The Effect And Mechanism Of Initial Damage On Ductile Damage Evolution And Fracture In Steel

In this paper, the effect of initial damage on ductile damage evolution and fracture in metal materials is investigated by finite element method (FEM) calculation and a comparison with experiment. The ductile fracture in tensile specimens with various notch root radiuses for a C-Mn steel has been predicted by using GTN (Gurson, Tvergaard, Needleman) damage model. The results show that, in the case of notch radius R=2mm , the prediction values of the maximum load P_m fracture initiation load P_i fracture load P_f and fracture work E are close to the experimental values. But in the case of R>2mm, the difference between the prediction and experimental values becomes large. The reason for this is that GTN model is based on the ductile failure mechanism of void growth and coalescence, and it is suitable for the specimens with small notch radius that has high stress triaxiality for void growth. The values of GTN model prediction for the three characteristic loads, especially for the fracture work E characterizing toughness are generally higher than the experimental values. And also the prediction for the fracture initiation locations is not consistent with the experimental observation in the case of R=lmm. The reasons for these are that the effects of the size, morphology, distribution and orientation of the inclusions/voids on the damage evolution and fracture process have not been considered in the GTN model.A mechanical analysis has been conducted on a notched specimen to investigate the effect of various distributions of large initial inclusions/voids on the crack propagation process. The results show that, when the distributions of initial inclusions/voids with the same sizes are period and uniform, the zone covered by high stress triaxiality dominated the fracture initiation, and the crack propagation is very straight, and the crack propagation resistance and toughness are lowest. When the distribution of the initial inclusions/voids with the same sizes is random, both the distances between voids and the zone covered by high stress triaxiality determine the fracture initiation location; and the crack growth path is zigzag, which is agreement with the experiment, and the crack initiation and growth toughness are high. When the distribution of the initial inclusions/voids with the various sizes is random, the dimension of voids, the zone covered by high stress triaxiality and equivalent plastic strain e_p determine the fracture initiation location, and the big voids determine the crack growth path, which is very close with the experimental observation, and crack initiation and growth toughness are highest. In order to increase the toughness of material, the number and size of big voids should be reduced, the distribution of the initial inclusions/voids should be random.A meso mechanical analysis has been conducted on a hot-rolling steel of 16MnR toinvestigate the effect of the elongated pre-damage voids on the subsequent damage evolution and fracture process under the maximum normal stresses with various directions. The analysis results show that, when the elongated voids due to the decohesion of the inclusions from base material are loaded subsequently, the local high stress and strain concentration around the voids and their interactions promote the growth of voids. There is distinct difference between the behaviors of voids' growth. The rapid growth and coalescence of a few big voids predominates the failure process of the material. When the elongated voids are loaded by the maximum normal stress with different directions, the distributions of local high stress and strain around the voids and their interactions are different, which causes different growth rates of voids and the critically applied normal strain of coalescence, thus the correspondent ductile fracture initiation toughness is different. When the maximum normal stress is parallel to the elongated voids, the toughness is highest; when perpendicular to them, the toughness is lowest.The effects of the amount and distribution of the initial damage and mechanical properties of the base metals on the subsequent damage evolution and fracture process have been investigated. The results show that, when area fraction of initial inclusions/voids and the inclusion sizes are small, the nucleation of voids is difficult, and the consequent void growth rate is small, and the toughness is high. In order to enhance the toughness of material, the number and size of initial voids should be decreased. When the distribution of inclusions is uniform, the nucleation of voids is not easy; when clustered, the nucleation of voids is easy. The distribution of voids is determined by the distribution of inclusions. When the distribution of the initial voids is uniform, the process of void growth includes two stages, that is, uniform growth stage and no-uniform growth stage, and the fracture initiation toughness of the materials is high. However, when the distribution of the initial voids is clustered, a few voids dominate the failure of material, and the fracture initiation toughness of the materials is low. Although the inclusions with large sizes dominate the damage evolution process (voids nucleation, growth and coalescence), the carbides also have some effect on the toughness of material. When the mechanical properties of the base metals are changed, the distributions of equivalent plastic strain e_p and maximum normal stress S₁₁ are almost the same. But when the work hardening of the base material is strong, the nucleation of the voids is easy. When the base materials are soft, the rate of the growth and coalescence of larger voids is large, and the toughness of the materials is low.