On Tensile Failure Mechanism And Strength Of Unidirectional Fiber Reinforced Composite Materials

The tensile failure mechanism and strength of fiber reinforced composite materials were studied theoretically, numerically and experimentally in this thesis. The thesis has three parts:First, the extended mode of matrix microcracks and its influences on the failure mode of fiber reinforced composite materials was studied by extended finite element method. The accuracy of extended finite element method crack tip stress fieldwas investigated. A method was suggested in order to improve the accuracy of crack tip fields and convergence rate. A simplified model for composites and bimaterial systems has been presented. The stress field in the zone included between the crack tip and the interface and the stress intensity factors of crack tip have been obtained by using Extended Finite Element Method. According to the result, the influences of transversal stress concentration and longitudinal stress concentration due to the crack on different kinds of fibers have been investigated. Consequently, one of the most common causes of failure in composites - debonding phenomena was analyzed.Second, the relationship of the property of interface between fiber and matrix and the tensile behavior of composites was researched. Interface is one of the most important factors of composites synergistic effect. A FEM model has been built for modeling the tensile behavior of fiber reinforced composites. The constitutive response of interface was described by cohesive damage model. Taking the distribution of fiber strength into account, the process of local damage evolvement was modeled. Farther, the effect of interfacial strength on the macro tensile strength of fiber reinforced composites has been studied.Third, a two-scale 3D shear-lag model was proposed. The tensile strength of unidirectional hybrid composites was predicted. Analyses are based on Monte-Carlo simulation and taking the fiber/resin interfacial toughness into account. The proposed model was then used in simulation the tensile behavior and strength of hybrid unidirectional composite bars reinforced by fibers of glass,high strength glass and carbon. The model was validated by micro and macro result of experiment. Numerical simulations agree well with experiment results.