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Factor of Safety question?

Question asked by Michael Palumbo on Nov 9, 2009
Latest reply on Nov 9, 2009 by Anthony Botting

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I need to attend a meeting today regarding a tank design I ran some analysis on. We distributed a draft of the analysis last week and one of the questions was whether the FOS is based on the Stress or the Strain. I believe that it is based on the the yield stress but I just wanted to double check with others. Also he wanted to know if the Von Mises stress is w/i the limits. I would also assume that if my stress calculated was Von Mises then the FOS would be calculated with Von Mises values and therefore would tell whether it is w/i the limits.

Anthony Botting Nov 9, 2009 11:22 AM

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It is an option in the PM dialog box to pick the limiting criterion. Alternatively, you can use the limiting criterion with a factor (also shown in the PM). Hope this helps.

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Anthony Botting Nov 9, 2009 10:41 AM

It is stress-based and you can mix and match. For example, you can instruct the plot to use Maximum Normal stress, maximum shear stress, or maximum von Mises stress (and more) against your criterion, which is typically a Yield Strength for many metals. This criterion is the "stress limit", and some people use a limiting value that is some fraction of the published yield strength (such as 1/2 or 1/4). Some people use a stress limit as the published Ultimate Strength. Just pay very close attention to the Factor of Safety plot dialogs as it walks you through the setup and definitions of what stress component(s) to use, and what limiting criteria to use. Hope that helps.

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Jim Sculley Nov 9, 2009 10:46 AM

According to the Simulation help file, if the material is ductile, the max vonMises or max shear stress is used for the FOS calculation. For brittle materials and composites, other formulae are used. When you create the FOS plot, you have the option of changing the criterion used from 'Automatic' to whatever you want.

There is a 'Summary of Failure Criteria' table in the help file that lists the actual formulas used.

Jim S.

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Michael Palumbo Nov 9, 2009 11:09 AM

I noticed if I change the FOS criteria from Automatic to Max Von Mises it displays the calculation equation as the Max Von Mises over the Stress "Limit" is this the UTS or the Yield value?

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Anthony Botting @ Michael Palumbo on Nov 9, 2009 11:22 AM

It is an option in the PM dialog box to pick the limiting criterion. Alternatively, you can use the limiting criterion with a factor (also shown in the PM). Hope this helps.

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Anthony Botting Nov 9, 2009 10:41 AM
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It is stress-based and you can mix and match. For example, you can instruct the plot to use Maximum Normal stress, maximum shear stress, or maximum von Mises stress (and more) against your criterion, which is typically a Yield Strength for many metals. This criterion is the "stress limit", and some people use a limiting value that is some fraction of the published yield strength (such as 1/2 or 1/4). Some people use a stress limit as the published Ultimate Strength. Just pay very close attention to the Factor of Safety plot dialogs as it walks you through the setup and definitions of what stress component(s) to use, and what limiting criteria to use. Hope that helps.
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Jim Sculley Nov 9, 2009 10:46 AM
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According to the Simulation help file, if the material is ductile, the max vonMises or max shear stress is used for the FOS calculation. For brittle materials and composites, other formulae are used. When you create the FOS plot, you have the option of changing the criterion used from 'Automatic' to whatever you want.

There is a 'Summary of Failure Criteria' table in the help file that lists the actual formulas used.

Jim S.
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Michael Palumbo Nov 9, 2009 11:09 AM
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I noticed if I change the FOS criteria from Automatic to Max Von Mises it displays the calculation equation as the Max Von Mises over the Stress "Limit" is this the UTS or the Yield value?
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- Anthony Botting @ Michael Palumbo on Nov 9, 2009 11:22 AM
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  It is an option in the PM dialog box to pick the limiting criterion. Alternatively, you can use the limiting criterion with a factor (also shown in the PM). Hope this helps.
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