Fatigue assessment of high‐frequency mechanical impact (HFMI)‐treated welded joints subjected to high mean stresses and spectrum loading

Abstract

The fatigue strength of welded joints can be improved with various post‐weld treatment methods. High‐frequency mechanical impact treatment is a residual stress modification technique that creates compressive residual stresses at the weld toe. However, these beneficial residual stresses may relax under certain loading conditions. In this paper, previously published fatigue data for butt and fillet welded joints subjected to high stress ratios and variable amplitude cyclic stresses were evaluated in relation to the current International Institute of Welding (IIW) recommendations on fatigue strength improvement and a proposed IIW design guideline for high‐frequency mechanical impact‐treated welded joints. The evaluation showed that the current IIW recommendations resulted in both non‐conservative and overly conservative fatigue strength estimations depending on the applied stress level, whereas the proposed fatigue assessment guideline fitted the current data well.

A finite element study on residual stress stability and fatigue damage in high-frequency mechanical impact (HFMI)-treated welded joint

“…However, benefit from HFMI-treatment remained even for Smin = -0.8fy with respect to similarly loaded AW state. This is in agreement with experimentally observed benefit from HFMI-treatment under VA loading [14,21,35,36]. Similarly, both the fatigue damage analysis and experiments [8][9][10] have indicated benefit from HFMI under R = 0.5 CA loading.…”

“…Current International Institute of Welding (IIW) fatigue design recommendations on post-weld improvement take residual stress relaxation into account by limiting allowable nominal stresses in these structures [5]. However, previous work [14], where available experimental high stress ratio and variable amplitude (VA) loading data were analysed, has indicated that the proposed peak stress limit for HFMI-treated joints subjected to fully-reversed loading [2] is too conservative in the case of variable amplitude (VA) loading.…”

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

“…From residual stress relaxation point of view, it is expected that relaxation will occur during one critical cycle due to either high mean stress or large compressive stress. As stated in [6], Equation 2and (3) fit available high stress ratio data reasonably well without being overly conservative.…”

“…damage sum FAT characteristic fatigue class in MPa corresponding to 2×10 6 cycles at failure with a survival probability of 95% based on two-sided confidence limits at a confidence level of 75% ( High-frequency mechanical impact (HFMI) is an effective and user-friendly means for improving the fatigue strength of welded steel joints. In addition, the method provides potential for lightweight design, as the fatigue strength of HFMI-treated joints increases with increasing steel strength [1,2].…”

“…To determine the applicability of the allowable stress ratio and stress range limits proposed in [3], fatigue data on high stress ratio and variable amplitude (VA) loading were analysed statistically by Mikkola et al [6,7]. The data were compared to the proposed characteristic curves [3] using nominal stress method as described in [6]. Three joint types were investigated: double-sided non-load-carrying transverse attachments, double-sided longitudinal attachments and butt joints.…”