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Allowable Stresses

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Allowable Stresses

Allowable Stresses

(OP)
How to fix the allowable stresse for the following stress conditions, for mechanical equipment and pressure vessel ? The basis has to be realistic based on research findings since we cannot assume high factors of safety.

Tensile Loading
Compressiver Loading
Shear Loading - Direct Shear
- Torsional Shear
- Thread Shear
Contact Stresses - Surface Contact - Flat to Flat
- Point Contact - Ball to Flat
- Rolling Contact - Flat to Flat
It does not seem to be prudent to take the allowable stresses as per ASME for non-pressure containing members where only a pure mechanical load can be expected.
The allowable stresses recommended by the other texts seem to be too conservative/unrealistic. Hence a rational approach is desired. Will be thankful if some one could throw more light on this subject in detail.

RE: Allowable Stresses

Conservative design will account for things that are often "taken for granted" like corrosion, wear, fracture properties, overloading, excessive vibration, etc. If you can control these variables, you can justify a reduction in safety factor and use existing theory. Uncertianty regarding material properties, assumptions for stress estimation and effects of combined stress can be mitigated using more accurate analysis techniques like finite element and actual lab testing.

Contact ASTM, ASME, NASA and various universities to see if they have literature available that describes your loading and materials sufficiently to allow for safety factor reduction.

RE: Allowable Stresses

MikeVV is correct. Additionally, you will have to increase the inspection of your material for manufacturing flaws and add stress concentration factors for flaws that may escape detection. ASME, based on code dictated inspections, accounts for flaws that may have escaped detection. The unrealistic limits on allowable stress are there to simplify design and material costs.

Tom Worthington
pworthi@astro.as.utexas.edu

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