Fatigue strength of AL-6XN stainless steel weldments

This study presents an assessment of the fatigue strength limit state of large scale welded joints fabricated from nonmagnetic AL-6XN superaustenitic stainless steel. Three welded details were tested under constant amplitude loading, subjected to both compressive and tensile stresses. The experimental program included longitudinal fillet welds, transverse groove welds, and simulated bulkhead attachments. Fabrication was of shipyard quality using the GMAW weld process. Two Ni-based weld metals, IN625 and IN622, were used. A total of 66 large-scale I-beams were tested. Over 186 fatigue cracks were observed, resulting in 82 through flange thickness fatigue cracks used in the assessment of fatigue strength, in addition to many uncracked details related to the constant amplitude fatigue limit (CAFL).; Discontinuities associated with initiation of fatigue cracks were identified from the fracture surfaces and included: lack of fusion areas at the weld root of longitudinal fillet welds; internal lack of fusion planes, internal volumetric discontinuities, and micro-discontinuities at the reinforcement weld toe of transverse groove welds; and micro-discontinuities (tears or inclusions) at the bulkhead attachment weld toe in combination with regions of sharp weld toe reentry angle.; Fatigue strength was evaluated analytically using fracture mechanics concepts. Initial discontinuities and fatigue propagation behavior observed during the experimental program were used to assess the fatigue strength of longitudinal fillet welds and simulated bulkhead attachments. The analytical models attempt to estimate the lower bound finite life fatigue strength curves for the longitudinal fillet weld and bulkhead attachment details, as well as the CAFL. Based on the experimental and analytical results, the following design recommendations were made: (1) Use AASHTO Category B for standard double sided longitudinal fillet welds, with a CAFL of 100 MPa. (2) Use AASHTO Category C for transverse groove welds subjected to nondestructive evaluation to avoid large internal defects, with a CAFL of 83 MPa. (3) Use AASHTO Category E for 200 mm long bulkhead attachments, with a CAFL of 41 MPa. (4) Use AASHTO fatigue classification categories to develop preliminary fatigue resistance curves for details not yet tested with AL-6XN stainless steel.