Study On The Reason Of Crack Branching And The Prediction Of Branched Angle

With the development of modern industry, fracture mechanics is more and more widely used in analyzing and solving various practical engineering problems. In the present fracture theories, it is always believed that cracks propagate along their extended lines. Therefore, several fracture criterions such as the G criterion and the K criterion are based on the former premise. However, the phenomenon of secondary crack is always been neglected. The definition of secondary crack is that cracks may change their propagation directions after they propagate along their extended lines. For example, the phenomenon of crack branching ((?)) and two kinds of crack kinking ((?)),((?)) etc..In this paper, the reason of crack branching with its arrestment effects is analyzed and the branched angle and the kinked angle are predicted. In addition, the reasons why crack branching and kinking are hardly found by researchers are explained. Then, the critical crack lengths of crack branching and kinking are calculated. Finally, the methods on how to predict the propagation direction after the secondary fracture happens are given, and the summary of the secondary facture observed in plenty of references is made.Firstly, the reasons why crack branching and kinking happen are analyzed by the method of energy release rate. The physical mechanism of cracks will propagate along the direction where the energy release rate is maximum is used to evaluate the relationships among the crack branching and kinking critical stress intensity factors KICb, KICk and the mode I plain strain critical stress intensity factor KIC, respectively. Therefore, the reason why cracks always propagate along the straight line firstly and then branch or kink is explained. At the same time, the theoretical branched and kinked angles are predicted. Then, the conclusion that the relationship between the crack branching stress intensity factor KICb is1.3times as big as the mode I plain strain stress intensity factor KIC and the branched angle is about65°are proved to be accurate by former experimental results. Besides, the arrestment caused by crack branching and kinking is analyzed by two ways that crack branching and kinking may reduce the stress intensity factor rapidly and enhance the mode I plain strain critical stress intensity factor KIC. Therefore, the conclusions that crack branching has magnificent arrestment effect, while crack kinking contributes little to crack arrestment are made. Furthermore, the critical crack length is also calculated so that the reason why secondary fracture is hardly found is explained. Then aiming at the problem about the propagation direction after the secondary fracture happens, the difficulties are pointed out, and research ideas and methods are analyzed. Finally, the summary of the secondary facture observed in plenty of references is made and the reason why the branched and kinked angles in these references are more or less different from the result in this paper is analyzed, then the modified methods are given. In all, this paper has some important meaning in predicting whether the crack will branch or kink and the crack propagating direction.