Fracture criterion considering notch brittleness of polytetrafluoroethylene under quasi-static loading

Abstract

This paper introduces a simple and effective method for predicting the static strength of polytetrafluoroethylene (PTFE). Plane and notched specimens were subjected to tensile and bending tests to better understand the effect of notch brittleness. A finite element method (FEM) model, validated by comparing its analysis results with the bending test results, was used to consider the fracture criterion for PTFE as a material exhibiting notch brittleness. Swift's model can appropriately represent the true stress-strain curve up to a strain of 200%. The notch brittleness was qualitatively determined by comparing the force-displacement curves for various notch tip radii. The FEM analysis was conducted in order to acquire the stress triaxiality and equivalent plastic strain of specimens. The fracture criterion was obtained by fitting with an exponential function the relationship between stress triaxiality and equivalent plastic strain just before fracture calculated by the FEM analysis. The mesh sensitivity was also analyzed by re-meshing around notched and holed regions with a minimum element size on the order of 10−1, 10−2, and 10−3 mm. Finally, the fracture criterion was appropriately validated by using a holed round bar (HRB) specimen. The critical point of the HRB specimen was clustered around every estimated fracture portion while indicating less mesh sensitivity. The fracture criterion gave us the estimate to break near the edge or inner surface of the center hole of the HRB specimen, which agreed with the actual fracture behavior of HRB specimens. The proposed approach had a weaker mesh-dependence on the estimation of fracture limit with notch brittleness for PTFE.