Test MethodOn the measurement of true fracture strain of thermoplastics materials

In tensile testing, engineering thermoplastic materials can sustain large deformation after necking. This property can be advantageous to the manufacturing process and mechanical performance. However, it causes difficulties in strain measurement and in the determination of true stress and true strain, particularly at higher strain rates. This paper discussed the measurement of true fracture strain. The true fracture strain was estimated using three different methods: the area reduction, elongation over the necked zone, and elongation over the gage length. With strain localization, strain measured by an extensometer becomes gage length dependent. To address the issue, an algorithm was formulated that estimates the average fracture strain over an arbitrary gage length from the fracture strain measured over the necked zone. The proposed algorithm provides a way to estimate the mesh dependence of the fracture strain for finite element (FE) analysis. This paper also presents an analysis-based two-step correction scheme for the crosshead extension to be used to supplement the strain measurement from the extensometer.