T-butt welded joints are found in many structural steel applications including buildings, bridges and offshore structures and are susceptible to fatigue crack initiation and propagation,which often leads ultimately to fast fracture failure. An example ofthis was the I-35W bridge in Minneapolis, which collapsed in 2007 resulting in 13 fatalities, as shown in Figure 1 . The experimental work for this project was conducted using A350 grade black mild steel plate. An ultrasonic peening treatment was applied to one T-butt specimen to introduce compressive residual stress at the weld toe, in order to reduce the effective fatigue crack propagation rate. The results generated from 3-D FEA modelling plus a FORTRAN program (implementing parametric stress intensity factor and crack propagation equations) will be compared with experimental fatigue test results.