Predictions of Tensile Strain Capacity for Strain-Based Pipelines With a Circumferential and Internal Surface Flaw

Y. Jang, Ju Kang, N. Huh, Ik-joong Kim, Cheol-man Kim, Young-pyo Kim
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Abstract

Strain-based design assessment (SBDA) has been known for suitable assessment concept for pipelines subjected to displacement-controlled load and high plastic deformation rather than conventional stress-based design assessment. Tensile strain capacity (TSC) has been used for one of important factors to indicate limit state in strain-based design, so that it is main concern to predict accurate TSC to ensure the structural safety and integrity of pipelines. For the pipeline containing a flaw, especially a girth weld flaw, TSC based on fracture mechanics can be determined. Crack-tip opening displacement (CTOD) has been widely used for typical elastic-plastic fracture parameter, representing crack-driving force and crack-resistance curve, which are required to assess unstable crack propagation. The one of the main principles of crack assessment is that the definitions of crack-driving force and crack resistance curve should be coincident. However, there exist two kinds of the definitions of CTOD, which are based on 90° and original crack-tip concept, and these have been not unified in practical regions until now. Moreover, it is reported that the deviations of crack-resistance curve can occur in the same specimen and experiment, caused by the different definitions of CTOD. Therefore, CTOD solutions based on each of different definitions of CTOD should be highly required since inaccurate TSC would be assessed when using not the identical definition of that. In the present study, CTOD solutions of pipelines with a circumferential and internal surface flaw are suggested by using two kinds of definitions of CTOD based on 90° and original crack-tip concept. For this purpose, FE analyses were systematically carried out considering various pipe geometries and material properties. And single-edge notched tension (SENT) specimen was used for representing resistance curve of API X70/X65 material. Moreover, the effect of the choice of each CTOD definitions on TSC was investigated through crack-driving force diagram (CDFD) assessment.
含周向和内表面缺陷的应变管道拉伸应变能力预测
基于应变的设计评估(SBDA)以其适合于位移控制载荷和高塑性变形管道的评估概念而闻名,而不是传统的基于应力的设计评估。在基于应变的设计中,拉应变能力(TSC)已被作为指示管道极限状态的重要因素之一,如何准确预测TSC是保证管道结构安全和完整性的重要问题。对于含缺陷的管道,特别是含环焊缝缺陷的管道,可以根据断裂力学原理确定TSC。裂纹尖端开度位移(CTOD)被广泛用作典型弹塑性断裂参数,代表裂纹驱动力和裂纹阻力曲线,是评估不稳定裂纹扩展所需的参数。裂纹评定的主要原则之一是裂纹驱动力和裂纹阻力曲线的定义必须一致。然而,目前存在着基于90°和原始裂纹尖端概念的两种CTOD定义,在实际地区尚未统一。此外,由于CTOD定义的不同,在同一试样和实验中,抗裂曲线也会出现偏差。因此,应该高度要求基于每种不同的CTOD定义的CTOD解决方案,因为当使用不相同的定义时,将评估不准确的TSC。本文采用基于90°和原始裂纹尖端概念的两种CTOD定义,给出了含周向和内表面缺陷管道的CTOD解。为此,考虑到各种管道几何形状和材料性能,系统地进行了有限元分析。API X70/X65材料的电阻曲线采用单棱缺口拉伸试样(SENT)表示。此外,通过裂纹驱动力图(CDFD)评价,研究了各CTOD定义的选择对TSC的影响。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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