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FEA; Some questions - Desige according to ASME VIII Div2 Part5

GHEISAR (Mechanical)
3 Nov 11 8:35
Dear friends,
I've analyzed a part of pressure vessel (quick opening closure) with ANSYS and design it according to ASME VIII Div2 Part5. So I have some questions:

1- In some zones like small fillets and sharp edges (see pic1), there are some local stresses higher than yield strength of material. I have used linear elastic material in analysis. I just checked the elastic criteria of Asme2 (calculating membrane and bending stress in critical stress classification lines (SCL) and comparing with allowable limits).  Do I have to check the limit load and elastic plastic criteria too? (Note that there isn't so much difference in SCL's stress and total deformations when I use nonlinear material and turning on the large deformation option)

2-I have classified the stress classification lines (SCL) in some critical zones but I can't determine which zone considered as general membrane and which one is in local membrane and secondary stress zone. For example (see pics 2,3,4): Door of closure has some teethes. In edge of teeth there is higher stress than in middle of it. Should I consider it as local membrane and secondary stress? (The software just tells me membrane and bending data and can't classify it as general or local) Is it right?

3- If in hydrotest we use the local stress to check in test criteria, test criteria shows that the part can't tolerate the test pressure in local zones. (My allowable stress is Fy/1.5 and if I design the model for this allowable, the stress will exceed the yield of material in test because the pressure's load is 1.5 times the design load!)
Is it true to omit the local SCL's in test conditions and just use SCLs which are in General mem and ben zone!?

4- I derive the allowable fatigue stress from fatigue curves for steel in 2000 cycle. It is 888 Mpa. I just check this number with my peak stress. Is it true or I must calculate the complicated procedure that mentioned in Asme?

Thanks a lot for your attention.
 
TGS4 (Mechanical)
4 Nov 11 13:04
1 - per 5.1.1.2

Quote (ASME Section VIII, Division 2, Part 5):

If multiple assessment procedures are provided for a failure mode, only one of these procedures must be satisfied to qualify the design of a component.
  (Note that you NEVER use SCLs for non-linear analysis - please follow the procedures exactly as described in the Code).

2 - per 5.2.1.2

Quote (ASME Section VIII, Division 2, Part 5):

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

3 - Following the analysis method of 5.2.2, please inform me where it is required to evaluate the hydrostatic test case.  Such a load case combination does not exist in Table 5.3.

4 - If you are performing an analysis to ASME Section VIII, Division 2, Part 5, then your fatigue analysis must follow one of the methods provided in Article 5.5.  Since such factors as load reversal (stress reversal) may occur, it is possible that your stress range may be substantially higher than could be assumed from a simple review of peak stresses.  The procedure is there for a reason - because it works.  It is complicated because fatigue evaluations are complicated.  Deviate at your peril.
GHEISAR (Mechanical)
8 Nov 11 14:20
Thanks a lot TGS4 for your attention and nice answers.

1-It is true that when we use SCLs, we can't use non-linear analysis but when we define contacts, the analysis automatically changed to non-linear although we use linear elastic steel. what should we do when we want to perform linear analysis with contacts?

2-when we use SCLs, we can accurately optimize our thickness of parts. but in limit load and elastic-plastic analysis, i understand that if the analysis converged, we just estimate that our parts are satisfactory and so we can't exactly optimize our parts.

3- See part 4.1.6.2, allowable stresses for hydrotest.
I think you told me that it doesn't need to evaluate the hydrotest conditions. Is it true?!

Best Regards  

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