铁素体管线钢的分层裂纹及其对断裂的影响

The delamination crack in ferrite pipeline steel and the effect on the fracture

通过对针状铁索体管线钢缺口根部三维应力状态的有限元分析和不同形式的断裂实验,研究了管线钢分层裂纹产生的条件及其对断裂性能的影响．结果表明裂纹或缺口根部的三维应力状态是产生分层裂纹的必要条件,材料的强度分布影响分层裂纹的形式和方向．分层裂纹均为主裂纹扩展前材料中的弱界面在垂直该弱界面的拉应力作用下产生的,其数量和方向受裂纹端部三维应力场和材料的强度分布状态控制．分层裂纹面上的应力为零,分层裂纹有一定的间距．在断裂过程中产生的分层裂纹使裂纹或缺口根部的构形发生改变,从而对裂尖的应力状态和材料的断裂性能产生巨大的影响．穿透裂纹体的分层裂纹使其有效厚度减小,表面裂纹体的分层裂纹与裂纹扩展方向垂直．在断裂过程中产生分层裂纹需要消耗更多的能量、降低裂端三维应力约束、有效厚度降低或裂尖钝化．这些因素均使断裂扩展更加困难,而使材料韧性得到提高．

The delamination cracks and its effects on the fracture of pipeline steel were investigated. The results show that the three-dimensional stress state is prerequisite for delamination crack and the strength distribution of material influences the form and direction of delamination crack. The delamination cracks are produced on the weak interfaces in the material by the tensile stress perpendicular to them before main crack propagation. The occurrence of delamination crack and its direction depend on the three-dimensional stress fields and strength distribution of material near the crack tip or notch root. The delamination cracks change the form of the crack and influence the three-dimensional stress fields of the crack tip; this can influence the fracture obviously. The delamination cracks of the fracture through wall thickness make the effective thickness decrease and the delamination cracks of surface crack are perpendicular to the direction of fracture propagation direction. The more energy is needed to produce delamination crack, the three-dimensional constraint of crack tip decreases, the effective thickness decreases or main crack tip is blunt. These factors all make fracture more difficulty, the toughness of the material will be improved.