Metallic Materials Cyclic Plasticity And Fatigue Of A Preliminary Study

Tensile test and cyclic strain test of copper single crystal were performed. There are three stages in the plastic deformation. The first stage is primary slip then followed by secondary slip; the last stage is due to dislocation passing the slips. The cyclic stress strain (CSS) curve exhibits a stronger hardening occurred in cyclic plastic deformation than that in tensile test. The structure of the deformation bands was observed and photographed by a microscope with a camera accessory.A user material mechanical behavior (UMAT) subroutine was developed based on the crystal plasticity model, and with it, the anisotropic elasto-viscoplastic behavior in single crystals was described. Slip systems' evolution was simulated by using finite element method with the subroutine. At the same time, the relationship between macro mechanical behavior and meso mechanism was investigated.Polycrystalline structure was possessed by most engineering materials. Under load the anisotropy of the constituent grains leads to strong inhomogeneities of stresses and strains on the grain level. Simulating on polycrystalline cyclic deformation indicates that hardening first appears at grain boundary then spreads to grain inside. Finally, hardening is not well-distributed in the specimen when deformation is limited. Meanwhile, Bauschinger phenomena could be modeled because the grains have random orientation.It was demonstrated that crack could grow under far field cyclic compression load in experiments. However, this kind of crack growth phenomena is difficult to be observed and described accurately by mathematic methods because the tensile residual stress area is too small and crack face contact exists. Finite element method was used to calculate the residual stress fields of three crack tip geometries. It was found that tensile residual stress is a key factor for crack growth under cyclic compression. The rationality of different crack tip geometries is discussed. Meanwhile, this work shows that crack propagation is limited and stable under cyclic compression. At last, some experiments were made to verity the conclusions.Flame tube in aircraft engine undergoes high temperature and high speed airflow washing. A hypothesis was suggested, which takes the transient heat transferring as the main reason to the cracking and damage on flame tube. Finite element method was adopted to simulate the thermal fatigue and the hypothesis was reproduced. As a result, the life of the flame tube was calculated by using Coffin-Manson equation.