An LEFM Analysis for the Effects of Weld-Repair-Induced Residual Stresses on the Fracture of the HSST ITV-8 Vessel

Abstract

A fracture mechanics analysis for an intermediate size pressure vessel is described. The vessel was tested as part of the Heavy Section Steel Technology program at Oak Ridge National Laboratory. The purpose of the test was to evaluate the effects of residual stresses on the fracture of the vessel. Residual stresses were due to a half-bead weld repair located near the fabrication seam weld of the vessel. An external part-through flaw was placed in the seam weld near the weld repair. The test was conducted at −23.3°C to cause nonductile fracture behavior and thus accent the effects of residual stresses. The vessel was loaded with internal pressure. Test results showed that crack growth initiated at a significantly lower internal pressure than would be expected if there were no residual stresses. The crack arrested before growing through the thickness of the vessel. The pressure was increased until crack growth was reinitiated. The crack arrested after causing the vessel to leak but not break. The fracture analysis of the test presented here is based on static linear elastic fracture mechanics. Stress intensity factors were evaluated for a range of crack depths using a finite element representation and an energy release rate computation. The effects of residual stresses due to the weld repair were included in the fracture analysis based on residual stress data. There are two important findings of this study. First, good agreement was obtained between the results of the computational fracture analysis and the test data for three characteristics of the test; the pressure at which crack growth initiated, the amount of crack growth at crack arrest and the increase in pressure required to reinitiate crack growth. Thus the observed effects of residual stresses are predictable. The second finding is that the combination of laboratory data and computational results indicate a significant effect of residual stresses on both the crack growth initiation pressure and the crack arrest behavior for these particular test conditions. Estimates are that residual stresses reduced the critical pressure for flaw initiation to 25 percent of the value for the case with no residual stresses. These results are discussed in terms of available data and the computational model.