Application of photoelasticity to determine mode one and mode two orthotropic stress intensity factors

摘要

With the advancement of technology and application of orthotropic materials in modern industry, the development of an experimental technique to analyze and determine the stress intensity factors in orthotropic materials was needed. To address this necessity, a new experimental method of combining orthotropic photoelasticity and orthotropic linear elastic fracture mechanics laws are developed. A new set of equations are obtained by combining orthotropic photoelasticity laws and orthotropic fracture mechanics formulas. These equations along with half-fringe photoelasticity techniques are used to determine the stress intensity factors. To model orthotropic materials, unidirectional fiberglas/epoxy composites, made at IITRI are used. Compact tension specimens and a strip with a slanted edge crack are employed to determine mode one and mode two stress intensity factors. The optical and mechanical properties of the materials are determined by the use of tension specimens. A microcomputer is employed to collect and analyze the experimental data, and a finite element solution is used to verify the experimental results. Solid SAP, a finite element program capable of analyzing orthotropic materials is utilized for the finite element solution. The experimental results show that photoelasticity can be used effectively in the determination of mode one stress intensity factors. However, problems arise from the low sensitivity and low toughness in the case where the cracks are parallel with the fibers in unidirectional composites. Further studies at higher loads, which are difficult because of the low toughness and crack tip plasticity effects, need to be undertaken. In the case of mixed mode stresses, photoelasticity is not as effective in determining stress intensity factors, as in the pure mode one case.