Study Of The Characteristics Of Fatigue Crack Propagation Based On Dislocation Theory

Fatigue crack is an important reason for failure of work piece, and the problem is solved by fracture mechanics theory at present, but the characteristics of fatigue crack propagation can't be explained by fracture mechanics theory. Based on the dislocation theory, the characteristics of fatigue crack propagation can be well studied, but existing dislocation theory only can reflect the feature of stage I of crack propagation. The study to propagation characteristics of the whole process is incompletely studied from the dislocation theory, so it is significant to study fatigue crack propagation.As the actual situation of the dislocation structure in crack tip is very complicated, an appropriate mathematical model to simplify the dislocation group in crack tip is necessary. This super-dislocation model to replace the actual discrete dislocations make it be more convenient to calculate the stress field caused by interaction of dislocations and crack, the results show that the stress distribution in crack tip is affected by applied stress factor and lattice friction. The result shows that peak stress in crack tip will change with the applied stress factor increasing and the friction stress decreasing. By means of stress calculation in every slip face, it's easy found that the position of nucleation where the most serious concentration happens won't change, the direction of model I crack propagation will not change.At present, the study of fatigue crack growth properties based on microscopic dislocation theory only show the characteristics of stage I of fatigue crack propagation, so there are good prospects for the study. According to energy conservation principle, the total energy of crack- dislocation system will be constant in every cycle and the expression for fatigue crack propagation can be induced. Considering of energy caused by dislocation-dislocation interaction and lattice friction respectively, the total energy of crack- dislocation system can be cacullated.Through the computation to the stress field of crack tip, the density of strain energy can be caculated, the expression for velocity of fatigue crack propagation can be improved. Combining energy conservation principle with Continuous Dislocation Model, the results are universally agree with traditional formulas, and coincide with the existing empirical results and experimental facts, and reflect the characters of stages II and III in fatigue crack propagation. The formula can also directly reflect effects of the frequency and stress ratio on fatigue crack growth, so the formula can be directly applied to predict fatigue crack growth in different frequency and stress ratio rate.