Crack driving force prediction in heterogeneous welds using Vickers hardness maps and hardness transfer functions

Flawed welds often require an Engineering Critical Assessment (ECA) to judge on the necessity for weld repair. ECA is a fracture mechanics based prediction of the integrity of structural components with defects under operating conditions. Adding to the complexity of a weld ECA is the occurrence of local constitutive property variations in the weldment ('weld heterogeneity'). Their quantification allows for a more accurate assessment compared to common (standardized) practice, which assumes welds to be homogeneous. Hardness measurements allow to quantify weld strength heterogeneity given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of relations ('transfer functions') between hardness and strength, and recognize substantial scatter in hardness based predictions of strength. Within this context, this paper investigates the suitability of Vickers hardness mapping to perform an accurate weld ECA for high strength low alloy steel. To overcome the scatter associated with standardized transfer functions, this paper suggests an experimental calibration procedure based on all weld metal tensile tests. Finite Element (FE) analysis has been conducted on welds originating from steels to simulate their crack driving force response in Single-Edge notched Tension (SE(T)) specimens. Vickers hardness maps and hardness transfer functions are combined to assign element-specific constitutive properties to the model. The transfer function calibrated by all weld metal tensile tests yields a better agreement with experimental load-CTOD curves than transfer functions mentioned in standards and codes. Finally, a step-by-step procedure facilitates a practical adoption of the methodology.