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，从细观角度研究了钢中解理裂纹的扩展机理，考察了一些因素对解理裂纹路径和断口形貌的影响。模拟中采用的假设是，裂纹通过每个颗粒的解理面扩展，裂纹的萌生由该解理面的最大拉应力控制。模拟中使用了裂纹的有限元应力场解和Cottrel&莱斯的扭结裂纹分析。对于多轴应力模式下的小范围屈服裂纹，也采用了Shih的解。将模拟结果与Charpy试验和其他断裂试验结果进行了比较。对于晶粒度，任意尺寸的断口比恒定尺寸的断口粗糙，弹塑性裂纹大于弹性裂纹。与随机分布的颗粒相比，晶体织构断口的不规则性增加。从趋势的角度来看，这与实验结果相吻合。虽然弹入现象被认为是由材料的非均匀韧性引起的，但结果表明，只有裂纹扭结才能阻止小裂纹的产生。只要裂缝很小，上面的事情就可以说。但是，当初始裂纹尺寸较大时，无论晶粒尺寸和晶体织构如何，裂纹路径几乎保持不变。

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.