Enhancing prediction accuracy in SIFT theory

The SIFT failure theory separably predicts fibre and matrix failure in a fibre reinforced composite material. Matrix yield prediction is based on a Von-Mises strain and first strain invariant criteria. Improvement of the matrix failure criteria for prediction accuracy is discussed and demonstrated in this paper. For two typical resin materials, Von-Mises yield criterion is unable to fit both yield strength values from uni-axial compression and pure-shear tests. When calibrated using the measured shear yield strength, the discrepancy between the predicted and measured uni-axial compression strength is found to be 11% and 60% respectively, whilst using a Drucker-Prager criterion these discrepancies could be removed. Use of the critical value of the first invariant strain, when calibrated using the uni-axial tensile yield strength, under-predicts the biaxial tensile strength significantly by over 30% on average, compared with available measurement data of neat resin materials. A revised criterion proposed in this paper could reduce the discrepancy to less than 10%. The proposed failure envelope contains Drucker-Prager failure criteria, that intersect with all the uni-axial compression, pure shear and uni-axial tensile test data points, and a revised tensile failure criterion. The areas governed by these criteria join each other smoothly. For a unit cell with a fibre and surrounding matrix from a lamina with 50% fibre volume fraction and typical material properties, a FEM analysis conducted in this study indicates that the difference between the yield strength of the matrix material predicted using Von-Mises and Drucker-Prager type criteria is over 60% in the pure shear load case, when the critical values of these yield criteria are determined in the uni-axial compression load case. The FEM analysis showed that the difference between the yield strength of the matrix material in a unit cell predicted using the first strain variant and the revised criterion reaches 6.1% in the biaxial tensile load case, when the critical values of these yield criteria are determined in the uniaxial tensile load case. By using ultimate strengths and secant stiffness to replace yield strengths and initial linear stiffness, the analysis in this paper may be extended to prediction of matrix ultimate failure using linear analyses.