Computational Meso-mechanical Model Of Short Carbon Fiber/copper Matrix Composites

Short carbon fiber/copper matrix composites have both the high electrical and thermal conductivity of copper and the high strength and toughness of carbon fibers,wear resistance,high temperature resistance,arc ablation resistance and good self-lubricating properties.It is one of the candidate materials for friction materials.With the shortening of the short carbon fiber length from millimeters to micrometers,the friction coefficient of the composite material is significantly reduced,and the friction stability is significantly improved,but the reduction of the carbon fiber length will inevitably affect the fiber reinforcement and toughening effect.The short carbon fiber/copper matrix composite material with carbon fiber content of 2wt.% was used as the research object.The tensile stress-strain curve of the composite material was calculated on the ABAQUS platform by the meso-mechanical analysis method of the composite material,and the axial stress distribution was analyzed.The change rule of short carbon fiber length(20～140 μm)was studied,and the effect of carbon fiber length on the meso-mechanical response of short carbon fiber/copper matrix composites was studied,and the mechanism of the effect of carbon fiber length on the failure mechanism of composites was expounded.The main conclusions of the full text are as follows:(1)The stress-strain curves of short carbon fiber/copper matrix composites can be divided into four stages according to their micromechanical behavior characteristics: elastic stage,plastic hardening stage,initial damage stage and damage evolution stage.During the elastic phase,the fibers bear most of the stress.In the plastic hardening stage,the equivalent plastic strain of the matrix near the end of the fiber and the axial direction is large.In the initial damage stage,the stress concentration of the material and the earliest interfacial damage mainly occurs at the interface near the fiber tip.In the process of damage evolution,the cracks continue to expand to the inside of the matrix and along the fiber axial interface,eventually resulting in the communication of cracks generated by several crack sources.(2)In the plastic hardening stage,the initial damage stage,and the damage evolution stage,when the fiber length is less than 20 μm,the stress in the middle of the fiber is lower than the end stress at each stage,and the fiber has almost no bearing capacity at this time.When the fiber length is greater than 60 μm,the axial stress of the fiber presents a “w” shape,that is,due to the discontinuity of the material,there is a high stress area near the end of the fiber,and the load on the middle of the fiber increases from the two ends to the middle.In the damage evolution stage,there are two main paths for crack propagation in short carbon fiber/copper matrix composites:(1)the axial interface of the fiber;(2)the tip of the crack propagates toward the matrix.The longer the fiber length is,the more the crack propagation of the composite tends to propagate along the axial interface of the fiber,and vice versa,the more tends to propagate along the tip of the crack to the matrix.(3)As the carbon fiber length increased from 20 μm to 140 μm,the tensile strength of the composite decreased from 146 MPa to 102 MPa.Although long fibers can bear higher loads during the stressing process and strengthen the matrix to a certain extent;in view of the weak carbon/copper interface,the stress concentration is obvious at the interface,and the fibers are debonded,resulting in a decrease in the strength of the material.The stress concentration at the fiber end causes the debonding of the end interface,the damage of the axial interface,the fiber bridging,the damage of the matrix,and the damage of the matrix caused by the concomitant interface damage,which are the main mechanisms of material fracture.The failure mechanism obtained by the finite element model simulation verifies the experimental results and verifies the reliability of the model.