Three numerical analysis methods are available and current editions of the code should be consulted for further details.

Elastic Stress Method

In this method, equivalent stresses (von Mises) are calculated using an elastic analysis, classified into categories and compared to allowable values. The different categories are: general primary membrane, local primary membrane and local primary membrane plus bending.

Limit Load Method

This method involves determining a lower bound to the limit load of a component. The allowable load involves applying design factors to the limit load such that the onset of plastic collapse will not occur.

The code consists of using scaled loads and an elastic perfectly plastic material model. Acceptability of the design is based on numerical convergence for the scaled loads.

Elastic Plastic

The collapse load is determined from both the deformation characteristics of the component and the applied loading. The allowable load is established by applying design factors to the plastic collapse load. The material model is elastic plastic and scaled loads are used. The analysis includes non linear geometry and convergence must be obtained for the scaled loads for the design to be considered compliant.

Conclusion

It is not always straight forward deciding which method to use and an experienced engineer can help. It is important to note that any of the three methods are acceptable by ASME code and a component design is considered acceptable if any of the three analysis methods are satisfied.