Early development of fatigue cracking at manual fillet welds

An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking dong 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 mu m 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''.