Fracture behaviours of fracture toughness testing specimens with metallurgical heterogeneity along crack front

Safe use of welded structures is dependent on fracture mechanics properties of welded joints. In present research, high strength low alloyed HSLA stee l in a quenched and tempered condition, corresponding to the grade HT 80, w as used. The fluxo cored are welding process (FCAW), with CO2 as shielding gas, was used and two different tubular wires were selected. The aim of this paper is to analyse fracture behaviour of undermatched weld ed joints, and also to determine relevant parameters which contribute to hi gher critical values of fracture toughness. Towards this end three differen tly undermatched welded joints were analysed using results of testing the c omposite notched specimens with through thickness crack front positioned pa rtly in the weld metal, partly in heat affected zone (HAZ) and partly in ba se material (BM). The presence of different microstructures along the pre-c rack fatigue front has an important effect on the critical crack tip openin g displacement (CTOD). This value is the relevant parameter for safe servic e of welded structure. In the case of specimens with through thickness notch partly in the weld me tal, partly in the heat affected zone and partly in the base material, i.e. using the composite notched specimen, fracture behaviour strongly depends on a partition of ductile base material, size and distribution of mismatchi ng factor along vicinity of crack front. Ii local brittle zones occur in th e process zone, ductile base metal can not prevent pop-in instability, but it can reduce it to an insignificant level while the fracture toughness par ameter is higher and the weakest link concept can not be applied.