Research On Fatigue Performance Of Twill Braided Composite Laminates

Composite materials are widely used in aerospace,marine,automotive and other engineering fields because of their high strength,high stiffness,long fatigue life and adaptability to the expected function of the structure.Due to the composite structure is inevitably subjected to cyclic load during service,its durability is a key design consideration in many structures,and the research on fatigue load of composite material has always been a concern of all kinds of engineers.This paper takes the twill braided composite laminates as the research object,based on the fatigue damage theory of unidirectional composite laminates,the fatigue progressive damage analysis method for twill braided composites was extended.The fatigue damage propagation and fatigue life simulation and experimental verification of typical mixed laminated twill braided laminates were carried out.Firstly,based on the fatigue damage theory of unidirectional laminates,considering that the layer of twill braided composites has the characteristics of bi-directional reinforcement and orthotropy,the fatigue failure criterions applied for unidirectional laminates are improved and the fatigue failure criterions suitable for twill braided composites are developed,the sudden drop rule of material properties corresponding to the fatigue failure mode is proposed.Combined with the normalized life model,the strength and stiffness degradation models of twill braided composites under arbitrary stress ratio are derived,and the fatigue progressive damage analysis method is established.Secondly,the basic parameters of the progressive fatigue damage model were obtained by testing the twill braided unidirectional laminates.Quasi-static and tension-tension fatigue tests were carried out on twill braided unidirectional laminates,and the S-N curve and the normalized fatigue life model curve were determined.For the residual strength and residual stiffness tests,the least squares method was used to obtain the expressions of the residual strength and residual stiffness of the laminate under tensile-tensile fatigue load.By comparing the failure morphology of the static test and fatigue test specimens,it can be found that the fatigue failure area is wider,the delamination phenomenon is more serious,and the damage accumulation process is analyzed.Lastly,fatigue test was carried out on the twill laminate with the C layup to obtain the fatigue life under different load levels.The unidirectional laminates test data are used as the simulation input of UMAT subroutine,and the finite element software ABAQUS is used to simulate the fatigue life of the laminates.The errors between the simulation results and the test results are within an acceptable range,which shows that this method can reliably and effectively predict the fatigue life of arbitrary ply twill braided laminates.