An Evaluation of the Protection Against Local Failure in ASME Section VIII, Division 2: Finite Element Model Considerations

The local failure criteria was a relatively new inclusion in the 2007 Edition of ASME Section VIII, Division 2. The elastic-plastic evaluation criteria for this failure mechanism was brand new to the Code. This failure mechanism introduced to the Code the concept of local strain limits based on the triaxial state of stress.

In implementing the evaluation of this failure mechanism, an interesting phenomenon was discovered. The triaxiality was to be calculated “everywhere” in the model, and used to create a limiting strain to be compared to the plastic strain at the point where said triaxiality was calculated.

As is well known, in the finite element method, stresses and strains are calculated at the Gauss Points, and extrapolated to the nodes. This uses the element shape function. One question that arises is: if the triaxiality is calculated at the Gauss Points, can it be extrapolated to the nodes using the same element shape functions? Another question that arises is: can the extrapolated values of stress and strain be adequately used to calculate the triaxiality at the nodes?

This paper discusses the validity of both approaches, and introduces a third option: a material property called Ductile Damage Initiation Criteria, and provides and example that demonstrate all three methods. Recommendations are provided about the best approach to be implemented.