Fatigue Fracture Analysis Of Nano-twinned Copper Under Cyclic Tensile Loading

Nano-twinned copper(NT-Cu)has great properties such as high strength,high ductility,and high hardness.By adding twin boundary(TB)into nanocrystals,the strength of metallic components can be improved effectively.Compared with traditional strengthening methods,it has greater lifting capacity.However,the working environment of metallic components is often complex.Statistics show that nearly 90% of the failure of metallic components comes from fatigue,and it is difficult to predict.Results show that NT-Cu can effectively resist fatigue fracture,hinder the propagation of fatigue cracks,and has high fatigue strength.In this paper,the responses of the interface strength,the void effect,and the crack propagation of NT-Cu under fatigue loading are studied by the molecular dynamics(MD)simulation method.Firstly,the relationship between grain boundary(GB)and TB in NT-Cu under fatigue loading is analyzed quantitatively.Four NT-Cu models are built,two of which contain GB and TB,and the other two contain only TB.Selecting the cyclic tensile loading with phase varying peak value.The results of the four models are compared,and it is found that the strength and dislocation density from the effect of GB is about 2-2.5 times of that from TB.The model containing only TB is in a state of low energy and more stable.Secondly,in the NT-Cu model,two different kinds of voids are set up,one kind is throughout the TB(intergranular),another kind is between two twin lamellas(independent).The influence of void location and void size are analyzed.Selecting the cyclic tensile loading with continuous variable peak value.Results show that the existence of void provides the source of dislocation.The strength of intergranular void is greater than that of independent void.The shape of the independent void deforms and shrinks under the loading,and that of the intergranular void almost does not deform.With the increase of the diameter of the void,the decrease of fatigue strength.The initial stability time of the model is shortened by the effect of independent void,while that is almost not affected by the intergranular void.Finally,the effect of crack on the fatigue response of NT-Cu model is studied.Selecting the cyclic tensile loading with continuous variable peak value that has residual strain of 1% and the crack is parallel to the TB.The influence of the TB spacing and crack length are analyzed.Results show that the crack provides the dislocation source under the fatigue response.The larger the TB spacing is and the shorter the crack length is,the more obvious the crack passivation phenomenon is.The results of this paper provide a theoretical reference for the following study of the fatigue problem on NT-Cu.