Damage Mechanics Model Of Composite Laminates Under Low Speed Impact Loading Based On Equivalent Displacement Method

Advanced composite materials are widely used in aerospace,aviation,automotive and many other fields because of their excellent mechanical properties such as high strength,high rigidity and outstanding corrosion resistance.However,the impact resistance of composite materials still needs to be further improved,because low-energy impacts such as bumps often cause complex damages such as fiber breakage and matrix cracking.These damages are difficult to be found without the help of inspection tools,which significantly reduces the overall strength of composite structures.In recent years,scholars started to pay attention to the damage behavior of composite materials under low speed impact.For the damage behavior of composite materials,scholars have proposed many damage mechanics models,such as material performance degradation models,in which damage variables are defined based on the failure strain under the unidirectional stress state.The material failure in the correlation failure criterion is a result of the multi-directional stress components.In this thesis,an equivalent displacement method is used to improve the quality of damage variables based on traditional material performance degradation models.A damage mechanical model of composite laminates under low-speed impact load based on the equivalent displacement method is proposed.The results obtained by the numerical simulation by these new damage models are compared with the experimental results to validate the accuracy of the model.The details are as follows:1.To simulate the damage behavior of laminates: this thesis builds a damage mechanics model of composite laminates based on the theory of continuous damage mechanics.First,based on the orthogonal anisotropic linear elastic constitutive relationship,this paper introduces the shear nonlinear behavior in the Soutis layer to establish a nonlinear constitutive relationship;Secondly,Hashin criterion is used as the failure criterion in the laminate material.The material failure modes can be divided into fiber tensile fracture,compression wrinkle and matrix fracture(tensile and compression)at meso-scale,.Finally,the equivalent displacement method is used to improve the traditional damage variable definition method to obtain a better material evolution model based on the energy evolution.2 To simulate the delamination behavior of laminates,this thesis builds a cohesive model based on the delamination mechanical model of composite laminates.First,the interface unit is introduced between each single-layer board to simulate the interface layer;Secondly,the secondary stress criterion is selected at the initial stage of the laminate layer delamination;Finally,the BK criterion under the mixed mode is used to simulate the interlayer crack growth of the laminated board behavior.3.The finite element model is developed by the finite element method.First,user material subprograms is developed by using Python language,including material stress calculation,failure judgment and degradation of stiffness matrix;Second,the finite element model is associated with the user material subprogram through the user interface and perform finite element calculation;Finally,the finite element results and experimental results are compared to validate the accuracy of the damage mechanics model proposed in this paper.The damage mechanical model of composite laminates under low-speed impact load based on equivalent displacement method proposed in this dissertation can improve the prediction accuracy of composite load-bearing performance analysis,and has a significant role in the future analysis of composite material damage behavior.