Evaluation Of Typical Defects Of Carbon Fiber Reinforced Resin Matrix Composites By Autoclave Forming

Carbon fiber reinforced composites(CFRPC)are widely used in the production of aerospace structural parts because of their excellent mechanical properties and their ability to be used effectively in harsh working environments.Among the many molding processes for carbon fiber reinforced composites,autoclave forming is one of the most widely used and mature molding processes in the production of aerospace composites due to its advantages of uniform working pressure,wide range of temperature applicability and reliability.However,when composites are prepared by the process,manufacturing defects,including wrinkles and delamination,are often generated,and these defects can cause degradation of composite material properties to varying degrees.In the past,when manufacturing defects occurred during the production of composites,they were usually scrapped to avoid the risk from usage.However,as the application of composites in aircraft structures becomes more and more important,which requires the integrity of aerospace composites,it becomes increasingly unfeasible to avoid the risk by scrapping.Therefore,it is an urgent task to study the effect of manufacturing defects on the comprehensive performance of composites so as to provide the basis for the subsequent evaluation of defects.Based on the above research background,this task will build on the existing research by introducing artificial manufacturing defects to investigate the effects of out-of-plane wrinkles or delamination on the mechanical properties of CFRPC and using finite element simulations to predict the change patterns of out-of-plane wrinkles or delamination on the main mechanical properties of composites.(1)Preparation and characterization of test pieces containing artificial defects in composites:the epoxy resin strips of various sizes were manually pre-buried between different layers of prepreg to simulate fiber fold defects and succeeded in preparing specimens containing different dimension and position out-of-plane wrinkles;by manually pre-burring the PTFE films of various sizes were in the center of prepreg,the specimens containing delamination were prepared successfully,and the effects of delamination size on the mechanical properties of composites were investigated.Combined with the actual molding conditions of aerospace composites,the test parts of composites containing manufacturing defects were prepared using autoclave molding.The manual wrinkles and delamination in test parts were inspected using immersion ultrasonic scanning and the microscopic morphology method The detection results verified the successful preparation of the test pieces of composites containing defects.(2)The test pieces of composites containing artificial out-of-plane wrinkles were tested in tension,compression,short beam shear and flexural tests,and it was found that both the size and embedding position of the out-of-plane wrinkles led to the degradation of the mechanical properties of the composites,and the effect of the embedding position was greater than that of the size.The tensile,compressive,interlaminar shear and flexural strength of the laminate decreased up to 22%,65%,20% and 34%,respectively,with the increase of wrinkle size when the out-of-plane wrinkle was located in the middle of the layers;the tensile,compressive,interlaminar shear and flexural strength decreased up to 47%,77%,23% and 49%,respectively,with the increase of wrinkle size when the out-of-plane wrinkle was located in the second and third layers.(3)The same tests were conducted on the composite test pieces containing delamination,and it was found that the presence of delamination led to a decrease in the compressive,interlaminar shear and flexural strength,while it had little effect on the tensile strength.At the same time,the compressive and flexural strengths of the laminates decreased gradually with the increase of the delamination size,and when the delamination size reached the maximum,the compressive and flexural strengths of the laminates decreased by 24% and 15%,respectively.In addition,although the presence of delamination causes a significant decrease in the interlaminar shear strength of the laminate,there is no obvious pattern with the increase of the delamination size,and when the delamination is located in the center of the test piece,the interlaminar shear strength decreases by about 38% at most compared with that of the defect-free pieces.(4)According to the mechanical property test results,the compression properties of the composite laminate with the greatest influence on the out-of-plane wrinkle and delamination were simulated and predicted by using finite element simulation.The VUMAT subroutine required for the Abaqus explicit analysis was written and the finite element analysis model was established.The results obtained from the computational simulations are consistent with the mechanical property test results.By varying the size of the defects,the simulation results were fitted with Boltzmann model to obtain the quantitative relationship between the defect size and the compression strength of the laminate.