Improved J Estimation by GE/EPRI Method for the Thin-Walled Pipes With Small Constant-Depth Circumferential Surface Cracks

Application of thin-walled high strength steel has become a trend in the oil and gas transportation system over long distance. Failure assessment is an important issue in the construction and maintenance of the pipelines. This work provides an engineering estimation procedure to determine the J-integral for the thin-walled pipes with small constant-depth circumferential surface cracks subject to the tensile loading based upon the General Electric/Electric Power Research (GE/EPRI) method. The values of elastic influence functions for stress intensity factor and plastic influence functions for fully plastic J-integral are derived in tabulated forms through a series of three-dimensional (3D) finite element (FE) calculations for a wide range of crack geometries and material properties. Furthermore, the fit equations for elastic and plastic influence functions are developed, where the effects of crack geometries are explicitly revealed. The new influence functions lead to an efficient J estimation and can be well applied for structural integrity assessment of thin-walled pipes with small constant-depth circumferential surface cracks under tension.

Validation of Axial Crack Stability Assessment Methods in xLPR Version 2.0 Code

As part of the xLPR (Extremely Low Probability of Rupture) project, validation efforts were conducted to assess the predictive capability of axial surface crack (Ax-SC) and axial through-wall crack (Ax-TWC) stability modules employed for making the crack stability assessment. For the Ax-SC, the plastic collapse solution/criteria from the Ductile Fracture Handbook (DFH) was selected and employed after making detailed comparisons between the solutions obtained from the DFH and API-579/ASME FFS-1 solution methods. Experiments from past tests conducted for the Atomic Energy Commission (AEC) and by MPA-Stuttgart were used to validate the performance of the implemented axial SC stability module. Similarly, for the axial TWC stability assessment, a Limit Load solution, and a numerical analysis procedure for an elastic-plastic fracture mechanics (EPFM) based method employing a J-integral fracture mechanics parameter approach using a GE/EPRI type J-estimation equation, and tabulated plastic influence functions based on pipe dimension and Ramberg-Osgood material parameters, were used. Experimental results from past pipe tests conducted for the AEC, others conducted for the PRCI, and by Tokyo Gas involving larger diameter pipes were used to validate the performance of the implemented axial TWC stability module. In this paper, the selection of stability criteria/solutions, their performance and implications for the xLPR 2.0 code are discussed. The impact of using different flow stresses (function of yield and ultimate stresses), and stress magnification factor (Folias bulging factor) expressions, on the predictions is highlighted. Further, a conservative basis for the assessment of axial crack stability in welds is proposed. In closing, additional empirical corrections that can be incorporated without overly biasing the deterministic solutions/criteria that feed into a probabilistic analysis are highlighted.

Estimates of Plastic J-Integral and Crack Opening Displacement Considering Circumferential Transition Crack From Surface to Through-Wall Crack in Cylinder

The present paper provides the enhanced estimation of plastic J-integral and crack opening displacement (COD) for circumferential transition crack from surface to through-wall crack in cylinder based on detailed finite element analysis. The effects of circumferential transition crack on plastic J-integral and crack opening displacement have been systematically investigated for practical ranges of cylinder geometries and materials of interest. Then, the plastic influence functions (h1, h2) employed in the GE/EPRI method have been proposed to quantify those effects on plastic J-integral and COD. Furthermore, the J-integral and COD estimations based on the reference stress method using optimized reference load have also been introduced for circumferential non-idealized TWC in cylinder. Then, in order to gain the confidence in the proposed methods, the results from those proposed estimates were compared with elastic-plastic FE results by using actual stress-strain data and Ramberg-Osgood fit constants for TP 316 stainless steel. The present results can be expected to apply on various structure integrity assessments and to accurate plastic J-integral and COD estimation for circumferential non-idealized TWC in cylinder.

Elastic-Plastic Fracture Mechanics Method for Poor Penetration Crack in Welded Piping Branch Junction

This paper provides two types of engineering J estimation equations for welded piping branch junctions with poor penetration crack under internal pressure. The first type is the so-called GE/EPRI type J estimation equation based on Ramberg-Osgood (R-O) materials. Based on detailed 3-D FE results using deformation plasticity, plastic influence functions for fully plastic J components are tabulated for practical ranges of the inner radius of brace to the inner radius of chord ratio, the thickness of brace to the thickness of chord, the thickness of chord to the inner radius of chord ratio, the crack depth to the thickness of chord ratio, the strain hardening index for the R-O material, and the location along the poor penetration crack front. Based on tabulated plastic influence functions, the GE/EPRI-type J estimation equation along the crack front is proposed. For more general application, the effective remote stress method based on GE/EPRI-type solutions is provided. This method provides a simpler equation for J, which could be used for any stress-strain relationship material, including Ramborg-Osgood (R-O) material and non-R-O materials under monotonic increasing loading. The proposed effective remote stress based J estimation equation is compared with elastic-plastic 3-D FE results using actual stress–strain data for a Type 304 strainless steel. Good agreement between the FE results and the proposed reference stress based J estimation provides confidence in the use of the proposed method for elastic-plastic fracture mechanics of pressurized welded piping branch junction.

Approximate Elastic-Plastic J Estimate of Cylinders With Off-Centered Circumferential Through-Wall Cracks

This paper provides approximate J estimates for off-centred, circumferential through-wall cracks in cylinders under bending and under combined tension and bending. The proposed method is based on the reference stress approach, where the dependence of elastic and plastic influence functions of J on the cylinder/crack geometry, the off-centred angle and strain hardening is minimised through the use of a proper normalising load. Based on published limited FE results for off-centred, circumferential through-wall cracks under bending, such normalising load is found, based on which the reference stress based J estimates are proposed for more general cases, such as for a different cylinder geometry and for combined loading. Comparison of the estimated J with extensive FE J results shows overall good agreements for different crack/cylinder geometries and for combined tension and bending, which provides sufficient confidence in the use of the proposed method to fracture mechanics analyses of off-centred circumferential cracks. Furthermore, the proposed method is simple to use, giving significant merits in practice.