Theoretical Study Of Pushout Method On The Interfacial Mechanical Behaviors Of SiC_f/Ti Composites

SiC fiber reinforced Ti alloy matrix composites（SiC_f/Ti-matrix composites）have been considered as high temperature structural materials in many applications,such as aerospace and motor-mobile industries due to their low density,high performance,high specific strength and stiffness at room and elevated temperatures.It is well known that the failure mechanism and macroscopic mechanical properties of composites have been critically influenced by the mechanical behavior of fiber/matrix interface,such as interfacial bonding strength,the distribution of residual stresses,the initiation and propagation of microcrack,and the effect of transferring load.In order to better understand the damage and failure mechanism of SiC_f/Ti-matrix composites,and guide the interface optimization technology and preparation technology,it is necessary to study interfacial micromechanical properties of SiC_f/Ti-matrix composites and its various influence factors.For the push-out test of SiC_f/Ti-matrix composites phenomenon that interfacial debonding is likely to occur at the supported end,the paper develops four theoretical models,respectively,as a function of four different test conditions.With the help of ABAQUS software,thermal residual stresses are obtained by finite element method.Finally,interfacial shear strength and fracture toughness of several composites are evaluated by incorporating push-out test with finite element method and theoretical analysis.The model for single-fiber push out test is developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in SiC_f/Ti composites.In the model,it is assumed axial stress in the fiber is constant.The model is based on fracture mechanics,taking into consideration of the free-end surface and Poisson expansion.Theoretical solutions to G_IIc are obtained,and the effects of several key factors such as crack length,initial crack length,interface friction coefficient,and interfacial frictional sliding stress on the critical applied stress necessary for crack growth are discussed.The predictions by the model are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6Al-4V,SCS-6/Ti-6Al-4V,SCS-6/Timetal 834,SCS-6/Timetal 21s,SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr.The results show that the model can reliably predict the interfacial toughness of the titanium matrix composites.A fiber-matrix two-phase model for push-out test is developed and the non-uniform distribution of axial stress in the fiber and shear stress at the fiber/matrix interface along fiber axis in bonded region are obtained by shear-lag method.The expressions of interfacial shear strength and fracture toughness are deduced,which include the effects of some parameters such as Young’s modulus and Poisson ratio of materials,specimen thickness,fiber volume fraction,interface frictional coefficient,interfacial frictional sliding stress and thermal residual stresses.The influences of several key factors（such as the applied stress needed for crack advance,crack length,and interfacial frictional sliding stress）on interfacial shear strength and fracture toughness are discussed.By using the model,the interfacial shear strength and fracture toughness of typical composites including Sigma1240/Ti-6Al-4V,SCS-6/Ti-6Al-4V,SCS-6/Timetal 834,SCS-6/Timetal 21s,SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are successfully predicted and compared with previous results of these composites.It is verified that the model can reliably predict the interfacial shear strength and toughness of SiC_f/Ti-matrix composites as well as other metal matrix composites,due to interfacial debonding usually occurs at the supported end of the samples in such composites.A fiber-interface reaction zone-matrix three-phase model for push-out test is developed for SiC_f/Ti-matrix composites whose interface reaction zone is reasonably thicker.In the model,the effects of Young modulus and Poisson’s ratio of materials,specimen thickness,fiber volume fraction,interface frictional coefficient,interfacial frictional sliding stress and thermal residual stresses and so on are included.The non-uniform distribution of axial stress in the fiber and shear stress at the fiber/matrix interface along fiber axis in bonded region are obtained by shear-lag method.Based on Kalton’s basic energy equation,the expressions of interfacial fracture toughness are deduced.The effects of several key factors such as crack length,the interface reaction zone thickness,interfacial frictional sliding stress and axial residual stresses on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed.Finally,the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V,SCS-6/Ti-6Al-4V,SCS-6/Timetal 834,SCS-6/Timetal 21s,SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model.A fiber-matrix-composites three-phase model for push-out test is developed for SiC_f/Ti-matrix composites whose fiber volume fraction is up to 50%or more.In the model,the effects of Young’s modulus and Poisson ratio of materials,specimen thickness,fiber volume fraction,interface frictional coefficient,interfacial frictional sliding stress and thermal residual stresses and so on are included.Based on shear-lag method,the non-uniform distribution of axial stress in the fiber and shear stress at the fiber/matrix interface along fiber axis in bonded region are obtained,and the expressions of interfacial shear strength and fracture toughness are deduced.The effects of some parameters such as interface thickness,interfacial frictional sliding stress and thermal residual stresses on the critical applied stress necessary for crack growth are discussed.Finally,interfacial shear strength of typical composites including Sigma1240/Ti-6Al-4V,SCS-6/Ti-6Al-4V,SCS-6/Timetal 834,SCS-6/Timetal 21s,SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model.SiC_f/C/Mo/Ti-6Al-4V composite was prepared by Foil-Fiber-Foil（FFF）method combined with vacuum hot press method,and part of the as-prepared composites were thermally exposed under conditions of 800℃/196h in vacuum.Then push-out tests for the as-prepared and thermally exposed（800℃/196h）composites were carried out.The interface of fibers which were pushed out was observed by SEM.It is found that,during push-out test,interfacial debonding occurs between C-coating and Mo-coating for as-prepared composite and that between fiber and TiC reaction layer for thermally exposed（800℃/196h）composite.Finally,interfacial shear strength and fracture toughness of two composites are evaluated by the present model.The results show that the interfacial bonding of thermally exposed（800℃/196h）composite is more stronger.