| Continuous SiC fiber reinforced titanium matrix composites(TMCs)exhibit excellent mechanical properties along the axial direction(longitudinal direction)of the fiber,while the properties in the direction perpendicular to the fiber(transverse direction)are low.This becomes one of the potential dangers for the composite used in complex environment.Therefore,the study of transverse mechanical properties of TMCs is of great significance to the engineering application of TMCs components.In this paper,the transverse mechanical properties of SiCf/TC17 are investigated in three aspects:single fiber reinforced composites,single ply multi-fiber reinforced composites and multi-ply and multi-fiber reinforced composites.The transverse failure mode,crack propagation and macro cracking process of the composites are calculated and analyzed by finite element simulation.Meanwhile,the diffusion bonding processes of TC17 alloy were studied by uniform design experiments.For the single fiber reinforced composite,the transverse mechanical properties were tested and evaluated using cruciform specimen.The surface and cross-section of fractured specimen were investigated by SEM to determine the failure position during tensile test.The results showed that the initial non-linearity in the stress-strain curve occurred at the stress of 271±12MPa,which indicated the beginning of fiber-matrix interface failure.The finite element simulation results showed that transverse fracture of composite interface was shear failure mode.Before the occurrence of non-linearity in the stress-strain curve,the crack initiated at the circular interface between reaction layer and carbon coating with a 40°-50° angle to the applied loading direction.Then the crack propagated along both circumferential and axial directions because of the shear stress.As the crack growing,the interface close to 0° angle to the applied loading direction failed first caused by the radial tensile stress,whereas the interface near 90°failed later as a result of circumferential shear stress.After complete failure of the interface,stress redistribution occurred around the SiC fiber and the interface separation increased with the increasing of the applied load,which gave rise to the yielding and deforming of the matrix near fiber until the final fracture of the composite.For the single ply multi-fiber reinforced composite,the transverse mechanical properties were tested with both straight-sided and dog-bone-shaped specimen.The results showed that the initial non-linearity in the stress-strain curve occurred at the stress of 188± 1 1MPa for the straight-sided specimen,and the value was lower than that of the single fiber cruciform specimen,which was due to the stress concentration at the free surface of the fiber.SEM observation showed that fibers sticked out from theend of the fracture specimen and interface obviously cracked at the center of the specimen.The crack position was consistent with that of the single fiber specimen.For the dog-bone-shaped specimen,the interfacial crack initiated at the end of a fiber,and then the crack propagated along the fiber axial direction to the center of the specimen.Due to the redistribution of the stress,the interfacial crack initiating and propagating process of the adjacent fibers occurred repeatedly.The matrix around the interfacial cracked fiber began to shrink inward and caused the matrix nearby to shrink inward as well,which gave rise to the fiber breaking.Finally,the crack of the adjacent fibers in the main fracture area crossed the matrix,resulting in the final failure of the material.For the multi-ply and multi-fiber reinforced composite,transverse tensile properties and fracture mechanisms of SiCf/TC17 composites at room temperature was also evaluated using cruciform specimen.The results showed that the stress-strain curve at room temperature was affected by the interaction of linear elastic deformation of SiC fiber region and plastic deformation of TC1 7 can.There were four stages of the curve,the initiation and formation of crack in the fiber region,quiescence of crack,gradually propagation of crack and instable fracture of composite,respectively.The first stage reflected the transverse tensile strength of SiCf/TC17 composites.The main fracture mechanism of SiCf/TC17 composite under transverse load included the interfacial debonding,brittle cracking of the matrix and fiber shear fracture etc.The initial debonding of interface excited the damage of composite,then the tiny crack at interface penetrated the matrix and contacted with other neighboring tiny cracks,which formed the initial crack surface of composite.The crack surface connected with each other through propagation,which finally formed the macro fracture.Yielding and deforming of TC17 matrix can occurred after the complete destruction of SiC fiber region of composite.The relationships between the tensile strength of the diffusion bonded TC17 alloy and surface roughness,heating temperature,heat preservation pressure and holding time were studied by the uniform design experiment.The results showed that if the roughness of the diffusion surface was reduced,the tensile strength of the specimen increases after diffusion bonding.When the roughness reaches Ra=0.1?m,the tensile strength of the specimen after diffusion bonding was almost the same as that of the matrix treated with the same process.Micro-holes are easily formed at the diffusion interface if the diffusion bonding temperature is low.Because of the different diffusion rate of the titanium atom in the ? and ? phase,the holes mainly appeared at the?/? interface,few appeared at the ?/? interface,hardly any holes appeared at the ?/?interface.The regression model was obtained through the regression analysis of the tensile strength of diffusion bonded alloy.The model showed that the heating temperature had the most significant influence on the tensile strength of the diffusion bonded specimen,followed by the holding time and the minimum influence of the heat preservation pressure.Considering the interaction between the three parameters and the trend of the whole model,for the diffusion bonding of TC17,even the fabrication process of composite,it could be taken into consideration in terms of the extent of reaction between fiber and matrix and the difficulty of processing.Finally,the values of the three parameters can be determined so as to obtain the best combination property. |
PDF Full Text Request