Ductile fracture of WAAM steel: Experiments, simulations and model calibration

• Ductile fracture tests on WAAM steel specimens covering various stress states. • Superior fracture ductility compared to equivalent, conventional steels observed. • Development and validation of numerical models for WAAM fracture specimens. • Calibrated fracture models provide accurate predictions for WAAM steel plates. Utilising robotic welding technology, wire arc additive manufacturing (WAAM) is developing into a viable method for the production of structural steel elements. However, there remains a lack of knowledge on the fracture ductility of WAAM steels, which is of particular importance under extreme loading conditions, such as severe earthquakes. An experimental and numerical study into the ductile fracture behaviour of WAAM ER70S-6 carbon steel plates has therefore been undertaken and is reported herein. ER70S-6 steel was selected for this study for its widespread use in WAAM and its well-characterised and desirable mechanical properties, facilitating comparisons of fracture ductility with conventionally-produced steels. Following mechanical and microstructural characterisation, a series of WAAM fracture specimens of two thicknesses and various shapes (corresponding to various stress states) was tested under quasi-static tensile loading. Digital image correlation (DIC) was employed to monitor the evolution of the deformations and strains of the specimens during testing. The WAAM steel exhibited superior fracture ductility compared to equivalent, conventionally-produced steels, with the thicker material showing higher ductility than the thinner material. Finite element (FE) models of the fracture specimens were developed and validated, with good agreement between the simulated and measured load-displacement curves and strain fields demonstrated. Through the experiments and simulations, the equivalent plastic strain at fracture and the corresponding stress triaxiality and Lode angle parameter were determined for each specimen. The obtained fracture data were used to calibrate three typical ductile fracture models – the Modified Mohr-Coulomb (MMC), Hosford-Coulomb (H-C) and Lou-Huh (L-H) models. All the models were shown to provide accurate predictions of fracture initiation for the WAAM ER70S-6 steel. The calibrated fracture loci represent a step forward for the prediction of ductile fracture in WAAM steels.