采用API X90管线钢,对不同厚度的单边缺口拉伸(SENT)试样进行断裂韧性实验,并结合全场应变测量技术和断口分析,研究SENT试样面外拘束(试样厚度)对裂纹尖端张开位移(CTOD)的影响。结果表明,在相同的载荷水平下,随着试样厚度的增加,最大纵向应变峰值急速下降,高纵向应变区域由裂纹尖端迁移至距裂纹面一定距离的未开裂侧,裂纹尖端的塑性变形能力降低。CTOD值对厚度变化很敏感,随着试样厚度的增加而降低。当试样厚度宽度比≥4时,临界CTOD值达到下平台并保持基本不变。因此,在管线工程设计中,可将试样厚度宽度比等于4作为SENT试样断裂韧性与试样厚度无关的一个参考试样尺寸。

The crack-tip stress and strain fields of single edge notch tension (SENT) specimen are similar to those of the full-scale pipe containing surface cracks under longitudinal tension and/or internal pressure. It is well known that material's fracture toughness is not constant, and the specimen size has a significant influence on fracture toughness. It is thus essential to consider the transferability from fracture specimens in laboratory testing to practical structures, i.e., size effects or constraint effects. However, the specimen dimensions for SENT specimens recommended by current design procedures have not validated the out-of-plane constraint effect on the fracture toughness. In this work, the effect of specimen thickness on the crack tip opening displacement (CTOD) of SENT specimen was investigated using an API X90 grade steel. Full-field deformation measurement by digital image correlation (DIC) technique and stretching zone width (SZW) examination were performed to analyze the size effects on fracture toughness. The results show that the critical crack initiation toughness is highly sensitive to specimen thickness, and decreases significantly as specimen thickness increases until the thickness-to-width ratio (B/W) equals to 4, beyond which the effect of specimen thickness becomes relatively weak. As the specimen thickness increases, the maximum longitudinal strain and stretching zone width decrease sharply, and the location of high-strain zones changes significantly; when B/W≥3, strain is initiated from the area opposite the cracked side rather than from the crack tip, indicating a strong loss of plasticity for thicker specimens. A dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SENT specimen.