Early Development of Fatigue Cracking at Manual Fillet Welds

Abstract

An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as-welded cruciform joints were tested under R = -1 and R = 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10-20 pm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the "initiation life" (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as-welded specimens are faster than in stress relieved specimens, which results in shorter "initiation lives". This is associated with a higher effective stress range, particularly under R = -1 loading where cracks are open over nearly the full stress range. The V-notch stress intensity factor is a promising parameter to rationalize the crack "initiation life". It takes into account the thickness effect experimentally observed. Under R = -1 loading of as-welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack "initiation life".

Fatigue strength of steel and aluminium welded joints based on generalised stress intensity factors and local strain energy values

“…Initially thought of as parameters suitable for predicting only the fatigue limit (Atzori, 1985) or the fatigue crack initiation phase (Boukharouba et al, 1995;Verreman and Nie, 1996), NSIFs were found capable of predicting also total fatigue life (Lazzarin and Tovo, 1998;Atzori et al, 1999aAtzori et al, , b, 2002Lazzarin et al, 2003Lazzarin et al, , 2004. This happens when a large amount of life is consumed at short crack depth, within the zone governed by the V-notch singularity.…”

On the accuracy of nominal, structural, and local stress based approaches in designing aluminium welded joints against fatigue

“…Accordingly, Verreman and Nie argued that this stress parameters could be employed directly to model the crack-initiation process in fillet welded joints subjected to fatigue loading [63]. A couple of years later, this approach was further developed by Tovo and Lazzarin who devised a more rigorous theoretical framework by proposing a formal definition for the N-SIFs [15].…”

Fatigue damage criteria classification, modelling developments and trends for welded joints in marine structures

“…Lassen 1990;Verreman and Nie 1996;Darcis et al 2006;Zhang and Maddox 2009;Hobbacher 2010;Chattopadhyay et al 2011;Zerbst and Madia 2015) and (calculation) values decreased from 1.00 [mm] down to 0.05 [mm], meaning even for welded joints, although the life time is often assumed to be growth dominated, the crack initiation contribution to the total life time increases, in particular in the HCF region.…”

“…For decreasing ρ the linear elastic notch stress becomes asymptotic and rather than a(n effective) local value a Williams' solution σ(r λ−1 , θ) based weld notch stress intensity factor (NSIF) can be introduced (Verreman and Nie 1996); an intact geometry-and notch stress gradient (area/volume) criterion:…”