Research On In-Plane Mechanical Properties Of Stitched Composite Laminates Under Complex Loads

Because of the improvement of interlaminar toughness and superior delamination resistance, the stitched laminates, as a new type of textile composite, have great potential applications in the aeronautics and astronautics industries especially for the primary loading-bearing structures. To fully take advantage of the mechanical properties, it is necessary to investigate the macro and micro mechanical properties and its correlative influential factors in-depth, study the damage evolution and failure mechanisms in detail and build a macro failure criterion, which would lay a solid foundation for optimization of the structure design. Therefore, to predict the mechanical performance of three-dimensional stitched composites has been focused by many international researchers in the domain of composite mechanics.Based on the meso-scale structure of stitched laminates and experimental conclusion of some researchers', a representative volume element (RVE) with the consideration of the distortion law of fiber and the volume content is presented. Then, the RVE-based micromechanical damage model is established by commercial software ABAQUS. In this model, Hashin criteria and maximum stress criteria are selected to indentify the initial damage occurrence of the lamina and matrix, respectively. The effects of damage judged by the proposed failure criteria on the mechanical properties are estimated by sudden stiffness degradation model. Tensile, compressive and shear strength of the RVE are studied by using the nonlinear finite element method coupled with periodical boundary conditions. A shear non-linear constitutive model of stitched lamina is put forward. The influence of the maximum distortion angle of fiber and the radius of stitch to the strength of different RVE are discussed.Based on the classical laminate theory (CLT) and the hypothesis of equal strain in different layers, the strength prediction model of stitched laminates are presented combined with the user-defined materials subroutines (UMAT) of the ABAQUS software. In this model, shear non-linearity is considered in each layer. Tsai-Wu criteria are used to judge the damage occurring and sudden damage manner and progressive damage manner are adopted to study the post-damage process of the laminates. The strengths of stitched laminates under complex loads are obtained. Effects of the nonlinear shear relationship in each layer and damage accumulation on the macro-mechanical behavior are discussed in detail.The failure envelopes of stitched laminates under various complicated loads are obtained by fitting the final strength values in least square method. Compared to the unstitched laminates'failure envelopes, the disparities of strength are expressed vividly. Finally, macro and phenomenological failure criterion about the stitched laminates are established.