress is not appropriate for the stable necking. Instead, an iterative process is proposed to determine the effective yield stress, based on which the loading level and the deformation behaviour can be simulated accurately. The simulation also considered anisotropic yielding in the stable necking process, which was verified by the simulation of DENT test. The study showed that anisotropic work-hardening occurred in the necking process. An empirical parameter, shear stress ratio, was implemented in anisotropic yield function, which successfully reproduced the load-displacement curves of DENT test in the FEM simulation.