**Preface to "Numerical and Experimental Analysis of the Fracture Behaviour of Heterogeneous Welded Structures"**

Welding is the most widespread technology for the connection of different materials, elements and structures. Loading capacity and knowledge about stress–strain behaviour is most important for safety and the reliable use of structures for different purposes in energy supplies and the transport of good and people. Filler material used in welding should be the same class as the base material; however, welding codex often uses a filler with a higher yield strength than the base material (such as by repair welding). Welded joints are also often the location of potential flaws, where flaw assessments assume homogeneous material properties, although welds are heterogeneous. There is a compendium of yield load solutions for mismatched strength fracture mechanics specimens developed to address the heterogeneity in the weld joint. However, solutions for the yield load at different combinations of the strength mismatch within the weld are missing, where mechanical testing and finite element simulations are necessary. In addition to more conventional approaches, a multi-scale approach recently introduced in the assessment of weld heterogeneity sounds very promising. It is also efficient to consider residual stresses, which can strongly affect the stress distribution around flaws in heterogeneous weldments. The multi-scale methodology is computationally efficient and provides a possible means to bridge multiple length scales (from 10 nm in MD simulation to 10 mm in FE models). This could be a useful tool by considering an acceptable level of accuracy with respect to yield load in heterogeneous welds.

In this book, modern trends in testing and simulating heterogeneous welded joints, including multi-scale approaches, resulting in appropriate flaw assessment procedures, are highlighted and discussed. The eleven research papers presented in this book give some overview of recent analytical, numerical, and experimental investigations in the field of yield strength mismatched welded joint behaviour. The papers cover several important issues to more accurately characterise the fracture mechanics behaviour and structural integrity assessment, as follows:







The editors of this book hope that the readers will find this chapters interesting and useful for their everyday work in this challenging area.

### **Drazan Kozak, Nenad Gubeljak, Aleksandar Sedmak ˇ**

*Editors*
