An Improved Axial Surface Crack Model to Predict Crack Growth and Burst Pressures Based on FE Analyses For Jplastic And J-R Curve Methodology

G. Wilkowski, J. Hong, F. Brust, Y. Hioe
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Abstract

During an in-depth review of the MAT-8 method, it was noted that there were assumptions for determining the FAD-curve shape by matching three points along FAD curves from FE analyses to get the plastic contribution of the crack-driving force (Jpl). Because the FAD-curve shapes from the FE analyses for different flaw sizes (depths and lengths) of interest for liquid line operation were quite different from both the MAT-8 assumed FAD-curve shapes and the FAD-curve fitting function for the FE-based FAD curves, an alternative procedure was pursued. For the cases of deeper and longer flaws, FAD-curve shapes cannot be found easily due to the highly nonlinear plastic behavior. To develop a new FE-based crack-driving force equation, a modification of the GE/EPRI basic formulation with contributions for elastic and plastic crack-driving force relationships was implemented. Several hundred FE analyses for axial surface cracks in pipes were performed. The new procedure was developed first by separating the elastic contribution from the total Japplied, then using the plastic part of Japplied versus pressure (Jpl-P) curve to establish an analytical solution. A nonlinear-regression analysis of the FE results from the entire Jpl-P curve was used, not just a few selected points. The new Jpl-P analytical solution showed good agreement with the carefully conducted FE-based solution from short, shallow flaws to deep, long flaws. The MAT-8 based solutions were good for short flaws, but deviated from the FE-based solution with both larger flaw depths and lengths. In addition to the new procedure, we included the use of the J-R curve for surface cracks which has been found to be a function of the surface crack a/t in pipe and SEN(T) tests, i.e., toughness is not a constant for all surface crack geometries which is discussed in other papers. Predicted burst pressures from the various methods including the new procedure were compared to pipe tests are presented.
基于j塑性有限元分析和J-R曲线法的轴向表面裂纹扩展和破裂压力预测改进模型
在对MAT-8方法的深入审查中,注意到存在通过匹配有限元分析中FAD曲线上的三个点来确定FAD曲线形状以获得裂纹驱动力(Jpl)的塑性贡献的假设。由于液线运行中不同缺陷尺寸(深度和长度)的有限元分析得到的FAD曲线形状与MAT-8假设的FAD曲线形状和基于FE的FAD曲线的FAD曲线拟合函数都有很大不同,因此采用了一种替代程序。对于较深较长的缺陷,由于其高度非线性的塑性行为,不易发现fad曲线形状。为了建立基于fe的裂纹驱动力方程,对GE/EPRI基本公式进行了修改,并考虑了弹性和塑性裂纹驱动力关系。对管道轴向表面裂纹进行了数百次有限元分析。新方法首先从总施加的压力中分离出弹性贡献,然后使用施加的压力与压力(Jpl-P)曲线的塑性部分建立解析解。对整个Jpl-P曲线的FE结果进行非线性回归分析,而不仅仅是选定的几个点。从短而浅的缺陷到深而长的缺陷,新的Jpl-P解析解与精心制备的fe基解析解具有良好的一致性。基于MAT-8的解决方案适用于短缺陷,但与基于fe的解决方案相比,缺陷深度和长度都更大。除了新程序之外,我们还将J-R曲线用于表面裂纹,该曲线已被发现是管道和SEN(t)试验中表面裂纹a/t的函数,即韧性不是所有表面裂纹几何形状的常数,这在其他论文中已经讨论过。介绍了包括新方法在内的各种方法的预测爆破压力与管道试验的比较。
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