Secondary Stress according to ASME

[dcattoni]

Hi Guys

I would like to know how to identify and calculate the secondary stress in a pressure vessel using FEM.

Thank!

 

[TGS4]

Oh my. First off, welcome to eng-tips. We have a search engine that will allow you to find previous responses to similar questions.

Now, to answer your question... You are asking a question in regards to how does one categorize (i.e. put into categories of primary, secondary, peak) linearized and classified stresses (membrane, bending, peak). Entire books have been written on this topic, so I will try to provide a concise summary.

FEA will provide you with total stresses. It is your job to first linearize the stresses, then classify the stresses. To do so, you will need to locate valid and validated stress classification lines - guidance on how to do this is provided in ASME Section VIII, Division 2, Part 5, Annex 5-A. Then, based on the nature of the loads (or load ranges) and these classified stresses, you will need to categorize these stresses. Some guidance on how to do this is provided in Table 5.6, but that table is a bit of a mess.

Read and understand the following statements from Part 5:

For components with a complex geometry and/or complex loading, the categorization of stresses requires significant knowledge and judgment. This is especially true for three-dimensional stress fields. Application of the limit load or elastic-plastic analysis methods in 5.2.3 and 5.2.4, respectively, is recommended for cases where the categorization process may produce ambiguous results.

The use of elastic stress analysis combined with stress classification procedures to demonstrate structural integrity for heavy-wall (R/t<4) pressure containing components, especially around structural discontinuities, may produce non-conservative results and is not recommended. The reason for the non-conservatism is that the nonlinear stress distributions associated with heavy wall sections are not accurately represented by the implicit linear stress distribution utilized in the stress categorization and classification procedure. The misrepresentation of the stress distribution is enhanced if yielding occurs. For example, in cases where calculated peak stresses are above yield over a through thickness dimension which is more than five percent of the wall thickness, linear elastic analysis may give a non-conservative result. In these cases, the elastic-plastic stress analysis procedures in 5.2.3 or 5.2.4 shall be used.

The structural evaluation procedures based on elastic stress analysis in 5.2.2 provide an approximation of the protection against plastic collapse. A more accurate estimate of the protection against plastic collapse of a component can be obtained using elastic-plastic stress analysis to develop limit and plastic collapse loads. The limits on the general membrane equivalent stress, local membrane equivalent stress and primary membrane plus primary bending equivalent stress in 5.2.2 have been placed at a level which conservatively assures the prevention of collapse as determined by the principles of limit analysis. These limits need not be satisfied if the requirements of 5.2.3 or 5.2.4 are satisfied.

 

[dcattoni]

Hi TGS4

Thank for the answer!

When I finish the fem pressure vessel model and I get the result, surely I'll be consult you. If you do not mind!!

Thanks!!!