Elastic-Plastic Fracture at Surface Flaws in HSLA Steel Weldments

The combined effects of weld-strength-mismatch and crack size on strength and ductility were studied experimentally using tensile panels. Two high-strength, low-alloy steel base metals were welded with two different electrodes to produce a specimen matrix which contained one each of undermatched, normal-matched, and overmatched welds and one base metal. Two nominal crack sizes were used in two different locations, the weld center and the heat-affected-zone. The basic observable quantities were the strength and ductility of the tensile panels. Measurements of the applied J-integral were performed by instrumenting a contour on the specimen surface with strain gages. It was demonstrated that cracks reduce weld strength in proportion to their area, but that their effect on ductility can be much larger, far out of proportion to the crack area. Overmatched welds were observed to shield cracks from plastic strains when the cracks had a relative area of about 2.5 percent of the cross section; but this effect was lost for cracks with a relative area of 12.6 percent. The J-integral measurements indicated that the differences in behavior among the various weld-strength-mismatch conditions could be explained by the applied J-integral, without the need to invoke differences in fracture resistance. The measured values of fracture resistance, in units of J-integral and crack-mouth-opening-displacement, were comparable for three-point-bend specimens of all the specimen types.