裂尖约束效应对高钢级管线钢断裂韧性影响的研究

Fracture toughness of pipeline steel is an essential parameter in fracture assessment and safety evaluation for pipeline. The effect of crack tip constraint on the fracture toughness is an important consideration in fracture mechanics analysis. It is meaningful and valuable for accurately evaluaing the fracture toughness of high grade pipeline steel by studying crack tip constraint effect on the fracture toughness so as to identify the appropriate fracture mechanics specimen of pipeline. Considering the inapplicability in the existing standard of fracture toughness assessment for the high grade pipeline steel, the effect of crack tip constraint on the fracture toughness for high grade pipeline steel is investigated through experimental tests together with finite element analysis. The standard SENB (single edge notch bend) and SENT (single edge notch tension) specimens are selected as the fracture mechanics specimens in measurement of the fracture toughness and crack growth resistance curves for inland high grade pipeline steel. The effects of specimen size, crack depth, loading modes on the measured fracture touhgness and resistance curves are investigated. Crack growth resistance curves under large scale yielding conditions for pipes with internal and external circumferential surface cracks and its fracture mechanics specimens are calculated using finite element method. The results are compared with those of corresponding SENT (with same crack depth) and standard SENB specimens. A practical procedure of fracutre toughness measurement and reasonable selection of fracture mechanics specimen is expected to be recommended for high grade pipeline steel in the engineering critical assessment (ECA).

管线钢断裂韧性是进行管道断裂评估和安全评定的重要参数。裂尖约束效应对断裂韧性的影响在断裂力学分析中是一个重要的考虑因素。通过研究裂尖约束对高钢级管线钢断裂韧性的影响规律，找出与管子裂尖约束水平相匹配的力学试样来对其进行断裂力学测试，对高钢级管线钢断裂韧性的准确评估具有重要的意义和工程应用价值。鉴于现行的断裂韧度测试标准对目前高级别管线钢并不适用，本项目拟采用试验测试和有限元模拟相结合，选取标准中规定的SENB（单边缺口弯曲）和SENT（单边缺口拉伸）试样测试法对国产高级别管线钢的断裂韧度及裂纹扩展阻力曲线进行测试，考查不同试样尺寸、裂纹大小、加载模式等对管线钢断裂韧性及阻力曲线的影响，同时借助有限元法计算分析内外周向表面裂纹管在大范围屈服条件下的裂纹扩展阻力曲线，并与其相应的断裂力学试样的结果进行比较分析，为高钢级管线钢断裂韧性测试方法的合理可行与断裂力学试样的合理选择提供理论依据与参考。

管线钢断裂韧性是进行管道断裂评估和安全评定的重要参数。裂尖约束效应对断裂韧性的影响在断裂力学分析中是一个重要的考虑因素。通过研究裂尖约束对高钢级管线钢断裂韧性的影响规律，找出与管子裂尖约束水平相匹配的断裂力学试样来对其进行断裂力学测试，对高钢级管线钢断裂韧性的准确评估具有重要的意义和工程应用价值。鉴于现行的断裂韧度测试标准对目前高级别管线钢并不适用，本研究以弹塑性断裂力学理论为基础，基于细观损伤模型，采用试验测试和有限元模拟相结合的方法，选取标准中规定的SENB（单边缺口弯曲）和SENT（单边缺口拉伸）试样测试法对国产高级别管线钢X80钢的断裂韧度(CTOD值)进行测试，同时对测试样进行三维有限元建模计算，分析裂尖约束对X80钢的断裂韧性的影响。为了进一步研究裂尖约束的影响，采用有限元法对内、外周向表面裂纹管及其断裂力学试样在大范围屈服条件下的韧性裂纹扩展阻力曲线行为进行模拟计算，并分别考查不同试样尺寸、裂纹大小、加载模式以及管径大小、管内压等因素对管子韧性裂纹扩展阻力曲线行为的影响规律，并与其相应的断裂力学试样的结果进行比较分析，结果表明：标准SENB试样的结果过于保守，而SENT试样因具有与周向表面裂纹管中裂纹尖端相似的几何约束水平而具有与裂纹管更为接近的阻力曲线行为，因此，在对周向表面裂纹管进行工程临界评估（ECA）中，可用来代替标准SENB试样对管子进行断裂力学测试，从而在保证安全的同时亦可大大降低结果的保守性，该结论为高钢级管线钢断裂韧性测试方法的合理可行及断裂力学试样的合理选择提供理论依据与参考。