Study On Fatigue And Dynamic Mechanical Properties Of Powder Metallurgy Cu/WCp Laminates

Functionally graded materials(FGMs)has received widespread attention since it was published,which is with the properties of gradient spatial variation of components and microstructures,and the material designability and high adaptability of complex environment.According to the gradient distribution,it can be divided into continuous gradient and laminated gradient material.Due to the addition of the external phase,the existence of the interface between the layers and different preparation process,the meso damage source and forms of FGMs more abundant.The microscopic damage evolution mechanism under external field as well as the relationship between the macroscopic mechanical properties and the microstructure of the material were more complicated.Secondly,due to the strain rate effect,the dynamic mechanical properties and quasi-static mechanical properties of the materials are different,which depends not only on the basic properties of components,but also on the dynamic loading rate.Therefore,in-depth study on the damage,fracture and dynamic impact behavior,explore the relationship between microstructure and macroscopic properties of FGMs and optimize design theory and fabrication process of FGMs has important academic significance and engineering application value.The main work and conclusions are as follows:1.The Cu/WCp homogeneous composites were prepared by powder metallurgy method.The effect of WCp content on the microstructure,microhardness and mechanical properties of the composites were investigated.The results showed that the particle distribution was relatively uniform.The micropores,hardness,strength and elastic modulus of composites increased with the increase of content of WCp particles,although ductility decreased correspondingly.Through the study of the basic mechanical properties of homogeneous composites,it can be used for the study of fatigue and dynamic impact properties of laminated materials.2.Firstly,the effects of particle content on fatigue crack growth behavior of the composites were investigated by using a single edge notched specimen and then the effects of stress ratio on fatigue crack growth behavior of Cu/WCp/15p composites was studied.The results showed that the larger the particle content,the greater the crack growth rate of the composites.The main reason was the low bonding strength of the interface,which lead to the premature debonding under cyclic loading.The larger the particle content,the more serious of the interfacial debonding,leading to the faster fatigue crack growth rate.The fatigue crack growth rate increased with the increase of stress ratio.The main reason was that the larger the stress ratio was,the more serious damage of crack tip was caused by Kmax.In addition,the crack closure caused by particles decreases the driving force of crack growth at low stress ratio.Secondly,The fatigue crack growth behavior of the laminates Cu/WCp/15p-Cu-Cu/WCp/15p were investigated by using a single edge notched specimen.From the results of laminates,it could be seen when the crack grew from the Cu/WCp/15p layer to the pure Cu layer,the elastic mismatch E15p/ECu caused by interface 1 increases the crack tip driving force,which lead to the crack growth rate of the laminated material faster than that of homogeneous Cu/WCp/15p composite.When it grew crosses the interface1 into pure Cu layer,the elastic mismatch ECu/E15p caused by interface 2 reduced the crack tip driving force,which lead to the crack growth rate of the laminated material lower than that of homogeneous Cu.As the crack grows and approaches the interface2,the elastic mismatch effect was the biggest,which made the crack fail to cross the interface2 and arrest.3.The small crack growth behavior of composites under different temperatures(25 and 200 ?)were studied by in-situ SEM fatigue testing machine.And then the meso damage characteristics of composites under thermal mechanical coupling and their effects on small crack growth rate were discussed.Moreover,the effects of particle and microstructure on fatigue crack initiation and small crack propagation were analyzed.The results indicated that the small crack growth rate at 200 ? was higher than that of 25 ??The main reason was that the crack tip opening displacement and plastic zone size increased due to the degradation of the composites under thermal mechanical coupling.Interfacial debonding was the main form of small crack initiation.The small crack growth was sensitive to microstructure,The retardarce of large angle deflection on crack propagation was the most significant and then the crack branching and large size plastic zone.However,the interfacial debonding accelerated the crack propagation.4.Based on the cohesive interface element,the interfacial debonding damage characteristics of a circular inclusion and a real microstructure particle were simulated.And then,the effect of interface properties on crack propagation behavior was studied.Results showed that the interfacial debonding characteristics of simulation were in well agreement with the experimental results.From the simulation of crack propagation path,the crack propagation path with weak interface coincided with the experimental results.It was verified that the interface of the composite was a weak interface,which was not conducive to the improvement of fatigue crack initiation and propagation resistance.5.The dynamic mechanical properties of laminates were studied by using split Hopkinson bar(SHPB)and the microscopic damage mechanism was analyzed.Results showed that the Cu/WCp composite was a kind of strain rate dependence material,which had a certain effect of work hardening.The larger the particle content,the greater the flow stress of the composite,even though the corresponding ductility was sacrificed,which mainly reflected in that when the particle content was 15%,the shear cracks occurs at Cu/WCp/15p under high strain rate.For the laminated material,when the layers interface perpendicular to load direction,the laminates had a certain strain rate effect.It can be found from the SEM analysis that the shear crack inititated at Cu/WCp/15p layer,then grew through the interface to Cu/WCp/3p and arrest.When the layer interface parallel to the load direction,the flow stress of laminates first increased and then stabilized with the increase of strain rate.The main reason was that the layers interface cracking under high strain rate,leading to the flow stress can not continue to be improved.It can be seen from the SEM analysis that the interface cracks occur at the interface,and the shear cracks and shear bands appear on the Cu/WCp/15p side under high strain rate.