Evaluation of Orthogonal Cutting as a Method for Determining Mechanical Properties of Polymers

Abstract

The purpose of this thesis is to study and evaluate orthogonal cutting as a novel testing method for simultaneously determining the yield stress and fracture toughness of polymeric materials. The work is of particular importance to the fracture toughness characterisation of the tough and ductile thermoplastic polymers as the toughness of such materials is difficult to measure via conventional fracture mechanics testing techniques. In addition, the new cutting theory is applied to the study of machining characteristics of the thermosetting epoxies and epoxy composites. The main focus is to understand the mechanism of brittle- to- ductile transition in the cutting-induced material separation process. An energy based analysis of the orthogonal cutting process was adopted for the determination of the yield stress and fracture toughness, i.e. σY and Gc of a polymeric workpiece material. It is found that varying tool rake angle from 0° to 30° does not exert significant influence on the determination of Gc. However, Gc shows a decreased tendency with decreasing depth of cut as the crack-tip plastic zone ahead of the tool cutting edge is constrained by the limited depths of cut. It is also observed that simultaneous bending and shearing take place on the shear plane during the orthogonal cutting chip formation process. The inclusion of the bending energy in the cutting analysis can correct the previously derived high values of yield stress. In addition, the brittle- to- ductile chip formation behaviour in machining of the brittle epoxies is clarified. The transition is ascribed to ductile-regime cutting as the depth of cut is reduced to be comparable with the size of plane strain crack-tip plastic zone. Furthermore, not only the depth of cut, but also the ratio of Gc/σY of a material controls the characteristics of the cutting-induced material separation process.