The FOS is defined as: (Allowable Value)/(Calculated Value). The allowable value is based on the failure theory (Tresca, von-Mises, Mohr, etc.) and the property value associated with the respective failure theory (maximum shear stress at yielding, yield strength, ultimate strength, etc.)
For example, if you have a part made of A-36 steel:
- A-36 steel -> ductile material -> Tresca or von-Mises Failure Theory (Tresca is more conservative, so we'll use von-Mises).
- Maximum von-Mises stress in model = 18 ksi.
- Minimum von-Mises stress in model = 1 ksi
- Yield strength of A-36 = 36 ksi.
- Maximum FOS = (36 ksi)/(1 ksi) = 36
- Minimum FOS = (36 ksi)/(18 ksi) = 2
Several things to keep in mind:
- Default FOS plots take singularities into account.
- FOS plots are based on a linear input/output relation.
shauns explanation of what fos is is great.
regarding the color plot, it simply gives better resolution of the fos under 100 if you have some point above that. So with Shaun's example, the chart would show 2 to 36. If the max was 1000, it would be 2 to 1000.
I personally prefer to show the FOS in 2 colors only ("Areas below the factor of safety"), and define the desired FOS.
This way a get a better picture of the problematic regions when they are below the desired FOS, since we already get a palette of colors in all the other plots.