When working with the Inventor's Stress Analysis module, you have to make a conscious decision whether your material is ductile or brittle.
Inventor will not make this decision for you as this property or classification is not part of the Inventor material properties.
This might be new to some and has led to some interesting discussions on the newsgroup.
Just to be complete, this is not the only decision or assumption to make. There are several other implicit stress analysis assumptions that you make when using Inventor's Stress Analyis software :)
When assigning materials in the Stress Analysis environment, the Safety Factor column allows you to choose between two calculation methods
Fig 1: Rather subtle choice between brittle and ductile material
What the dialog does not explain is why you would need to make this choice.
The reason is that in order to calculate an accurate safety factor, Inventor uses a different formula depending on the brittleness of the material.
For ductile materials
For brittle materials
UTS=Ultimate Tensile Strength (as can be found in the material properties)
For the max. principal stress, we compare the s_{1}_{ }and s_{3} values to account for special combinations of tension, bending, torsion or shear:
Max. principal stress = s_{1 } (if s_{3}>0)
= s_{3} (if s_{1}<0)
= s_{1} s_{3 }(in all other cases)
s_{1 }= 1^{st} principle stress
s_{3 }= 3^{rd} principle stress
It is also important to note that the Max. principal stress is more of a local stress value that has to be measured at the same point where the global maximum Von Mises stress occurs.
The best way to accomplish this is by using the probe tool.
Let's put all this in action with this sample part that contains 3 pressure loads and is clamped down with a fixed constraint on the red face. Skip all existing FEA references on open (I did not attach the calculation results to keep the download size down)
Fig 2: Sample part used for the comparison calculations
Case 1: use of a ductile material (steel)
If we activate the first stress analysis in the downloaded part, you will see it has a steel type material with
Yield strength = 260 MPa
UTS = 485 Mpa
Fig 3: Material properties for a ductile material
Make sure to select the Yield Strength calculation for the Safety Factor
Fig 4: Appropriate Safety factor setting for ductile material
After running the stress calculation, we see that the Max. Von Mises stress = 359 Mpa
This means that theoretically
Min. Safety factor = Yield Strength / Max Von Mises stress = 260 MPa/359 MPa = 0.72
This value gets confirmed by looking at the Safety Factor calculation scale.
Fig 5: Safety factors for steel part
Case 2: use of a brittle material (glass)
Now let's change the material from steel to glass.
This can easily be done by copying the previous simulation to preserve existing load and boundary conditions.
For your convenience, you can just activate the second simulation that is present in the file.
Assume that we override the steel material and use polished glass with UTS= 50 MPa
Fig 6: Appropriate Safety factor setting for brittle material
After running the simulation, we want to determine the max. principal stress value.
We first place a probe on the location of the max. Von Mises stress (zoom in to place the probe more precisely).
We then switch to the 1^{st} principle stress and 3^{rd} principle stress results and note down the values for s_{1}_{ }and s_{3}.


1^{st} principle stress

3^{rd} principle stress

s_{1} = 258.7 MPa
s_{3} = 82.9 MPa
s_{1}  s_{3} = 258.7 + 82.9 = 341.6 MPa
This means that
Min. safety factor = UTS / Max. principal stress= 50 MPa/341.6 MPa = 0.146
This value gets confirmed by looking at the Safety Factor calculation scale.
Fig 7: Safety factors for glass part
Needless to say that the glass design (0.15) will break much sooner than the steel design (0.72) …
Enough math for today and let's enjoy the remainder of spring break.
Bob