Research On Analysis Model For Fatigue Life Of The Stitched Composite Laminates

Stitched composite material is enhanced its impact resistance, inter-laminar tensile and shear strength, inter-laminar fracture toughness in the thickness direction by using stitching on the basis of traditional composite materials. As a new material, its fatigue performance is a very important assessment indicator. However, researches on the mechanical properties, such as fatigue life, are limited in our country. Therefore, it is necessary to test on the fatigue properties of the stitched composites, and analyze its fatigue damage mode and establish the fatigue life prediction model to provide performance parameters and theoretical guidance in practical engineering applications. The article adopts theoretical of fatigue damage accumulation and experimental verification approach to research on the fatigue characteristics of stitched composites. The main work is as follows:1. Experimental research on the mechanical properties of stitched composite laminates is carried out. The static strength tensile and tension-tension fatigue behavior of two different overlay(4s[0/90] and2s[0/45/90/-45]) in the three kinds of stitching angle of the T700/QY8911 stitched and unstitched composites are tested. It is also tested the variation of stiffness degradation of the composite laminates at different stress levels. The experimental research provides a data support for the theoretical predictions on fatigue characteristics of stitched composites.2. The fatigue behavior and fatigue life of stitched composite laminates are taken. The effects of stress level, layer mode and stitching angle on the tension-tension fatigue performance of stitched composite laminates under constant amplitude tension-tension load is analyzed. Combined with the test data, two different layers4s[0/90] and2s[0/45/90/-45] were stitched under stitching angle 10 °, 45 ° and 90 °, which give the fatigue life of the S-N curves and stiffness degradation curves. The results show that stitching has important impacts on the tensile fatigue damage and propagation of composite laminates. Different stress level, layer mode and stitching angle have effect on deterioration laws of fatigue stiffness and critical fatigue failure of stitched composite laminates.3. The model for estimating fatigue damage and tension-tension fatigue life of stitched composite laminates has been studied. Based on fatigue cumulative damage theory, a stiffness degradation model of two-stage function and a two parameters residual stiffness model of stitched composite laminates are developed and analyzed. The tension-tension fatigue life of stitched composite laminates can be predicted by using the model and do an experimental verification. The results show that damage analysis model is effective to predict fatigue damage occurs, expansion and fatigue life under fatigue load.4. The damage expansion rate analysis model and the mechanism of stitched composite laminates are discussed. According to the damage expansion modes and fracture location of the material under fatigue loading conditions, the mechanism of tensile fatigue damage and propagation of stitched composite laminates on the differences between stress level and stitching angle was analyzed.