Determination of Fracture Arrest Stress

by David A. Osage, P.E., ASME Fellow / July 2009

 

Pressure vessel components constructed to the ASME B&PV Code, Section VIII, Divisions 1 and 2 that are not subject to Post Weld Heat Treatment (PWHT) have a fracture arrest stress value set equal to 10% of the Ultimate Tensile Strength (UTS) of the material. The fracture arrest stress designates an applied stress value for a material at or below which crack propagation would cease in a component with a small flaw. The fracture arrest stress value in the code is based upon work conducted by T. S. Robertson [1] and William S Pellini [2]. This work included testing of base metal and welded joints without significant constraint. Pellini concluded that for welded plates without constraint, localized residual stress fields, resulting from restraint of longitudinal shrinkage during cooling of the weld metal, act in a direction parallel to the weld and thus can only propagate cracks oriented in a direction normal to the localized residual stress (i.e. perpendicular to the weld joint). The perpendicular cracks may propagate into the base metal; however, these cracks will arrest due to lower or negligible residual stresses outside of the weld zone in the base metal. Therefore, for manufacture of components welded without significant constraint, i.e. manufacture of ships and atmospheric storage tanks, the fracture arrest stress value equal to 10% of the UTS appears to be valid. In the manufacture of pressure vessels, the geometrical constraint of typical weld joints results in a different residual stress distribution that have stress components that are orientated both parallel and perpendicular to the weld. Due to the presence of residual stresses perpendicular to the weld, small cracks parallel to the weld joint may result in unstable crack propagation or brittle fracture because they remain in the residual stress field. Therefore, the use of a fracture arrest stress value equal to 10% of the UTS detailed in paragraph 3.11.5.1 of the ASME Section VIII, Division 2, Part 3 criteria document may not be valid. A review of the work conducted by Robertson and Pellini and current code rules is provided and compared to fracture assessments results using procedures from API 579-1/ASME FFS-1 and ASME Section VIII, Division 2, Part 3.

 

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