Experimental and numerical analysis of fillet-welded joints under low-cycle fatigue loading by means of full-field techniques

The welded structures used in the naval field are generally subjected to fluctuating stress over time. In some structural welded details, due to changing loading conditions, significant elastic-plastic deformation can arise, which may lead to the failure of the structure after a relatively low number of cycles. The aim of this scientific work was to investigate the behavior of welded T-joints under low-cycle fatigue using full-field techniques: digital image correlation and infrared thermography. Low-cycle fatigue tests were carried out on welded small-scale specimens with the aim of obtaining loading and boundary conditions similar to those that occur in large-scale components in their real operating conditions. A nonlinear finite element analysis was also performed. The material curves, relative to different zones (base material, heat-affected zone, weld), were obtained by hardness measurements, which were done by means of a fully automated hardness scanner with high resolution. This innovative technique, based on the ultrasonic contact impedance method, allowed to identify the different zones (base material, heat-affected zone, weld metal) and to assess their cyclic curves, which were considered in the finite element model. Finally, the finite element model was validated experimentally comparing the results with the measurements obtained using the digital image correlation technique. The applied procedure allows providing useful information to the development of models for the prediction of fracture and fatigue behavior of the welded joints under the low-cycle fatigue loading.