A Study on mechanisms of cleavage crack propagation of polycrystal metallic materials in crystal grain size level

采用二维数值模拟方法，从细观角度研究了解理裂纹在钢中的扩展机理，并探讨了一些因素对裂纹路径和断口形貌的影响。在模拟中使用的假设是，裂纹通过每个晶粒的解理面扩展，并且断裂的起裂主要由该平面的最大拉应力决定。采用有限元法求解裂纹应力场和Cottrel & Rice的扭结-裂纹分析方法进行了数值模拟。对于多轴应力模式下的小尺度屈服裂纹，也采用了Shih的解。模拟结果与夏比试验及其它断裂试验结果进行了比较。晶粒直径方面，随机晶粒尺寸的断口比固定晶粒尺寸的断口粗，弹塑性裂纹比弹性裂纹粗。与晶粒随机分布的情况相比，晶构断裂表面的不规则性增加。从趋势的角度看，这与实验结果是一致的。虽然认为弹跳现象是由材料韧性不均匀引起的，但研究表明，仅通过裂纹扭结就可以阻止小的弹跳现象。只要裂缝小，以上事情就可以说了。而当初始裂纹尺寸较大时，无论晶粒尺寸和晶构如何，裂纹路径基本不变。

We studied on mechanisms of cleavage crack propagation in steels from mesoscopic view by 2-D numerical simulations and investigated the effects of some factors on crack path and morphology of the fractured surface. Assumptions used in simulations are that a crack propagates through cleavage plane of each grains and fracture initiation is dominated by maximum tensile stress of that plane. Stress field solutions of crack by FEM and the kink-crack analysis of Cottrel & Rice were used in simulation. For small scale yielding crack in the multi-axial stress mode, the solutions by Shih was used, too. The results simulated were compared with Charpy test and other fracture tests. As for the crystal grain diameter, fracture surface for random size of grains was rougher than constant one, and elastic-plastic cracks than elastic. Irregularity of fractured surface of cristal textures was increased as compared to the case of random distributed grains. These correspond with the experimental results from the viewpoint of tendency. Although the pop-in phenomena have been considered to take place by non-homogenious toughness of materials, it was shown that a small ckack could stop by only crack kinks. As far as crack is small, the above things can be said. But, when initial crack size is large, crack path becomes almost constant despite of grain sizes and cristal textures.