Standardized weld flaw assessment techniques assume the weld region to be homogeneous which is a strong idealisation of reality. Characterising the effects of heterogeneous properties of welds through the analysis of deformation patterns and slip lines is the major concern of this research. It is the goal to investigate which effects these variations in properties within the weld material have on the propagation of cracks within the weld material. Performed experiments are SENT tests on strongly heterogeneous welded connections. The same material is also simulated with a weld heterogenisation model in ABAQUS®. Results from both experiments and simulations are discussed and compared. It is shown that slip lines tend to avoid zones of high hardness in a way that a path of least resistance is found. Related to this, it is seen that the slip line angles deviate from the theoretical 45° for homogeneous material. Obtained results validate the numerical model used.