Numerical ductile fracture assessment of weldments in direct-quenched, ultra-highstrength steel

Abstract

Many studies conducted in recent years have raised concerns about the ultimate loadcarrying and elongation capacities of weldments made of direct-quenched, ultra-highstrength steels (UHSS). The reason for this is the formation of a soft region in the heataffected zone (HAZ) of the welds. Also, undermatching filler wire must be used in the welding of some of the highest available steel grades. Hence, the welds in these steels have a complex metallurgy with regions that have mechanically distinct zones. Numerical methods, especially finite element analysis (FEA), comprise a common tool in the analysis of steel structures. In many fields, for example in the military and the automotive and aeronautics industries, FEA is used in the assessment of ductile fracture. While multiple different material models capable of assessing ductile fracture have been developed for base materials, these models have not been used for welded structures. The literature indicates that analysing the mechanical behaviour of welds with different zones requires accurate material models that consider hardening behaviour. To investigate this complex phenomenon, numerical methods must be used. In response, this thesis conducts a numerical ductile fracture assessment of friction-welded joints. Prior to the assessment, accurate material models for the used materials were acquired using experimental tensile test specimens. The data from the experiments were inputted into an optimisation routine to identify the material parameters for the constitutive and fracture models. To generate an accurate model of the HAZ, additional friction wielding specimens were manufactured. By using hardness measurements, thermal simulations and an experimental tensile test with a digital image correlation system, a discretisation of the HAZ was conducted and constitutive models were generated for the discretised HAZ regions. The findings of the thesis demonstrate the feasibility of numerical ductile fracture assessment. In addition to the fracture assessment, three weld details were analysed. The results from these specimens indicate that the soft HAZ might be detrimental to the load-carrying and deformation capacities of the weldments. The parametric studies also indicate that by using numerical ductile fracture assessment, detrimental geometrical configurations can be avoided in weldments. This enhanced knowledge, in terms of numerical ductile fracture assessment, could benefit the steel industry in developing high-strength steels with enhanced weldability.