钝化的预裂纹尖端产生的短裂纹对解理断裂的影响

EFFECTS OF SHORT CRACKSPRODUCED AT BLUNTED PRE-CRACKTIP ON CLEAVAGE FRACTURE

结合-130℃下C-Mn钢三点弯曲(3PB)断裂韧度试验时预裂纹尖端构形的变化和断口观察,用有限元对预裂纹尖端起裂并扩展的短裂纹进行模拟与计算,通过计算得到的应力、应变、三向应力度来说明短裂纹的产生和钝化对钢最终发生解理断裂的影响。结果表明:当预裂纹仅仅被钝化时,预裂纹尖端一区域内积累的应变足以使裂纹形核;另一区域三向应力度和正应力也足以使一裂纹核扩展,但是这两个区域不重合,之间有一距离,因此形核的裂纹不能扩展,能扩展的区域内没有裂纹核,解理断裂也就不能引发。在钝化的预裂纹尖端产生短裂纹后将改变应力、应变、三向应力度的分布,特别是改变他们在裂尖前的相对位置。裂尖前短裂纹起裂、扩展及钝化产生更尖锐的构形,使应力及三向应力度区向钝化的裂尖方向靠近,从而使上述两个区域彼此重叠,这时解理裂纹起裂与扩展的条件同时具备,而在钝化的预裂纹尖端的一定距离处起裂与扩展。

FEM simulations of a short crack initiated, extended and blunted at a pre-crack tip and calculations of distributions of stress, strain and triaxiality are carried out to investigate their effects on the triggering of cleavage fracture of 3PB pre cracked steel specimen tested at-130℃. Combined with the observations of configuration changes and fracture surfaces of pre-crack tips, the results reveal that as long as the fatigue pre-crack is only blunted, in its vicinity a region, where the accumulated strain is sufficient to nucleate a crack and a region, where the stress (triaxiality) is sufficient to propagate a crack nucleus is separated by a distance. The nucleated crack cannot be propagated and the cleavage fracture cannot be triggered. A short crack produced at the fully blunted fatigue pre-crack will change the distributions of strain, stress and triaxiality and specially their relative locations ahead of the pre-crack. While the strain retains, the stress (triaxiality) is rebuilt. The initiated, extended and then blunted short crack makes a tip configuration, which is much sharper than that of the fully blunted original pre-crack tip. The sharpened crack tip configuration re-builds a 'sharper' distribution of stress (triaxiality) and make above two regions closer. Finally the two regions overlap each other and a cleavage crack can be initiated and propagated at a distance ahead of the blunted pre-crack.