Numerical Simulation of Dynamic Ductile Fracture by lap14150


									IPC 2008 - Abstract

    Reality Check on Girth Weld Defect Acceptance Criteria
              G. M. Wilkowski, D.-J Shim, F. W. Brust, and S. Kalyanam
                    Engineering Mechanics Corporation of Columbus
               3518 Riverside Dr. Suite 202, Columbus, Ohio 43221, USA


         This paper examines the inherent conservatisms of girth weld defect acceptance
criteria from the 2007 API 1104 Appendix A, CSA Z662 Appendix K, and the proposed
EPRG Tier 2 criteria. The API and CSA codes have the same empirically adjusted limit-
load criteria in them, where it has previously been shown that the conservatism on the
failure stress is about 30 to 50 percent compared to pipe test data prior to applying any
safety factors. In terms of crack length it was found that the API/CSA limit-load
equation might allow a flaw of 5% of the pipe circumference, where the properly
validated limit-load equation would allow a flaw of 75% of the circumference, i.e., a
safety factor of 30 percent on load corresponded to a safety factor of 15 on flaw length.
         Similarly, there are conservatisms in a proposed EPRG Tier 2 girth weld defect
acceptance criterion. That criterion was directly based on flat-plate data. However, the
flat plates are really an intermediate-scale test, and still require proper scaling to pipes of
different diameters. The EPRG allowable flaw length is 7T from a large database of flat
plate tests with the a/t value of less than 0.5 (or a < 3mm), and the failure stress being the
yield strength of the base metal (also a Charpy energy limitation of minimum > 30 J and
average > 40 J). However, the widths of those wide-plate tests are typically a factor 5 to
12 times less than typical large-diameter pipes. The proper limit-load/fracture mechanics
scaling solution would have the flaw length proportioned to the plate width, not the
specimen thickness. This difference in the scaling parameters results in an underestimate
of the critical flaw length by a factor of ~5 to 10.
         Examples of the conservatisms of each criterion and comparisons of the
magnitudes of the conservatism will be illustrated.
         An improved procedure will be presented that accounts for proper limit-load
solutions with pipe tests, effects of pipe diameter, effects of internal pressure, and a much
simpler approach to also incorporate the material toughness. The fracture analyses could
evoke SENB, SENT testing, or have relatively simple Charpy data to asses the transition
temperatures to ensure ductile initiation will occur.

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