Failure assessment through various uncoupled damage models in flow forming processes

Flow forming is a unique metal forming process that utilizes specially designed rollers and a mandrel to shape metal parts with high dimensional accuracy and structural integrity, especially for thin-walled, seamless tubes. Due to the incremental nature of the process, material is subjected to complex stress states and significant plastic deformation that can lead to many types of defects and cracks, and therefore it is crucial to make accurate forming limit predictions to optimize the manufacturing process. This study investigates the capabilities of uncoupled damage models in predicting fracture initiation and formability limits during the flow forming of IN718 alloy. In this regards ten different damage criteria are employed in Finite Element (FE) simulations including Ayada, Ayada-m, Brozzo, KH, Le-Roy (LR), McClintock (MC), Oh (OH), Rice-Tracey (RT), which are calibrated with tensile tests. Subsequently, these models are applied to flow forming simulations at varying thickness reduction ratios (37.5%, 50% and 70%). The results are compared with experimental trials to assess the prediction accuracy of each model regarding formability limits and fracture initiation. The initial investigations demonstrate that the Ayada criteria are better in predicting damage at all reduction ratios compared to other criteria, making them particularly suitable for this process. In addition, the KH model gives comparable predictions, although it is not successful at all reduction ratios.