Fraunhofer Institute for Mechanics of Materials IWM
The crack initiation phase in welded joints — that is, where cracks form and how they grow — is of central importance for assessing component safety. Due to the heat input generated during welding, several microstructural zones (base metal, heat-affected zone, weld metal) with different mechanical properties form side by side. Precise knowledge of crack initiation is necessary, as the fracture-mechanical fatigue life assessment is based on an existing crack with a defined depth and width. Early fatigue crack propagation occurs along the slip planes of the individual grains and can be described by microplastic cyclic deformation. The micromechanical properties of the polycrystalline microstructure are decisive in this context. Numerical modeling of short-crack growth accounts for this microstructure and allows for a clear distinction between the effects of local materials properties and weld geometry, as well as the notch effect at the weld transition. Such damage mechanisms were demonstrated using fine-grained structural steel (S355NL) as an example.
The challenges lie in differentiating and characteristically evaluating the various microstructural zones of the weld, which exhibit significant differences in mechanical properties. In particular, the correct preparation of microprobes from the different microstructural regions to determine the respective mechanical properties is crucial. Furthermore, transgranular crack initiation within the individual grains and the transition of the crack plane between adjacent grains must be taken into account, as it has a direct impact on the macroscopic properties of the welded joint.