I - Ii Complex Crack Fracture Criteria And The Microscopic Process Of Relationship Research

The ductile fracture of metal is one of the most important problems in fracture mechanics. There are two fracture modes ?void -mode fracture and shear-mode fracture in I - II mixed-mode fracture of ductile metal. The void-mode fracture is resulted from initiation, dilatation and coalescence of voids in metal material; the shear-mode fracture, however, is believed that is related to shear belt in the structure. These two modes exist simultaneously in the process of fracture and compete with each other.Based on lots of experiments, by combining the progress in mesomechanics with FEA computation and analysis, this thesis investigated many parameters' effect on crack with an arc-tip in plain mixed-mode fracture, and compared existing criteria for fracture initiation, fracture direction and fracture mode transition.The thesis studied the change of stress triaxiality in the four-point curved beam made in 16MnR with arc-tip crack and its important effect. In the beam , the points of peak value of stress triaxiality distribute along a line, whose direction is well consistent with the fracture direction got by experiment when So >3mm.Research about CTOD and its components 8 ' , " shows that the direction angle of CTOD vector is identical with the fracture angle of void-mode fracture , which is obtained through experiments, and it is also identical with the result got by peak value line of stress triaxiality .Besides, 8 i' was compare with VGC as criterion for initiation of void-mode fracture. Comparatively , VGC seems to be better than i l. Still, the change of VG parameter in the beam was investigated in this thesis.Dilation of plastic belt in the beam is also studied. Analysis show that there is clear direction during the dilation of plastic belt, which is similar with the path of shear-mode fracture.In short , the analysis of arc-tip fracture shows that: the initiation of void-mode fracture can be judged by VGC criterion ,or by j' .according to SX criterion ;fracture direction of void-mode fracture can be determined by CTOD and peak value line of stress triaxiality . The mesomechanics mechanism of shear-mode fracture need further study, and analysis found direction of plastic belt is consistent with the direction of shear-mode fracture.In conclusion, the research of this thesis indicate that the behavior of crack with a arc-tip is the same as that of a spiculate crack, but its load of fracture initiation is higher than the latter.