Experimental evaluation of HFMI effectiveness for high-strength steel butt welds at R = 0.55–0.70

Abstract High-frequency mechanical impact (HFMI) is among the most effective post-weld treatments for improving fatigue performance. However, the current IIW Recommendation provides design guidance only up to a stress ratio of R = 0.52, leaving high-R applications without codified rules. This paper quantifies HFMI effectiveness at R = 0.55 and R = 0.70 under constant-amplitude four-point bending. A total of 92 transverse V- and X-butt welds in S355J2+N, S460NL, S690QL, and S960QL were tested at 5 Hz to failure or runout. The fatigue life improvement factor λ reached mean values of $${\lambda }_{mean}\approx$$ λ mean ≈ 5.6 for S960 and $${\lambda }_{mean}\approx$$ λ mean ≈ 2.2 for S690 at $$R=0.55$$ R = 0.55 . These values represent conservative lower-bound estimates, as runouts were treated as failures at the test termination limit. The benefit persists to $$R=0.70$$ R = 0.70 provided that the nominal maximum stress remains below the yield strength; cases exceeding the yield strength show reduced gains. Crucially, all HFMI data points—including those loaded beyond the yield strength—lie above the draft IIW design lines.