Influence Of Damage On Mechanical Properties Of Composite Grid Reinforced Structure

Since composite grid reinforced structure has the advantages of lightweight,high specific strength and high specific stiffness,it is more and more introduced into the application of aerospace structures.The manufacturing process of composite grid reinforced structure is a process of material curing and component forming at the same time,which is prone to manufacturing defects such as porosity and delamination.Drilling is a common processing technology of carbon fiber reinforced composites.However,the influence of machining damage on the mechanical properties of composite grid reinforced structures has not been fully understood.The composite grid reinforced structure is impacted by foreign matters in the process of use,which may lead to serious damage such as fiber fracture,matrix crack or delamination in the composite structure.For the composite grid reinforced structure in the humid and hot storage environment,water enters the structure and this causes mechanical damage as time goes on,and the gradually accumulated microcracks will also destroy the integrity of the composite interface.Worldwide,existing research on damage models focuses on laminated plates and skin truss structures.A few damage models of grid reinforced structures only analyze the skin with defects,which has certain limitations.With the increasing application proportion of composite grid reinforced structures in the important aerospace structures,the impact of damage caused during the processing and enlistment on structural mechanical properties has attracted more and more attention.Aiming at the technical problems such as uneven pressure and many internal defects in the traditional hard mold forming process of composite grid reinforced structure,two different process methods,one-time mold turnover process and two-time mold turnover process,were proposed in the thesis based on the characteristics of sufficient thermal expansion and easy demoulding of silicone rubber soft mold forming process.And the grid plate specimens were prepared accordingly.Furthermore,the forming molds of grid reinforced cylinder and other special-shaped structures can be assembled,which provides a basis for the further research work of the thesis.For the interlaminar damage model of grid reinforced structure,the generation and propagation of damaged interface layer are traditionally analyzed based on the method of fracture mechanics,and the defects in skin are mainly focused on.In the thesis,exponential softening cohesion theory model is introduced to simulate the interlaminar damage interface characteristics,therefore,the interlayer damage model is extended to the interface between the skin and the stiffener,and the stiffener interlayer structure.The exponential softening cohesion model can accurately simulate the interlaminar damage of grid reinforced structures by the comparing the numerical simulation results and experimental results.Aiming at various layered problems of grid reinforced structure in the production and processing process,different defect types in the actual drilling process and forming process are simulated by presetting layered defects in the preparation process of test pieces.The simulation and prediction model of the interlayer damage of the composite grid reinforced structure is established based on the theoretical model.the comparison between the test results and the simulation analysis results shows that the simulation prediction model can accurately predict the compressive residual bearing capacity of different damaged structures,which verifies the effectiveness of the simulation prediction model.On this basis,the variation curves of the compressive residual strength of grid reinforced structure and skin structure with the defect size of drilling delamination defects and manufacturing defects are obtained.The comparative analysis shows that the grid stiffener improves the ability of laminates to resist delamination defects.In order to verify the impact damage resistance of the grid reinforced structure formed by the new epoxy resin,the impact damage analysis model is established to predict the damage in the impact process based on the simulation and prediction model of the composite grid reinforced structure with forming layered defects and the above interlayer damage analysis model.The results of impact test and axial compression test at different impact positions verifies the accuracy of the simulation prediction model.The variation curves of compressive residual bearing capacity of grid reinforced structure and skin structure with impact energy are obtained by the method of numerical simulation analysis.The comparative analysis shows that grid stiffener improves the impact resistance of laminates.In addition,aiming at the aging damage of grid reinforced structures,the positive correlation between load bearing capacity and moisture absorption of specimens is introduced,and the wet diffusion process of water is simulated through the heat conduction module to establish the simulation and prediction model of aging damage of composite grid reinforced structures.The results of aging test and axial compression test of grid reinforced structure verifies the effectiveness of the simulation prediction model.According to the simulation results,the curves of the compressive residual bearing capacity of the grid reinforced structure with the accelerated aging time are obtained.The comparative analysis shows that the grid reinforced structure does not improve the aging resistance of the skin structure.The thesis gives the damage tolerance of this kind of structures in service based on the research results of the effects of drilling delamination,forming delam ination,impact damage and aging damage on the residual bearing capacity of composite grid reinforced structures,which provides theoretical guidance and technical support for the technical standards for the detection,acceptance and repair of important layered defective structures in aerospace engineering.