Phase Field Crystal Method For Crack Propagation Under Dynamic And Static Loading

Crystal defect control techniques have been paid much attention in the field of materials because they can meet various requirements of material properties.In the process of material processing,defects such as cracks and dislocations are inevitable.Traditional experiments of nanoscale metal materials were usually unable to better reveal the microscopic mechanism of materials due to their limitations of the inability to dynamically display the evolution process of microstructure and the difficulty in preparing samples.Therefore,it was necessary to use Phase Field Crystal method to study the micro-crack propagation behavior.Phase Field Crystal method was an effective numerical simulation method developed in recent years to explore the evolution process of the microstructure of metal materials in the time scale and spatial scale.In this paper,the single-mode Phase Field Crystal model was used to explore the microcrack propagation mechanism of nano-single crystal samples under uniaxial dynamic and static tension.Considering the influence of orientation angle and temperature on crack propagation.And the variation laws of crack propagation form,dislocation motion,system free energy and crack propagation area were observed.Under uniaxial dynamic tensile conditions,the free energy of the system,crack area fraction and crack propagation mode of different orientation angles samples were analyzed in the whole process by observing the evolution of cracks.The simulation results showed that the crack extension occurred when the applied strain reached the critical value by the curve of crack area fraction and strain.With the continuous increase of stress,the area of the crack increased slowly and tended to be almost a straight line.When the strain reached a critical value,the crack began to expand rapidly.The initial downward trend of the free energy curve of the system was due to the elimination of residual stress in the crystal.The rapid growth of cracks in the midterm made the free energy of the system increase rapidly.In the later stage,the growth rate of the crack gradually slowed down until the crack broke.The simulation evolution diagram showed that the orientation angle has a great influence on the crack propagation form.The propagation modes of the orientation angles of 9° and 14° were brittle-ductile mixed fracture,while the brittle fracture occurs when the orientation angles were 19° and 30°,and there was no dislocation formation.It is found that the crack tip propagation was sensitive to temperature under the same tensile mode.At lower temperatures(r=-0.8 and r=-0.85),the crack tip was always accompanied by dislocations and brittle fracture occurs.With the increase of temperature(r=-0.9 and r=-0.95),the crack tip began to emit dislocations and ductile fracture occurred.With the decrease of temperature,the free energy of the system increased with the increase of applied strain,but the lower the temperature,the slower the energy growth rate,and it was difficult for the crack propagation behavior.Under uniaxial static tension,the effects of orientation angle and temperature on the critical strain value and mode of crack propagation were analyzed.When the orientation angle was different and the tensile strain was constant,the atoms around the crack were staggered to form dislocations,and the movement of the dislocations induced a series of vacancies when the orientation angle was 14°.The vacancies were connected to the main crack and fracture,which was a ductile fracture.When the strain was higher than the critical strain,the free energy of the system decreases with time evolution.When the temperature was taken into account,it was found that under the condition of equal strain and orientation angle,the rate of decrease of free energy was greatly related to temperature with the continuous evolution of time.Increasing the temperature accelerates the atomic vibration.The easier the crack propagation was,the faster the free energy of the system decreased and the earlier the fracture behavior occurred.By observing the evolution diagram,it can be seen that the crack will turn several times,and the crack propagation path was “zigzag”.In this paper,the evolution process of crack propagation was discussed in detail,and the influence of orientation angle and temperature on the fracture process caused by crack propagation under dynamic and static tensile mode was analyzed,which provided theoretical support for preventing fracture failure of metal materials.