Numerical Simulation Of Crack Initiation And Propagation In Nano-scale Materials

The deformation and failure of materials are derived from the micro structure of materials,such as dislocation,vacancy,grain boundary and micro cracks.In order to further study the characteristics and mechanism of deformation and failure of materials,it is necessary to carry out multi-scale analysis.We should combine macro analysis with micro analysis to find the root of the problem and the mechanism of deformation in a deeper level.Recently,the study of crack propagation in metal materials is no longer confined to the macro and meso level,but has gradually penetrated into the micro nano level.But in the measurement of current conditions,it is difficult for nano crack propagation to realize real-time observation,so the computer simulation becomes extremely important.We use computer to study the nano-scale crack initiation and propagation,which can not only greatly reduce the cost of the test,but also provide a relatively accurate theoretical guidance.The phase field crystal(PFC)method is a new numerical method in the recent ten years.Compared with other numerical methods,PFC can not only simulate the evolution of the microstructure of materials from the time scale of atomic diffusion,but also can simulate the microstructure evolution from the spatial scale.In this paper,PFC method is used to study the effects of different temperatures,different liquid values and different strain rates on crack initiation and propagation.At the same time,we analyze the mechanism by combinating the change of free energy.In addition,this paper also studies the initiation and propagation of different initial crack shape and the effect of different orientation angle of crack initiation and propagation.In this paper,the crack tip propagation behavior is studied,the difference between brittle fracture propagation and ductile fracture propagation is studied.The mechanism of BDT(brittle-to-ductile transition)is also studied.The main results of this paper are as follows:1?For the samples with different initial crack shapes,their expansion is different.For the two or more crack samples,the size and location of one of the cracks effect the initiation and propagation of other cracks.2?The location of the triangle and the circular notch affects the initiation rate and the propagation mechanism of each other.Due to the stress concentration at the crack tip,the crack extends along the smooth line when the crack is brittle.When the crack is subjected to ductile fracture propagation,there is a plastic deformation zone at the front of the crack and the plastic deformation occurs because of the dislocation caused by the dislocation.The dislocations produce voids in the slip track during the slip process.The void grows?expands and connects under the tensile strain thus producing cracks.Crack is "the" font and is similar to "serrated" shape.3?The mechanism of crack initiation and propagation is different with different orientation angles.When the right orientation angle is taken,the crack fracture is ductile brittle transition,and the transition from ductile fracture to brittle fracture.4?Under the same conditions,the higher the liquid phase value and the higher the temperature and the larger the strain rate,the earlier the crack initiates and the smaller the critical strain and the faster the crack growth rate and the longer the crack length.In general,the crack propagation is brittle at low temperature and the crack propagation is ductile at high temperature.The ductile brittle transition(DBT)can be achieved by increasing the temperature.This research uses the phase field crystal method to simulate the initiation and propagation behavior of nano scale crack,and analyze its internal mechanism.The above theoretical results are consistent with the experimental results,which provide a theoretical guidance for the practical application of the material to prevent fracture failure and have an important guiding significance for the study of crack propagation mechanism and the prevention of fatigue fracture.