Fatigue strength of welded joints made of S1100 structural steel

Abstract

Fatigue strength of butt welded joints, non-load carrying T-joints, load carrying X-joints and longitudinal gusset joints made of SSAB’s Strenx® 1100 Plus structural steel is investigated in this study. There is no design code or standard for ultra-high strength steels at the moment so the target is to get necessary information for design recommendation. All the test specimens are welded with robotized GMAW except of two butt joints that are welded by fiber laser. In addition, the effect of post-weld treatments on the fatigue strength is investigated. Some of the joints are post-weld treated by high frequency impact treatment (HiFIT) or tungsten inert gas (TIG) treatment. Fatigue tests are performed with constant amplitude tensile loading with applied stress ratios R = 0.1 and R = 0.5. The total number of test specimens is 33. Experimental fatigue test results are compared with the results of fatigue strength assessment by the nominal stress, structural stress, effective notch stress (ENS) and 4R methods. FE-analysis is performed by Femap/NxNastran to obtain stress concentration factors for fatigue strength assessment by the ENS and 4R methods. In addition, 2D measurements, residual stress measurements and hardness measurements are carried out. Experimental results indicates that the applied stress ratio has substantial effect on the fatigue strength of test specimens. Post-weld treatments improves significantly fatigue strength of the joints with both applied stress ratios. Generally, nominal and structural stress methods provides conservative results with applied stress ratio R = 0.1 but even unsafe results with R = 0.5 in some cases. The ENS method provides accurate results for GMAW butt welded and non-load carrying T-joints in as-welded condition at low stress ratio but results are unsafe in other cases in as-welded condition. The 4R method takes in to account stress ratio and residual stresses which results in more accurate results especially at high stress ratios compared to other fatigue strength assessment methods.