Stress & Strain
Understanding von Mises stress, principal stresses, and what the numbers mean.
Von Mises stress
Von Mises stress consolidates the multi-axial stress state into a single scalar value. It's the standard metric for assessing yield risk in ductile materials like steel and aluminium.
Plastic deformation begins when the von Mises stress exceeds the material's yield strength (R_p0.2). The safety factor against yield is:
SF = Yield strength / Max von Mises stress
Principal stresses
The three principal stresses (σ₁ ≥ σ₂ ≥ σ₃) are the maximum normal stresses along the principal axes, directions with no shear stress.
Use principal stresses for:
- Brittle materials (cast iron, ceramics), use maximum principal stress σ₁
- Fatigue analysis, amplitude of principal stress drives crack initiation
- Detailed stress assessment in critical regions
Which to use?
Dr.Q reports von Mises, Tresca, and maximum principal stress (σ₁) directly; the table below is general guidance on which measure matters for each failure mode.
| Situation | Recommended metric |
|---|---|
| Ductile material yield check | Von Mises stress |
| Brittle material fracture | Maximum principal stress σ₁ |
| Fatigue assessment | Principal stress amplitude |
| Weld seam evaluation | Normal and shear stress components |
Interpreting the colour scale
The default colour scale runs from blue (minimum) to red (maximum). Check that the scale bounds are sensible, if the maximum is dominated by a singularity at a sharp corner, the rest of the part will appear uniformly blue even though it may be highly stressed.
Manually set the scale maximum to the yield strength of the material. This gives an immediate visual indication of where the part yields and where it doesn't.