Study On Fatigue Behavior And Interface Structural Evolution Of C/C Composites

Carbon fiber reinforced carbon composite,often known as carbon/carbon composites(C/C),is a new inorganic non-metallic composite material.The C/C composite exhibit many superior properties such as high modulus,high strength,low coefficient of thermal expansion,high temperature resistance,high wear resistance,high thermal shock resistance,high corrosion resistance and high temperature strength retention rate.The C/C composite has a wide application prospect in the field of aviation and military.As a kind of thermal structure material,mechanical properties of C/C composites has attracted many attentions from scholars,as well as stability of mechanical properties in application service.Because the C/C composite is composed of carbon fiber,carbon matrix and interfaces between different phases,micro-structure of C/C composite is a complex heterogeneous system.The complex micro-strcture makes damage accumulation and evolution of C/C composite is more complex than that of traditional materials.Bothe of the structure of carbon fiber preform,carbon matrix texture type and fiber/matrix interfacial bonding could affect the damage development and accumulation of the C/C composite.Therefore,studying the effects of interface evolution on the composite structural safety during cyclic loading is significant.Experimental objects of this thesis are based on three kinds of C/C composites with different fiber performs,i.e.unidirectional(1D),cross-ply(2D)and carbon felt(2.5D)reinforced C/C composites.The thesis studied mechanical properties and fracture modes of the experimental materials under different fatigue loading mode,different fatigue stress level and different fatigue loading cycls.The thesis also studied evolution of different C/C composite interfaces during the cyclic loading,and discussed the strengthening mechanism of C/C composite after fatigue.The main contents and results are as follows.The thesis studied the effects of tensile-tensile cyclic loading on mechanical properties,facture mode and micro-structural evolution of unidirectional C/C composite.After tensile cyclic loading,residual tensile strength of the unidirectional C/C composites hadbeen improved,and tensile fracture mode of the post-fatigue specimen exhibited more pseudoplastic than that of the original specimen.By means of SEM analysis and mercury porosimeter characterization,the study found that fatigue micro-cracks had generated at either the fiber/matrix interfaces orthe interfaces between different pyrolytic carbon matrix layers,and open poristy had been improved significant after fatigue test.The results indicated that the cyclic loading led the interfacial bonding inside the C/C composites to be weakened.The paper studied influences of bending-bending cyclic loading cycles on mechanical properties,fracture mode and micro-structure of unidirectional C/C composites.The cyclic loading cycle numbers conducted in this study were 10~4,10~5,5×10~5 and 10~6,repectively.The test results showed that,after the first 10~4 cyclic loading,residual flexural strength of the sample reached the maximum among all fatigue cycles,and residual flexural modulus of the sample decresed sharply.Flexural modulus of the post-fatigue specimens decreased slowly with increasing fatigue cycles mumber from 10~4 to higer value.The mercury porosimeter test results showed that the number of defect increased with increasing cyclic loading cycles,the defects with small size(diameter<1?m)increased significantly,but the defects with intermediate size(diameter in 1-10?m)and large size(diameter>10?m)increased indistinctively.The thesis also studied influence of fatigue stress level on mechanical properties,fracture mode and micro-structure of 2D cross-ply and 2.5D carbon felt reinforced C/C composites.Both of the cross-ply and carbon felt reinforcement C/C composites,residual strengths of the specimens under lower fatigue stress level were higher than that of the speciemns under their fatigue limit.Moreover,the results showed that residual flexural modulus decresed with increasing fatigue stress level.The SEM photos of the cross-ply C/C sample after fatigue cyclic laoding indicated that the interfaces inside fiber bundle were weakened firstly,the interface between fiber bundles debonded secondly,the interfaces between fiber bundle layers debonded finally.The SEM photos of the carbon felt reinforced C/C sample after cyclic loading showed that,lower fatigue stress led micro-cracks to generate near the defects pre-existed in the sample,and higher fatigue stress caused the micro-cracks connected with each other and formed a penetrating crack.The thesis used internal friction characterization to characterizemicro-structural damage evolution of the original and post-fatigue unidirectional C/C composites.The results indicated that internal friction values of the post-fatigue specimenswere much higher than that of the original specimens.Internal firction value had been improved significantly after the early stage of fatigue test(10~4 cyclic loading),but had remained unchanged after a certain number of fatigue cycles(5×10~5 cyclic loading).The results showed that open porosity of the post-fatigue specimen increased remarkably after the first 10~4 cyclic loading.By means of XRD and Raman characterizations,the paper studied pyrolytic carbon structural evolutionof the unidirectional(UD)and cross-ply(2D)composites during fatigue cyclic loading.The results showed that the pyrolytic carbon transformed from two-dimensional turbostratic structure into three-dimensinal ordered structure after fatigue test.The thesis work also researched the effct of pyrolytic carbon matrix sub-layer interfacial(the second inerfaces)fracture in residual strength improvement of the fatigued C/C composites,based on the experimental objects of the unidirectional and felt reinforcement C/C composite.The results showed that residual flexural strengths of the samples had been improved after fatigue tests,and the fracture mechanisms of the original and post-fatigue specimens were different.Fracture mechanism of the original specimens could be described as fiber/matrix interfacial de-bonding,and the dominant damage of the post-fatigue specimens could be regarded as pyrolytic carbon step-delamination.The degradation of matrix sub-layer interfacial bonding strength was beneficial to improve mechanical properties of C/C composites.