Modeling of ductile damage in metal forming

The prediction of ductile fracture during material forming processes has been of great interest to the scientific and engineering communities over the last century. Indeed, understanding and modelling ductile damage mechanisms remains a key issue for obtaining defect-free products using increasingly complex forming processes and for increasingly high-performance materials. A multitude of phenomenological and micromechanical models have been developed over the last fifty years to predict ductile fracture. These models are generally validated for a given loading path (mostly under monotonic loading) and specific materials. Thus, their ability to be extended to other configurations – in terms of loading and materials – is often questionable. Given the multitude of models proposed in the literature, it is difficult for an engineer to choose the most appropriate model for his process and material. What's more, the procedure for identifying model parameters (model calibration) is a very important step that should not be overlooked. As we shall see in this article, some models are very basic and easy to use. They require only a few tests to identify the model parameters, but often give only a qualitative response and no precise prediction of the moment of failure. Other models are much more sophisticated and more precise in terms of predicting the instant of failure. However, they require extensive testing and sometimes microstructural observations to correctly identify the model parameters. Let's get the suspense out of the way now: the perfect, universal, easy-to-calibrate model doesn't exist, and every engineer will have to choose from the multitude of models on offer.

The aim of this article is to help these engineers make the right choice, depending on the type of process being studied and the resulting state of stress. This choice should also take into account the accuracy required and the ability to carry out the (sometimes numerous) calibration tests. In this article, we will not consider Cohesive Zone Models ( – CZM), whose use is more suited to multilayer structures or bonded assemblies undergoing limited plastic deformation. These approaches are therefore little, if any, suited to the context of materials forming.

The article is divided into five main parts and a final synthesis. The first part recalls the fundamental mechanisms of ductile fracture in the context of metal forming. We place particular emphasis on the influence of the stress state on the mode of fracture, and strongly recommend reading the article by Montheillet and Briottet