Theory of critical distance combined with the generalized strain energy density criterion for mixed mode fracture assessment of PMMA dental materials

摘要

Polymethylmethacrylate (PMMA) is one of the well-known dental materials generally used for restoration, extension and relining of dental prostheses. Being subjected to periodic loads, while sustaining inherent voids and microcracks, dental materials are prone to the nucleation and extension of cracks under mixed mode loading condition. In this paper, the mixed mode fracture behavior of PMMA dental materials is studied both experimentally and theoretically. The experiments are carried out using a modified single edge notched bend (SENB) specimen. To theoretically assess the onset of fracture, the theory of critical distance (TCD) along with the generalized strain energy density (GSED) criterion is utilized. This fracture model includes the effect of the second stress term in the William's series expansion, known as the T-stress, in addition to the stress intensity factors (SIFs) to estimate the onset of fracture. Based on the theoretical model, crack propagation commences once the strain energy W reaches its critical value W_c over a critical distance r_c, with W_c and r_c both being material parameters. This article also suggests the implementation of a new critical distance model, derived from the GSED criterion, which ameliorates the theoretical estimates when compared to the traditional models for the critical distance devised specifically for stress-based fracture models. It is eventually demonstrated that the GSED criterion provides superior fracture predictions for the tested PMMA dental material in comparison with the classical SED criterion, mainly in view of the contribution of the T-stress and the critical distance concept.