Bearing Characteristics And Post-buckling Progressive Failure Behavior Of Composite Laminate With Cutout Under Shear Load

The involvement of cutout in aero structures is normally for satisfying manufacturing concerns and functional requirements,such as tube channel passage,repairing operation,etc.With the extensive application of Fiber Reinforce Polymer(FRP)composites in key bearing components of aero structure,the effect of cutout on the bearing and failure behavior of FRP structures has become one of the major concerns.The presence of cutout will locally cut the fiber of FRP composites,which may lead to decrease in structural stiffness and strength.Meanwhile,the change of stress/strain distribution and the stress concentration within the structure due to the existence of cutout may also have a significant weakening effect on structural stability.According to the difference of size,cutout in aero structures can be classified into three categories,i.e.,small cutout,medium cutout and large cutout.The stress/strain distribution and failure characteristics of composite laminates with cutout are closely related to the size and shape of cutout.Currently,the analysis and design of structure with cutout is a great challenge in aero structure design.FRP composites are typical heterogeneous,orthotropic materials,whose failure mechanisms are always the emphasis and difficulty in analysis.The presence of cutout brings more complexity into the failure mechanisms of composite structures,which may include delamination,matrix cracking,fibre breakage and more commonly,multiple failure mechanisms.Therefore,experimental and numerical investigations on composite structures with cutout of different scales have important academic value and significance in engineering practice.In this work,systematic and in-depth study on the bearing characteristics and failure behaviors of Carbon Fibre Reinforced Polymer(CFRP)composite laminate with small,medium and large cutout under specific loading condition are carried out.Firstly,the test of CFRP laminate with small cutout subjected to quasi-static tension is carried out.Two groups of specimen with different stacking sequences are tested,including the stacking sequence mainly with ±45° layers and the stacking sequence manly with 0° layers.A 3D progressive damage model based on Continuum damage mechanics(CDM)is constructed,and the shear nonlinearity is taken into account in this model.The progressive failure process of specimens is simulated with the developed model.The performance of three popular kinds of failure criteria to determine the failure initiation of composites,which are Hashin,Puck and La RC04 criteria,is evaluated by comparing the predictions and test results.The investigation on the bearing characteristics and failure mechanisms of CFRP laminate with small cutout is a fundamental research preparing for the following work on the mechanical performance of composite laminate with medium and large cutout.Secondly,a laminated level CDM model applicable for modelling large scale thinwalled laminated composites are is developed.The coupling between fiber failure and matrix failure is taken into account during failure evolution.The effect of delamination on the laminate stiffness is accounted via the strain invariant failure theory.Using this model,the bearing characteristic and failure behavior of CFRP laminate with medium cutout under shearing load are analyzed via both experiment and simulation.The prebuckling and post-buckling of laminate with three typical cutout shapes,i.e.,circular,rounded rectangular and diamond cutout,are discussed.Parametric analysis of cutout size on the stress concentration and buckling load of structure are conducted.Moreover,the effectiveness of reinforcement by inserting sub-merged co-curing layers is evaluated by both numerical modeling and experimental testing.It is found that the damage model is capable of reproducing the post-buckling progressive damage with complicated coupling effects within thin-walled composite structure costing low computational resources,and good agreements are in general obtained between the numerical simulations and experimental results.Thirdly,an experimental and numerical study is presented on the post-buckling damage process of multidirectional CFRP composite laminated specimen.The simply supported specimen is with a central large-sized elliptical cutout and subjected to shearing load.To the best knowledge of author,the experimental research on the mechanical behavior of CFRP laminate with large-sized elliptical cutout is carried out for the first time.Finally,a mesh objective computational model for quasi-brittle crack modelling is proposed.Resorting to the implementation of a local crack tracking algorithm,it is desirable that the spurious mesh bias dependence of CDM based model can be effectively addressed.A combined technique of a real-time crack bandwidth estimation and a nonlocal averaging scheme is implemented in the model.The new estimation of the real-time crack bandwidth can be not only depending on the element size and pattern,but also the physical crack path within each consolidate cracked element.Thus the energy dissipation during crack propagation can be considered in a more accurate,physically based way.By a classic numerical example,the proposed CDM model,empowered by the combined technique,demonstrates significant improvements in the prediction of damage propagating path and load-displacement response of quasi-brittle materials.The proposed model has also been extended to model failure behavior of composite materials,whose feasibility has been demonstrated by modelling the laminate with cutout under tension.