Fatigue of Welded Structures

Abstract: Fatigue of metals is yet an area of material science that is not completely well understood, and there are many topics that are still being investigated. However, fatigue of welded components is an even more complex phenomenon. Apart from thermal cycles during welding, which strongly affect the base material and induce residual stresses and distortion, the fusion process and the existence of a filler (in some processes) lead to very heterogeneous microstructures, varying in mechanical properties and chemical composition throughout the area of the joint. Additionally, weld defects like porosity, slag inclusions, undercuts and overlap among others, can significantly reduce the fatigue strength. All these features create suitable conditions for weld toe cracking that is apparently the most problematic region. Based on this, several improvement techniques have been developed and they have proven to enhance fatigue resistance by either inducing compressive residual stresses in the weld toe or modifying its shape into smoother geometries.
In the present work, fatigue strength of transverse non-carrying load fillet joint was analysed. High frequency axial fatigue tests were performed in as-welded and improved joints. TIG-dressing and shot peening techniques were employed to modify the weld toe into more convenient configurations. Microstructures obtained in both cases are discussed and related to crack initiation phase. Crack path and fracture surfaces were also investigated.
It was found that as-welded (AW) samples welded according specifications presents enhanced fatigue strength above FAT 90. TIG dressing fulfilled to create smoother toes and increased fatigue life by at least 50%. Testing of shot peened samples is still ongoing. An average increase of surface hardness around 10% was measured in the base metal (BM) with this technique. Compressive stresses can delay nucleation and growing of microcracks from the toe, which is translated into longer fatigue lives.