Failure Mechanism Based Strength Analysis Method For Laminated Composites

The application of composite materials in aerospace and other engineering structures is growing rapidly in recent years. The constitution complexity and damage diversification of composites challenge the conventional structural strength analysis methods since there exists strong and complicate interaction among different failure modes. Therefore, understanding the failure mechanisms of composite materials and establishing failure mechanism based strength analysis method are quite necessary for the design and maintenance of composite structures, and will contribute to reducing the structural testing cost, shortening the product development cycle and improving the structural service efficiency and safety. This dissertation dedicates to several major problems in the strength analysis of fiber-reinforced composite laminates, such as interfacial fracture, intralaminar strength theory and nonlinear constitutive relation.As the conventional modeling technique has the deficiency of low efficiency and high computational cost in analyzing the interfacial fracture of composite laminates, a new combined interface element which can be used with conventional shell elements is proposed to form three-dimensional shell delamination model for the laminates. The formulae of the proposed interface element take account of thickness offset, translational and rotational nodal movement of adjacent shell elements. The tangent stiffness matrix of the interface element is deduced and a bilinear mixed mode constitutive law is utilized to characterize the delamination initiation and propagation processes. The new interface element has been implemented in finite element code ABAQUS via its user defined element subroutine(UEL).The double cantilever beam(DCB) test is simulated using the proposed shell delamination model and parametric studies have been conducted. The tests of three-point end notched flexure(3ENF), mixed-mode bending(MMB) and adhesively bonded single lap joint are simulated numerically with the shell delamination models. The interfacial fracture of those test coupons are also calculated analytically with the author re-derived formulae. The simulation results indicate that the proposed shell delamination model can capture the single and mixed mode delamination initiation and propagation with good accuracy.The systematic intralaminar strength theory is established according to the failure physics of composite laminates. The in-situ effect in laminated composite is introduced and the in-situ strength calculation methods are derived according to the literature. With the experimental observation of failure mechanisms from literature, we propose a modified model for fiber compression prediction, in which the unique feature of fiber kinking model in LaRC series strength theory is retained but supplemented with a new strength criterion to characterize the initiation of shear-driven fiber compressive failure mode. As the matrix transverse compression and in-plane shear both show apparent nonlinear mechanical behavior, an equivalent strain method is proposed to consider the nonlinear coupling effect between them, and thus acquired the nonlinear constitutive relation. The theoretical predictions of failure envelopes and stress to strain curves are conducted for a number of unidirectional and multidirectional laminates under simple and complex loading states. Moreover, the influence of chosen parameters on the prediction results are investigated. The results indicate that the strength theory proposed in this dissertation behaves pretty well in most cases and can capture the actual test phenomena and tendency.A continuum damge mechnics(CDM) approach based effective stress degradation scheme is proposed to characterize the degree of damage under different failure modes, paying attention to the feature of continuum shell model. A nonlinear damage constitutive law is then obtained and implemented in user defined material subroutine VUMAT in ABAQUS explicit package. The mesh sensitivity is investigated by loading a cubic block with different mesh densities, and the off-axis compression, open hole compression, open hole tension and overheight compact tension tests are simulated to exhibit their failure processes. The results indicate that the structural response, failure patterns and final failure are predicted with satisfactory by the proposed strength analysis method.