Phase-field-crystal Study On The Crack Propagation Behavior Of Titanium Under Disturbance Strain

Titanium has been widely used in aerospace,marine and other fields because of its good physical properties.However,titanium is prone to crystal defects leading to fracture failure in complex working conditions,which will pose a serious threat to the application of the material.At present,although many traditional experiments on the fracture of titanium sample are carried out at the macroscopic scale,the evolution process of microcrack is rarely explored on the microscopic scale.In this thesis,phase-field-crystal(PFC)model is used to study the crack evolution behavior of titanium under complex working conditions,and the crack initiation and propagation process under biaxial static strain,biaxial dynamic strain and biaxial disturbance strain are deeply discussed.The effect of temperature on the crack growth of titanium is also studied.The main research contents and results are as follows:When biaxial static strain is applied,the titanium crack begins to expand along the direction of the closest atomic arrangement until fracture failure.The simulation results show that the crack propagation mode is mainly affected by temperature.The propagation mode of crack determines the fracture type of crack.Ductile extension of crack is accompanied by emission dislocation and dislocation slip.When biaxial disturbance strain is applied,the effects of different disturbance amplitude A and frequency(?)on the initiation time,crack growth type and growth area of titanium are analyzed.(1)The disturbance strain can change the type of crack propagation.When the disturbance amplitude A is small,changing the frequency(?)does not change the crack growth type;When the disturbance amplitude A is large,the change of frequency(?)will cause brittle-to-ductile transition(BDT)or ductile-to-brittle transition(DBT).(2)The disturbance strain can promote or hinder crack propagation.When the disturbance frequency(?)is small,increasing frequency(?)or amplitude A can promote crack propagation.Further increasing frequency(?)can reduce this acceleration effect and eventually hinder crack propagation.When the disturbance frequency(?)is large enough,increasing the amplitude A will slow down the crack propagation rate.The crack growth types,crack morphology and crack growth stability of titanium are studied under different temperature and disturbance frequency(?).(1)The fracture mode of crack is affected by temperature under certain disturbance strain.When the temperature is lower than the brittle-to-ductile transition temperatureT_BDT,increasing the disturbance frequency(?)can promote the plastic propagation of cracks and lead to BDT.After that,increasing the frequency(?)will promote the brittle crack propagation,but not achieve DBT.When the temperature is higher than the brittle-to-brittle transition temperatureT_BDT,the disturbance frequency does not significantly affect the crack propagation mode.(2)The stability of crack firstly decreases and then increases with the increase of disturbance frequency(?).When the disturbance frequency(?)is large enough,the stability of crack will be improved compared with that of undisturbed strain.On the other hand,the increasing temperature will lead to the decrease of crack stability under the same strain condition.In summary,this thesis discusses the mechanism of Titanium microcrack propagation under disturbance strain,explores the fracture mechanism of Titanium materials under different temperature and disturbance strains.broadens the application range of PFC and promote the theoretical development of Titanium material design and manufacture.