ASME VIII-2 Part 5 - 5.3.2 (Elastic Analysis - Triaxial Stress Limit) - Toe of Welds

[marty007]

I'm just curious how other members of the forum experienced with ASME VIII-1 Part 5 handle the local failure criteria limit at the toe of welds.

Take a nozzle for example, with external fillet welds. If the welds are modelled as simple 45deg fillets, then singularities exist at each toe of the weld. As the mesh is made finer and finer, the triaxial stress at this location will increase to infinity.

What methods do other members use to work around this issue?

 

[TGS4]

Two issues that are apparent is the use of an elastic stress basis for a local criterion and the stress category that is used with this criterion. It is not apparent how pseudo elastic stresses, i.e. elastically calculated stresses that exceed the yield strength can be used to evaluate a local fracture strain of a ductile material with strain hardening. In addition, the type of stress used in the criterion (i.e. linearized or average values verse stress at a point) and stress category (i.e. primary, secondary and peak) needs to be resolved. Since local failure is the failure mode being evaluated, the type of stress and stress category used in VIII-3 would appear to more correct. For ductile materials, a local criterion based on elastic analysis may not meaningful and the elastic-plastic method that follows is recommended for all applications.

First thing: the current rules state that you are to use the linearized primary principal stresses. So, the peak stresses at the toe of a weld would be essentially irrelevant to a linearized stress. You ought not to be using the stress-at-a-point.

Second: see the quote above from ASME PTB-1. Use the elastic-plastic method. Period.