Stress concentration factors and fatigue behaviour of welded thin-walled CHS-SHS T-joints under in-plane bending

Fatigue tests were carried out on welded thin-walled (t<4 mm) T-joints made of circular hollow section (CHS) braces welded onto square hollow section (SHS) chords, under the loading conditions of "in-plane bending in the brace". Stress distributions were measured at different hot spot locations around the chord-brace junction, where cracks were observed to initiate and grow causing fatigue failure. The degree of non-linearity from the measured stress distributions around the weld toes of the brace-chord junction showed that stress distribution is significantly non-linear, along specified locations for brace diameter to chord width ratios greater than 0.5. The quadratic method of extrapolation of local strains/stresses to determine hot spot stresses is recommended. Using the measured stress distributions from this investigation and results from previous research, hot spot location lines that produce the highest hot spot stresses are recommended. The stress concentration factors (SCFs) for the welded thin-walled CHS-SHS T-joints lie in between the predicted values of SCFs for CHS-CHS and SHS-SHS T-joints for given non-dimensional parameters. A method is suggested for estimating SCFs for welded thin-walled CHS-SHS T-joints using existing parametric equations for SHS-SHS T-joints. An end of test failure criterion is chosen and shown to be a reliable method for obtaining fatigue data that can be used for producing design S-N curves. A through thickness crack is shown to occur when a surface crack has grown to a length equal to about 40% of the circumference of the weld toe in the chord.