| Owing to high stiffness/weight ratio and corrosion resistance,fiber-reinforced composite has been widely used in many engineering sectors.In the manufacture and service phases,low-velocity impacts usually caused by weight dropping threaten the safety of composite parts because complex impact damage behavior,including matrix cracking,fiber breakage and delamination would dramatically reduce residual performance.As one of the important factors affecting the impact resistance of composite materials,fabric structure is the entry point for many scholars to study how to improve the impact resistance and impact response of composites.Braided composite laminates allow an improvement of low-velocity impact response through varying the intra/inter-laminar structure.Therefore,it is vital to study the influence of plies with different braiding angles on the low-velocity impact response and failure mode of braided composite laminates.This paper takes carbon fiber braided composite laminate as the main research body.The influence law of ply configuration on the impact resistance and damage mechanism of braided composite laminates were explored through the combination of experiment and finite element simulation.Firstly,three kinds of braided composite laminates with different stacking sequence were prepared(uniform structure:IS-45[45°6];non-uniform structure:IS-30[30°2/45°2/60°2],IS-60[60°2/45°2/30°2]).Secondly,low-velocity impact tests were performed at three different energy levels on the drop-weight impact testbench,and quasi-static indentation tests with different displacements were carried out correspondingly.Thirdly,a meso-scale finite element simulation model of a braided laminate with uniform structure was established to simulate the process of low-velocity impact.Then,combining the mechanical response curve and the post impact surface damage morphology,the impact response and surface damage mode of different specimens were compared and analyzed.Combined with the results of the quasi-static indentation test and the CT inspection,the influence of braided plies with different braiding angles on the impact response and damage mechanism of the braided laminate under various stages of low-velocity impact is discussed and analyzed.Finally,the finite element simulation model was employed to analyze the formation mechanism of edge damage which is common to all specimens in this study.The specific content of this study will be expanded from the following chapters:Chapter one mainly elaborates the related research on low-velocity impact of fabric-reinforced composite laminates.First,the typical mechanical response and damage mode of fabric-reinforced composite laminates under low-velocity impact were described.The optimization effect and mechanism of different fabric reinforcement structures on the impact resistance of composite materials are analyzed in detail through the reading of domestic and foreign literature,and then the research content and direction of this paper were determined.In chapter two,the principle of circular braiding and the structural design of braided composite laminates were first introduced,and the vacuum assisted resin transfer molding method for specimen preparation was described in detail.Three types of braided composite laminates are obtained.The structure of impregnated yarns and resin-rich region in the different braided plies were analyzed in connection with the measured parameters such as thickness and fiber volume fraction.Secondly,the low-velocity impact test bench and the test data processing method were introduced,and the whole impact process was recorded by a high-speed camera.According to the low-velocity impact tests,the quasi-static indentation tests with different displacements were performed to explore the damage development in various stages.Finally,the internal damage morphologies of post-impact specimens were obtained by non-destructive CT inspection.In chapter three,the establishment of finite element simulation model for low velocity impact of braided composite laminates were introduced.First,the meso-scale geometric model of the braided laminate was established by voxelization according to the architecture of real specimen.The ductile and shear criterion was used to model the damage initiation and damage evolution of resin matrix.3D Hashin criteria were employed for the damage initiation and failure evolution of impregnated yarn.In chapter four,the impact resistance of braided composite laminates was analyzed by the low-velocity impact responses and surface damage morphology of specimens.According to the mechanical response curve,the impact response of three kinds of braided composite laminates with different ply configuration were compared.It was found that the non-uniform braided composite laminates own a better impact response.In addition,whether it is located on the front or back surface,the influence of plies with different braiding angles on the impact response and damage mechanism of specimens was analyzed.In the fifth chapter,the influence law of different stacking sequence of braided plies on the failure mode and damage mechanism of braided composite laminates were analyzed and discussed.Combined with the results of quasi-static indentation tests and the CT images of the post-impact specimens,the effect of impacted side and non-impacted side in the non-uniform structures were analyzed,the internal damage mode of the uniform/non-uniform braided laminates was further compared and analyzed.In addition,the formation of specimen edge damage is the reason for the load drop in the mechanical response of all braided composite laminates.Therefore,the simulation and experimental results were compared and verified in this paper.The deformation of the specimen,the stress distribution and damage development of the matrix and impregnated yarn elements in the impact process were observed,and the edge damage mechanism leading to the load drop of the specimen was explored.The sixth chapter summarizes the research work of this paper and prospects the future research. |
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