Crack propagation simulations in steel welded joints for off-road vehicles

In a previous study, steel welded joints used in off-road vehicles and consisting of a pipe inserted into a plate through four intermittent fillet welds, were subjected to uniaxial fatigue testing. Two different joint configurations were examined: one with welds aligned (longitudinal joints) and the other with welds perpendicular (transverse joints) to the loading direction. The Peak Stress Method (PSM) was applied to estimate (i) the location where cracks would initiate and (ii) the fatigue life of the joints. The PSM correctly identified the crack initiation points, in agreement with the experimental observations. However, for the transverse joints, the experimental fatigue life was significantly longer than that predicted by PSM; this was attributed to the exceptionally long crack propagation phase that is inherently excluded by the PSM approach. On the other hand, the PSM provided an accurate estimation of the fatigue life for the longitudinal joints, for which the crack propagation phase was considerably shorter. To investigate such behaviour in more detail, crack propagation simulations have been performed in the present work using Abaqus® as FEM solver and FRANC3D® as pre- and post-processor. The fatigue crack propagation has been simulated for both joint geometries starting from a semi-circular pre-crack with depth ai = 0.1 mm and located at the experimental crack initiation location, up to final failure. The comparison of the trends of the crack driving force Ki as a function of the crack length a has allowed to justify the different crack propagation behaviour of longitudinal and transverse joints.