Damage Research And Life Prediction Of Polycrystalline Metal Materials Under Creep And Fatigue Condition

Modern industry is developing rapidly in the direction of high temperature and high pressure.In the petrochemical,aerospace and energy industry,more and more machineries,equipment and components,such as large scale gas turbine,aero-engine,nuclear power equipment,metallurgical machineries etc.They are suffering long-term work under high temperature and high pressure condition.Serious threats to the industry safety caused by creep and fatigue damage problem increase prominently under such circumstances.Especially many accidents which happened under the condition of creep and fatigue interaction have aroused people’s attention.Pure creep and pure fatigue problems are traditional engineering problems,so the problem under the interaction of creep-fatigue is more worthy of our attention,and in more accord with the actual engineering situation.In this paper,we analyze the damage problem under the condition of the interaction of creep and fatigue with several typical polycrystalline metal materials from the microscopic modelling,evolution simulation and life prediction.From the micro perspective,we improve and use void growth evolution equation,and we set up a grain & grain boundary distribution model of polycrystalline metal materials by using the Voronoi polygon subdivision method.It can randomly generated void initiation points in the grain boundary,and calculate the void evolution in the grain boundary under the condition of creep-fatigue interaction.We define the damage variable with the parameter of void percent of grain boundary,and research the changing rule of the influence factors such as load,temperature,hold time and grain size.In order to describe the void evolution process more accurately and visually,we set up a cyclic calculation method and process which can realize the micro void evolution model which is simulated by the finite element through the program.In this method,we have considered under each load cycle the stress and voids will influence each other and change instantly,which is more in line with the real condition.This kind of calculation method and process can be applied to a variety of typical polycrystalline metal void evolution analysis,and it has universal application significance.In the damage model of researching fatigue based on the Gurson constitutive,the void percentage is an important parameter.We introduce the increment of void percentage caused by creep effect into the damage model,and make it suitable for creep-fatigue interaction conditions.By a rolling contact model validation,we prove the damage under the condition of the improved model has increased significantly more than pure fatigue,and also build a scalar method analyzing creep fatigue damage through the void percentage.Finally,we evaluate and compare a variety of commonly used life prediction methods under creep-fatigue interaction,and we use a new improved method based on hysteretic energy method which is more precise and has a definite physical meaning to predict the creep-fatigue life of Grade91 better.