Research On Fatigue Behaviors Of Carbon Fiber Reinforced Resin Matrix Composites Considering The Effects Of Voids And Specimen Size

Carbon fiber reinforced polymer(CFRP)composites have many advantages in terms of their relative stiffness,strength,and fatigue resistance,therefore,the improvement of fuel economy and the reduction of costs can be achieved by reducing the weight of vihecles and airplanes with the implementation of CFRP compnents.Recent years,CFRP composites have been widely used(transportation,medicine,etc.).However,since the fatigue mechanisms of fiber reinforced polymer(FRP)composites are different from those of metals due to their anisotropy and complex microstructures,conventional fatigue models for metals cannot be utilized directly in the fatigue life prediction of composites;besides,as one of the inherent defects of resin matrix composites,voids have significant impact on their fatigue behaviors,the previous investigations are not sufficient,and an acceptable fatigue model has not been reported;lastly,researchers throughout the world noticed that the size of composite structures has significant impact on their fatigue performance.In this situation,the effects of voids and specimen size on the fatigue properties of CFRP composites under various loading conditions were investigated in this study.The contents and conclusion of this study can be summarized as follows:(1)Examination of the volume content and morphological parameters of voids in CFRP composites.An image-processing software was established for resin matrix composites to examine the voids' content and morphological parameters(aspect ratio,size,etc.),the void characterization for composites cured under various pressures was achived based on the present software.(2)Experimental investigation of effects of voids on the mechanical properties of CFRP composites.Quasi-static tests,tension-tension(T-T)/tension-compression(T-C)fatigue tests for T-300/924 unidirectional(UD)and laminated composites with various void contents were conducted,and the impacts of voids on the stiffness,strength,as well as the fatigue properties of the composite were investigated based on the experimental results.A significant degradation of the static strength and fatigue resistance of the material could be observed for composites with high void contents.Moreover,the influence of the presence of voids was also studied from the perspective of failure probability,stiffness reduction,as well as the evolution of transverse crack density.(3)Stiffness and strength modeling for CFRP composites with void defects.The equivalent stiffness of the matrix with voids was accessed with consideration of the shape of voids,and a stiffness prediction model for composites with void defects was proposed with the help of stiffness volume average method and finite elementary analysis of representative volume element(RVE)with fiber randomly distributied.In addition,a static strength prediction method for FRP composites with void defects was also established.In this model,the initial damages for five different failure modes are judged by an equivalent-strain-form failure critetion proposed in this paper,and the corresponding failure evolutions were characterized with the implementation of continuum damage model;meanwhile,the stiffness degradation of the material considering the influence of the five failure modes was accomplished by modifying the conventional linear degradation method.Finally,a gradual damage analysis method for composites considering the effect of voids was proposed,and a corresponding analytical software was established using Python language.(4)Fatigue modeling for CFRP composites with void defects.A fatigue model was proposed for CFRP composites with void defects.In detail,the fatigue index was first defined based on fatigue crack initiation model and conventional fatigue crack propagation model;meanwhile,the damage evolution(evolution of transverse crack density)from meso-scale perspective was combined with the stiffness degradation of composites from macro-scale perspective,and a tensile fatigue model was thus established for CFRP composites with the consideration of void content.The present model is capable of capturing the effect of voids duing the course of fatigue.Finally,the validation of the model was accomplished by comparing the predictive results with the experimental data.(5)Study on the size effects on the fatigue of CFRP composites.Fatigue tests for 0°,10°,and 90°UD composites based on fatigue samples of different sizes were conducted to investigate the size effects on the fatigue properties of CFRP composites,the test results show that for 90° UD composites,the increase in the specimen size can result in a significant deacease in their fatigue life;in addition,to incorporate the sample size into the fatigue prediction,a fatigue model considering the size effect was proposed for UD composites on the basis of Weibull statistics and Basquin's equation;and lastly,the mechanisms of the size effect on the fatigue of CFRP composites were studied with the aid of scanning electron microscope(SEM).(6)Study on the influence of stress ratio on the fatigue of CFRP composites.Based on the fatigue test results for UD composites at various stress ratios,constant fatigue life(CFL)diagrams for the UD laminates with various fiber orientations,which allow for an accurate fatigue life prediction over the whole range of stress ratios,were established;in addition,the influence of stress ratio on the fatigue resistance of the material was also assessed on the basis of the CFL diagrams.