Allowable stresses in high-frequency mechanical impact (HFMI)-treated joints subjected to variable amplitude loading

Abstract

The effectiveness of high-frequency mechanical impact (HFMI) is considered to rely on the existence of compressive residual stresses. To determine when residual stress relaxation occurs, and what the resulting influence on fatigue improvement is, local stress-strain response in as-welded and HFMI-treated weld toes was modelled under different peak stress conditions. Then, effective notch stress analysis was used to correlate these results with available experimental observations. The simulations showed that high stress ratios and compressive peak stresses were critical with respect to residual stress relaxation, as expected. A compressive peak stress of 0.6fy (nominal yield strength) resulted in full residual stress relaxation. The relative fatigue damage calculations and the notch stress analysis indicated, however, that fatigue improvement could be expected even after significant residual stress relaxation. Based on this and previously observed benefit for high stress ratios, an increase in maximum allowable stresses for HFMI-treated welded steel joints is suggested. The maximum stress ratio is proposed to be increased from R = 0.52 to R = 0.7 and the maximum stress range to limit compressive stresses is proposed to be increased from ΔSmax = 0.9fy to ΔSmax = 1.2fy, which corresponds to Smin =-0.6fy for stress ratio R =-1.

Mean Stress Effect in High-Frequency Mechanical Impact (HFMI)-Treated Steel Road Bridges

“…Figures [11][12][13] show that both the investigated fatigue load models underestimate λ HFMI values in many cases, which indicates that the mean stress effect could not be captured using one of these models in general. It is noteworthy that the maximum deviation corresponds to shorter span lengths, as shown in the figures.…”

“…The penalty method provided by the IIW is only applicable for stress ratios of up to 0.52. Nonetheless, several research articles have investigated the effect of HFMI-treatment on the fatigue strength of welded joints subjected to higher R-ratios [5][6][7][8][9][10][11]. An extension of the method proposed in the IIW recommendations was suggested in [7] to cover higher R-ratios, where four reduction classes were proposed for R-ratios greater than 0.5.…”

Fatigue properties of as-welded and post-weld-treated high-strength steel joints: The influence of constant and variable amplitude loads

“…So far in the literature, HFMI studies have mainly focused on the degree of improvement in fatigue strength at constant amplitude loading (CAL) [9] [5] [10]. Studies dealing with HFMI material characterization have also been performed in a limited manner including the investigations on highly cold-worked region [11], hardness dependency on steel strength [12], influence of operating parameters [13] and the microstructure after fatigue loading [14].…”

“…On the contrary the treatment may decrease the fatigue life at high-applied strains. Based on the numerical simulations, Mikkola et al [17] [9] have shown that full residual stress relaxation may occur at CAL for R = 0.5 and a compressive peak stress of 0.6 f y . Nevertheless, improvement in fatigue strength could be expected even after significant residual stress relaxation due to work hardening and modification of weld to profile.…”

“…For instance, Schubnell et al [18] have shown that the effect of work hardening may result in significant fatigue life improvement for S355 mild steel. Additionally, the study of Mikkola et al [9], considering S700 steel, has indicated that benefit from HFMI could be still expected due to improved weld geometry and strain hardening even full stress relaxation has been observed at R = 0.5.…”