This paper presents experimental and analytical results focusing on the strain limit of X80 linepipe. Ductile crack growth behavior from a girth weld notch is simulated by FE analysis based on a proposed damage model and is compared with the experimental results. The simulation model for ductile crack growth accompanied by penetration through the wall thickness consists of two criteria. One is a criterion for ductile crack initiation from the notch-tip, which is described by the plastic strain at the notch tip, because the onset of ductile cracking can be expressed by constant plastic strain independent of the shape and size of the components and the loading mode. The other is a damage-based criterion for simulating ductile crack extension associated with damage evolution influenced by plastic strain in accordance with the stress triaxiality ahead of the extending crack tip. The proposed simulation model is applicable to prediction of ductile crack growth behaviors from a circumferentially-notched girth welded pipe with high internal pressure, which is subjected to tensile loading or bending (post-buckling) deformation.
Fracture behavior of high strength linepipes with weld defects is of great interest for the integrity of pipeline system. Especially, in the seismic or permafrost area, where large grand displacement can be expected, linepipe materials need to have sufficient resistance against brittle and ductile fracture under large deformation. Since large weld flaw can be eliminated by recent advance in material, welding and inspection technology, ductile fracture must be a next concern for high strength linepipe. Therefore, ductile fracture behavior of girth weld joints of Grade X80 and X100 linepipes were investigated in this study. Wide plate tensile tests were conducted using girth weld joints with the surface notch in the weld metal. Close observations were conducted in order to determine crack initiation from the notch root of the wide plate specimens. Local stress-strain conditions were evaluated by 3-D FE analysis, and criterion for ductile cracking were compared with that was obtained by smallscale specimen. Results showed that ductile cracking behavior is strongly affected by hardening properties of the base materials. Resistance to cracking in girth weld metal can be improved by applying the materials with lower Y/T ratio even with the similar weld metal property, and over-matching of the weld metal should be important condition. Concepts for defect assessment in terms of preventing ductile cracking are discussed with regard to the effect of base metal properties and strength matching of girth weld joint.
For two series of API 5L X65 linepipes Pipes A and B , the critical condition for ductile cracking of the linepipe steel and the applicability of the critical condition to an axially notched linepipe were investigated. Static 3-point bending tests for Charpy Vnotch specimens were conducted in order to evaluate the critical condition of ductile cracking from the notch tip by using FEanalyses. At the position of ductile cracking from the notch tip for the Charpy type specimens, the stress triaxiality was approx. 0.6 for both linepipe steels, however the equivalent plastic strain p was different on each linepipe; the p for the ductile cracking was approx. 0.65 for Pipe A and approx. 1.47 for Pipe B. Hydraulic burst tests were then conducted for internally patched linepipes with an axial through-wall TW notch. The results of the FE-analyses for the hydraulic burst tests indicated the following: 1 the position of the ductile cracking at the TW notch tip was not the center of the wall-thickness WT , but a slightly shifted position to the inner surface from the center of WT, 2 the equivalent plastic strain at the position where a ductile crack was initiated for the TW notched linepipe was almost the same as that obtained from the 3-point bending test result for the Charpy V-notch specimen. The present study revealed that the critical strain for ductile cracking from a notch tip for a Charpy type specimen was in good agreement with that for an axially notched linepipe. It was therefore clarified that the critical condition for ductile cracking for linepipes with an actual flaw could be predicted from the results of a small-scale test and FE-analysis to evaluate the relationship between the stress triaxiality and the equivalent plastic strain at the position of the ductile cracking.
In order to establish a guideline for fracture evaluation without excessive conservatism by considering plastic constraint in the ductile-brittle transition temperature (DBTT) region, the CAF (Constraint-Based Assessment of Fracture in Ductile-Brittle Transition Temperature Region) subcommittee has been launched in 2018 in the Atomic Energy Research Committee of the Japan Welding Engineering Society with a five years’ term. In the committee, fracture tests are conducted using laboratory specimens of C(T), SE(B), and 50mm-thick flat plate with a surface flaw subjected to bending load or tensile load to verify fracture evaluation methods, which strongly depend on numerical simulation. Since simulation results are easily affected by analysis conditions, benchmark analysis is essential for the potential users of the guideline. Therefore, benchmark analyses are executed on brittle and ductile damages by Beremin and Gurson–Tvergaard–Needleman (GTN) models implemented in the FE codes owned by the committee members. The benchmark analyses are carried out in four steps; Step 0 is to confirm the output of finite element (FE) codes in each member with the same input data and the same FE model. Step 1 is to confirm the result of Weibull stress analysis for C(T) specimens tested at −125°C. The Weibull parameter, m, was fixed in this step. At step 2, sensitivity analyses are conducted on Weibull stresses in different conditions. The outputs by the GTN model are also confirmed. At the final step, the fracture simulation will be run for flat plate specimens with less plastic constraint than the standard fracture toughness specimen. As the results of the benchmark analyses up to step 2, a significant difference are not observed in the Weibull stress computed by committee members with the same input data and FE model and it is confirmed that the effects of element type, nonlinear deformation theory employed in FE analysis and convergence procedure for the Weibull parameter m are marginal by the sensitivity analyses. For the calculation of the Weibull parameter m by using the fracture toughness test results and the developed programs by committee members, the converged values of m show good agreement among them. The obtained knowledge by the benchmark analyses is used to establish the guideline for fracture assessment of structural components with different plastic constraints.
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