High‐cycle variable amplitude fatigue experiments and design framework for bridge welds with high‐frequency mechanical impact treatment

Abstract

Fatigue enhancement by way of high-frequency mechanical impact (HFMI) treatment can enable effective design and construction of steel bridges. However, bridges may experience high and varying mean stresses, the effects of which are not covered today by any design recommendation or in the literature on HFMI-treated joints. In this study, fatigue experiments were conducted with realistic in-service bridge loading, which revealed the same high fatigue performance as for constant amplitude loading. The effect of mean stress in spectrum loading was quantified and a method to account for it in an equivalent manner is proposed. A design framework has been developed for design and engineering purposes.

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

“…More fatigue tests were conducted under high-stress ratios (0.5 ≤ R ≤ 0.8) in [8], and the results supported the trend of the IIW method in terms of reducing the fatigue strength class depending on the R-ratio. In principle, the trend follows a decrease in one fatigue strength class per 0.12 increase in the R-ratio [12,13], as can be seen in Figure 1. In a design situation, the R-ratios from traffic loads are unknown to designers.…”

“…If not, a correction method can be used to account for the mean stress effect from traffic loads in design. The authors have, in a previous publication [12], suggested such a method to consider the mean stress effect in the design of HFMI-treated road bridges. This method considers the mean stress effects produced by the combined effects of self-weight and traffic load based on the collective mean stresses produced by measured traffic data.…”

“…The former denotes a modified equivalent stress range to account for stress ratio effects using a magnification factor f, as can be seen in Equations ( 1)- (3). ∆S eqR and ∆S eq were calculated with an S-N curve slope of m = 5, which is recommended for HFMI-treated weldments [1,12]. The factor, f, is a continuous representation of the step-wise mean stress effect.…”

“…The factor, f, is a continuous representation of the step-wise mean stress effect. However, it corresponds to a magnification factor on the stress range instead of a penalty on the fatigue strength [12]. In addition to the variation in R-ratios from traffic load, the self-weight effect is considered through a factor Φ which denotes the ratio between the self-weight stress, S SW , and the maximum stress range in the load spectrum, ∆S max :…”

“…In [12], 55,000 measured lorries were run on influence lines for simply supported and continuous bridges with span lengths from 10 to 80 m, and different positions along the span starting from the midspan to the support. The used traffic was a result of 87 days of "weight in motion" measurements from 12 different locations in Sweden including different types of roads.…”