Characterisation of equivalent HAZ in butt-welded high-strength and ultra-high-strength steels

Numerical and analytical models incorporated with detailed heat-affected zone (HAZ) subzones are impractical due to the intricate strength gradient. Furthermore, when performing numerical calculations using damage mechanics, they require a highly refined finite element mesh and enormous computational power. Using an equivalent HAZ with the characterised strength and width to represent the entire HAZ could be a practical approach to account for the HAZ strength reduction with high accuracy and low computational cost. In the presented study, the equivalent strength and width are characterised for HAZ in high-strength steel (HSS) and ultra-high-strength steel (UHSS) butt-welded connections. A parametric study is carried out through finite element (FE) analysis. The investigated parameters include the strength gradient, HAZ width, bevel angle, plate thickness, and width of butt welds. The first moment of area approach is proposed to quantify the equivalent strength of HAZ for a single layer of the connection. The HAZ area in each layer is used as a weighting factor to consider the strength variation in the thickness direction. A Macro+ model integrated with equivalent HAZ and the equivalent strength of the weld metal is proposed to predict the tensile resistance of HSS and UHSS butt welds. The Macro+ model is successfully validated against the FE and experimental results with a maximum of 4.7% and 8.3% deviation, respectively.