Quantitative correlation of microstructure and fracture toughness for high strength steels and their welds (355-890 MPa)

The Microtough project was targeted to postulate a correlation between microstructural characteristics and fracture toughness values to allow for quantification of a microstructure's influence on toughness without performing expensive fracture mechanics tests. Additionally, research work was carried out to modify, develop and test, fracture toughness models for modern high strength structural steels with high cleanliness. CEIT's lower shelf toughness model containing parameters to be determined in metallographic investigations assumes that different crack arrest barriers like particle-matrix interfaces and grain boundaries have to be overcome by a cleavage microcrack which has been nucleated in a brittle particle. Quantification of lower shelf fracture toughness has been performed successfully for three different low temperatures and three materials. Additionally, also the influence of different crack barriers at different temperatures could be quantified. RWTH and MPA employed damage models (GTN and Rousselier) for quantification of upper shelf load-deformation behaviour and characteristic crack initiation values Ji,phys of five base materials. Most model parameters were determined experimentally. The validity of experimentally determined parameters was verified by tensile tests with notched, round specimens. Prediction of the ductile material behaviour and quantification of materials' upper shelf toughness was performed successfully. MPA has correlated results of tensile tests and microstructural or fractographical investigations with characteristic upper shelf fracture toughness value Ji,phys according to different analytical models by Stroppe, Stüwe and Ritchie. For calculation of Ji,phys using quantitative microstructural as well as fractographical data, only two quantitative relations exist, and the limitations of these two relations could be shown in the Microtough project.