A material failure model is implemented using the user material option in the finite element program ABAQUS, to describe ductile failure. The model uses a continuum damage mechanics (CDM) based relation to describe the dependence of the failure strain on stress state, and simulates element failure via material softening. The utility of the model in predicting the first onset of failure is demonstrated by comparing finite element predictions with experimental results taken from the literature. Experiments describing the quasi-static ductile failure of tubular steel members under a variety of different conditions have been used.
The numerical simulation of ductile fracture is of value in many situations where ultimate material failure is possible under severe loading. Some examples are the piercing of pipelines due to accidental loads from excavation or dropped objects, the failure of offshore structures due to ship impact and the fracture of nuclear containment vessels under accidental impact. The J-integral based crack growth approach has been used as part of finite element codes for many years and has shown good agreement with measured results for cracking failure. This approach has major drawbacks in many practical engineering analyses of structures, however, as the crack path must be known a priori. Explicit codes such as ABAQUS/Explicit or LS/Dyna have been used to model ductile failure, commonly using a specified maximum equivalent plastic strain as a failure criterion. This, however, is not a very accurate failure model in any but uniaxial stress cases as it does not take into account the dependence of failure strain on the current stress state of the material. The (modified) Gurson ductile failure model (Gurson 1977; Tvergaard 1981) is more suitable for such analyses as it can predict the site of failure as well as failure load.
