Research On The Failure Of Carbon Fiber Braided Composites Under Dynamic Loading

Carbon fiber braided composites are widely used in aeronautical and aerospace fields due to their unique properties such as low density,high strength,high stiffness,low thermal expansion and good corrosion resistance.Thereinto,the carbon fiber reinforced carbon matrix composites(C/C composites)have the advantage of excellent high-temperature mechanical properties,which are considered as the ideal materials of aircraft thermal structures.However,the reliability of the aircraft structure is severely challenged in the complex service environment with vibration,noise and shock.This thesis aims at solving these problems.The main contributions of this work are performed as follows.Firstly,a novel statistical analysis method based on a Bayesian inference scheme,named Bdistribution,is proposed to evaluate the shear strength reliability of C/C composite pins.The shearstrength distributions of the C/C composites in different directions are calculated by the B-distribution and four traditional statistical models(three-parameter Weibull,two-parameter Weibull,log-normal,and gamma).These are critically analyzed by a Kolmogorov-Smirnov(K-S)test and a maximum likelihood test.The reliability of C/C composite pins’ shear strength is analyzed.The results show that the B-distribution behaves the highest precision for the shear strength of C/C composite pins,however,it is time-consuming.The three-parameter Weibull distribution shows a higher precision than the other three traditional distributions,and it costs less time than the B-distribution.The threeparameter Weibull distribution is considered as the ideal distribution for the shear strength of C/C composite pins.The 97.5% reliability shear strengths at 30°,45° and 60° are higher than those in other directions.Therefore,when installing a C/C composite pin,the primary shear bearing direction is preferred at the angle from 30° to 60°.Secondly,shear experiments with different strain rates are conducted to understand the dynamic shear properties of C/C composite pins.A scanning electron microscope(SEM)is used to analyze the strain rate effect on the failure mode and failure mechanism of C/C composite pins.The results show that the shear failure mode of C/C composite pins under low loading rate is the tensile failure of fiber bundles,and the shear stress-strain curve shows a “pseudo-plastic” characteristic.While under high loading rate,the shear failure mode of C/C composite pins is the shear failure in the shear section,and the shear stress-strain curve shows a “brittle fracture” characteristic.The shear failure mode of C/C composites is sensitive to the loading rate,which is owing to the strain rate effect on the defect propagation of C/C composites.Thirdly,the in-plane and out-of-plane compression experiments with the strain rate from0.0001/s to 1000/s are performed on a 3D fine weave pierced C/C composites.The Strain rate effect on the compressive properties and failure mechanism is analyzed.The loading curves are divided into two stages: the cumulative damage stage and the degradation stage.A piecewise strain rate dependent compressive constitutive model is proposed.The results show that the in-plane and out-of-plane compressive strength and the corresponding strain of fine weave pierced C/C composites increase with increasing strain rate.The quasi-static in-plane compressive failure mode of the fine weave pierced C/C composites is characterized by the shear failure at the angle of 45° and the local buckling of the x-direction fiber bundles,while under high strain rate in-plane compression,it is characterized by the compressive fracture of interlaminar matrix and the progressive compression failure of xdirection fiber bundles.Under quasi-static out-of-plane compression,the fiber bundles show poor integrity.Large amounts of “micro confining pressure structures” fail simultaneously,and there is a sudden failure in the stress-strain curves.The fiber bundles maintain good integrity under high strain rate out-of-plane compression,the “micro confining pressure structures” fail progressively,and there is no sudden failure in the stress-strain curves.The newly proposed piecewise strain rate dependent compressive constitutive model can be used to predict the compressive constitutive model of fine weave pierced C/C composites under the strain rate from 0.0001/s to 1000/s.Finally,a hat-shaped shear specimen is designed for dynamic in-plane and out-of-plane shear experiments of fine weave pierced C/C composites.A strain rate dependent double-damage shear constitutive model is proposed based on the two damage stages in the stress-strain curves.The strain rate dependent shear failure mechanism is analyzed.The results show that the quasi-static interlaminar shear failure mode of the 3D fine weave pierced C/C composites is characterized by the deflection and tensile fracture of z-direction fiber bundles and the shear fracture of interlaminar matrix,while under quasi-static in-plane shear loading,it is characterized by the deflection and tensile fracture of x-direction fiber bundles in the carbon fabric and the shear failure of “micro shear structure”.With the increase in strain rate,the fibers are more tightly bundled by the matrix,and the fiber bundles show better integrity.Therefore,with the increase in strain rate,the first damage stage,which mainly represents the damage of matrix,is gradually close to the second damage stage,which mainly represents the damage of fibers.The newly proposed strain rate dependent double-damage shear constitutive model can be used to predict the shear constitutive model of fine weave pierced C/C composites under the strain rate from 0.001/s to 1000/s.The prediction of dynamic response and failure of composite structures based on the static mechanical properties of composites may lead to a low prediction accuracy.It is indispensable to predict the composite structures’ dynamic response and failure based on the dynamic mechanical properties of composites.The key to solving this problem is the dynamic mechanical properties of composites.In this thesis,the failure of C/C composites under dynamic loading is studied.The research results in this thesis can improve the predition accuracy of dynamic response and failure of C/C composite structures.