<p>This study evaluated on the strain caused by the release of residual stress induced by welding as a fatigue crack initiates and propagates. The change in the strain in the unloading condition owing to fatigue crack initiation and propagation was confirmed by fatigue tests conducted on box-type specimens with vertical stiffeners and out-of-plane gusset welded-joint specimens. Next, a strain monitoring system employing small IoT datalogging equipment was demonstrated by detecting the change in strain owing to crack propagation during a fatigue test. Finally, the weld residual stress in the out-of-plane gusset welded-joint specimen was reproduced using a finite element analysis to confirm that the residual strain at the weld toe changed owing to the propagation of a fatigue crack.</p>