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 tests were performed at three R-ratios (-I, -2 and O) and different stress amplitudes on cruciform welded joints both in the as-welded and stress relieved conditions.A sensitive system using micro-strain gages installed close to the weld toe allowed monitoring of the crack length and opening level from 10 -20 pm. The effect of residual stresses on crack propagation and closure behavior is described for different loading conditions.The experimental data confirm the predictions that can be derived from an elastic analysis based on the crack closure concept, such as a more pronounced effect under partially compressive loading but reaching a saturation below a certain value of R. Further, they give a direct evaluation of the relative residual stress intensity KrlKmax and of its progressive decrease as the crack grows, as well as a better understanding of plasticity effects. G. Beck et al. (eds.), International Conference on Residual Stresses

An experimental study was undertaken on the early development of fatigue cracking along the wavy toe of a manual fillet weld between structural steel plates. Stress relieved and as-welded cruciform specimens were tested under fully reversed loading at two stress amplitudes. Miniature strain gages installed along the toe were used to monitor the crack depth and crack opening level from a crack depth of 10-20 pm. A calibration of gage response versus crack depth from a 2D finite element analysis is shown to be acceptable. The analysis of the gage responses and of the fracture surfaces including beach marks of short cracks has revealed that discontinuous straight-fronted cracks rapidly form along the edge of the successive weld ripples. Their aspect ratio increases until some coalescence results from a progressive bridging between the adjacent crack planes. As the step of the ridge line decreases, the cracks fully coalesce into one uniform through-crack. Faster growth rates associated with negative opening levels are recorded in as-welded specimens. The welding residual stress distribution makes initiation and early crack propagation concentrate at the center of the specimen which results in a long semi-elliptical crack.