Nonlinear analysis of woven fabric-reinforced graphite/PMR-15 composites under shear-dominated biaxial loads

An elastic-plastic, time-independent, macroscopic, homogenous model of an 8 HS woven graphite/PMR-15 composite material has been developed that predict s the nonlinear response of the material subjected to shear-dominated biaxi al loads. The model has been used to determine the response of woven compos ite off-axis and Iosipescu test specimens in nonlinear finite element analy ses using a multilinear averaging technique. The numerically calculated res ponse of the specimen was then compared to experimentally obtained data. It has been shown that the numerically calculated stress-strain diagrams of t he off-axis specimens are very close to the experimentally obtained curves. It has also been shown that the numerically determined shear stress-strain and load-displacement curves of the woven Iosipescu specimens are close to the experimentally obtained curves up to the point of significant interlam inar damage initiation and propagation. The results obtained in this study clearly demonstrate that the nonlinear material behavior of the graphite/po lyimide woven composites subjected to shear-dominated biaxial loading condi tions cannot be ignored and should be considered in any stress analysis. Th e linear-elastic approach grossly overestimates the loads and stresses at f ailure of these materials in the off-axis and Iosipescu tests. It can be as sumed that the same discrepancies will arise in the numerical analysis of t he woven composites tested under other biaxial shear-dominated loading cond itions using other biaxial test methods.