Investigation On Fatigue Damage Evolution And Fatigue Failure Model Of 3D Woven Composites

Compared with laminate composites,3D woven composites have stronger out-of-plane properties and stronger integrity,and have broad application prospects.However,due to the complex weave structure of 3D woven composites,the damage mechanism is very complex,which brings difficulties to the design and application of 3D woven composites.Through literature review,it is found that the fatigue damage mechanism of 3D woven composites is not clear,and the fatigue simulation still need to be further studied.Fatigue failure is an important issue that cannot be ignored in engineering applications.Therefore,the problems existing in the fatigue research of 3D woven composites will hinder the application and development of this material.In this thesis,the fatigue failure process,damage mechanism and fatigue simulation method under different load conditions are studied.The main works are as follows:A fatigue damage research method combining synchrotron radiation computed tomography(SRCT)and multiple fatigue damage information is proposed.Using this method,the fatigue damage mechanism of 3D woven composites under tension-tension fatigue load is studied.According to the stiffness degradation and temperature rise,the tension-tension fatigue damage process can be divided into three stages.Based on SRCT results,the typical failure modes in the above three stages are obtained.The main factors affecting the fatigue performance are disclosed.The fatigue damage research method based on multi-information is applied to the study of tension-compression(T-C)fatigue damage evolution of 3D woven composites.To prevent buckling under compression load,and considering the need of fatigue damage detection,a new anti-buckling device is designed.It is found that the damage process of 3D woven composites under T-C fatigue load can be divided into two stages.The damage in the first stage develops slowly,and the duration of this stage exceeds 95% of the fatigue life.The damage mode at the inflection point of the two stages is successfully observed.The compression fracture of the warp yarn causes the stiffness decreasing rapidly.The damage tolerance design requires that the structure subjected to lowspeed impact still have a certain bearing capacity and fatigue life.Therefore,the fatigue behavior of 3D woven composites after impact is studied.The influence of low-speed impact on the static properties and fatigue life of 3D woven composites is analyzed.The fatigue damage evolution after impact is analyzed using CT technology.The mechanism of low-speed impact affecting fatigue performance is found.Low-speed impact reduces the integrity of the material,thus causing fiber fracture in sequence and reducing the quasi-static and fatigue performance of 3D woven composites.In the above research,it is found that the interface property is an important factor affecting the fatigue performance of 3D woven composites.In this paper,an interface fatigue model is developed based on the trapezoidal cohesive constitutive model.The relationship between the interface crack growth rate and the interface fatigue damage rate in the model is established.The static damage caused by stress redistribution during fatigue is considered in the model.The model is verified by single element and experiments.This model is successfully applied in the fatigue simulation of 3D woven composites.A fatigue progressive damage model of 3D woven composites is proposed.A representative volume cell with high degree of reduction is built.In the fatigue damage model,the Puck criterion is used as the damage initiation criterion of fiber yarns,the paraboloid yield criterion is used to introduce the plasticity of the matrix,and the exponential damage evolution model is used for both fiber yarns and matrix.The interface fatigue damage model proposed above is used to simulate the delamination in 3D woven composites.The input parameters of the model are determined by the static and fatigue tests of component materials.The simulation results show that the model can accurately predict the quasi-static properties,fatigue life,and damage evolution of 3D woven composites.