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 |
3 |

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