Interlaminar fracture behavior of woven fabric composites and mode III delamination analysis

An experimental investigation was conducted using the double cantilever beam (DCB) test to evaluate the effects of fabric reinforcement and microfiber addition to the matrix on the interlaminar fracture behavior and other mechanical properties of woven fabric composites. A procedure for determination of the mode I fracture toughness {dollar}Gsb{lcub}rm IC{rcub}{dollar} for specimens undergoing large deflections on the DCB test was established, and a technique for interlaminar fracture toughness enhancement by matrix modification with microfiber additives was explored. Major mechanisms contributing to the fracture toughness and microfailure of the composite samples were identified by scanning electron microscopy (SEM) analysis. Results indicated that the addition of 1% to 3% different microfibers to the epoxy matrix increased the interlaminar fracture toughness of glass fabric composites by 75% to 108% and showed no significant effect on other mechanical properties.; A 3-D finite element analysis was performed on a (90,(+45/{dollar}{lcub}-{rcub}45)sb{lcub}n{rcub},(-45/{lcub}+{rcub}45)sb{lcub}n{rcub}90rbracksb{lcub}rm s{rcub}{dollar} class of laminated composites under the edge crack torsion (ECT) test configuration. Valid finite element delamination models were established and formulas for calculating the mode III fracture toughness from 3-D finite element models were developed. The relations between the interlaminar fracture behavior and various configuration parameters were investigated and the effects of point loads, specimen ends, geometry, mode II interference, and friction were evaluated. Results showed that the ECT specimen is mode III dominant, and the distribution of the mode III strain energy release rate {dollar}Gsb{lcub}rm III{rcub}{dollar} in the ECT specimen depends on crack length rather than layup. Specimen ends have significant effect on {dollar}Gsb{lcub}rm III{rcub}{dollar}. Friction between crackfaces induced by point loads is negligible. An ECT test with a new fixture was conducted on carbon/epoxy laminated composite samples to verify the analysis methods and improve the test method. A procedure for specimen layup/crack length selection for valid ECT mode III test was proposed.