X70管线钢DWTT试样的分层裂纹及其断口评价

Delamination Cracks and Fracture Evaluation in DWTT of X70 Pipeline Steel

通过对管线钢不同温度的落锤撕裂试验和微观断口分析,研究了DWTT试样断口的分层裂纹及其对断裂的影响。结果表明,分层裂纹是受力变形时管线钢内部的薄弱界面受到三维应力作用的结果。分层裂纹出现于主裂纹起裂或加速之前,裂纹稳定扩展或减速时不会产生新的分层裂纹。分层裂纹的数量、张开程度和分层裂纹间距与主裂纹起裂或加速时的应力状态有关,而分层裂纹的长度与裂纹扩展时裂尖的应力状态有关。分层裂纹表面为解理形貌,解理面较大。韧脆转变温度以下的脆性断裂断口或韧脆转变温度附近混合型断口的脆性断裂区,不出现分层裂纹,仅在韧性断口或断口的韧性区出现宏观分层裂纹。研究表明,产生分层裂纹处的断口,分层裂纹无论是三角形分布或是其它形状分布,该区域均应评价为韧性。

The delamination crack and its effect on the fracture of pipeline steel were investigated by drop-weight tear test （DWTT）at different temperature and micrograph analysis. It was indicated that the delamination cracking is caused by the effect of 3D stress on the weak interfaces during deformation and appears before beginning or acceleration of main crack. New delamination crack can not appear during stabile propagation or deceleration of crack. The quantity, opening degree of delamination cracks and the distance between two delamination cracks are influenced by the stress state at beginning or accelerating point of main crack , but the length of delamination crack is influenced by the stress state of crack tip during the propagation of crack. The surface of delamination crack is of cleavage fracture appearance with large cleavage face. There is no delamination crack in the brittle fracture below ductile and brittle transition temperature or in the brittle fracture region of mixed-mode fracture at about ductile and brittle transition temperature. Macro delamination cracks appear only on the ductile fracture or on the ductile part of fracture. The part of fracture with triangular or other shapes delamination cracks ought to be evaluated as ductile fracture.