Fatigue strength capacity of load-carrying fillet welds on ultra-high-strength steel plates subjected to out-of-plane bending

Weld root fatigue strength capacity is an important design criterion in load-carrying (LC) fillet welded joints subjected to cyclic loads. This paper elaborates on the weld root fatigue strength capacity of fillet welded LC joints made of ultra-high-strength steel (UHSS) and subjected to out-of-plane bending. Experimental fatigue tests are carried out using constant amplitude loading with an applied stress ratio of R = 0.1 with both pure axial, i.e. DOB = 0 (degree of bending, bending stress divided by total stress) and bending, i.e. DOB = 1.0, load conditions. The applicability of different approaches - nominal weld stress, effective notch stress concepts, and 2D linear elastic fracture mechanics (LEFM) - for the fatigue strength assessment of weld root capacity is evaluated. Furthermore, a parametric LEFM analysis is used to evaluate the effect of weld penetration on the root fatigue strength capacity in axial and bending loading. The results indicate that in the case of bending, nominal weld stress can be calculated using the linear stress distribution over the joint section and FAT36 as a reference curve. In the bending loading, for the joints failing from the weld toe, a mean fatigue strength of up to 185 MPa in the nominal stress system was achieved, indicating that the reference curve FAT63 is overly conservative. The ENS concept with FAT225 seemed to be slightly unconservative for assessing the root fatigue strength capacity. LEFM analyses revealed that in the case of increasing weld penetration and bending loading, weld root fatigue strength capacity seemed to correlate with the nominal weld stress calculated using effective weld throat thickness, while in axial loading, weld stress should be calculated using external throat thickness summed with penetration length.