KEVOS® MAKEOS · Materials
Equivalent Thickness for Equal Stiffness or Strength
How thick must a part in the new material be to match the stiffness and strength of the original?
Wall or plate thickness needed in a substitute material (for example plastic replacing metal) for the same bending stiffness and for the same bending strength as the original, the governing thickness, and the resulting mass ratio.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Original thickness | mm | Thickness of the existing part. |
| Original modulus | GPa | Elastic modulus of the existing material. |
| Original allowable stress | MPa | Allowable bending stress of the existing material. |
| Original density | kg/m³ | Density of the existing material. |
| New material modulus | GPa | Elastic modulus of the new material at service temperature (use a creep modulus for sustained loads on plastics). |
| New material allowable stress | MPa | Allowable bending stress of the new material for the service conditions. |
| New material density | kg/m³ | Density of the new material. |
Method and formulas
- Plates and walls of the same width in bending: stiffness ∝ E t³, so t₂ = t₁ (E₁ ÷ E₂)^(1/3).
- Strength ∝ S t², so t₂ = t₁ √(S₁ ÷ S₂).
- Governing thickness is the larger; mass ratio = ρ₂ t₂ ÷ ρ₁ t₁.
- Example: 2 mm of the original needs 8.1096 mm of the new material for equal stiffness.
Limitations
- Same shape and width, bending only; ribs and section changes are usually a better route for plastics.
- Plastics creep: use the long-term modulus for sustained loads.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Replace a metal panel or housing with plastic or composite
- Compare aluminium and steel panel thicknesses
- Estimate weight saving from a material change
Who it is for
- Design engineers
- Product engineers
- Materials engineers
What you get
- Thickness for equal stiffness
- Thickness for equal strength
- Mass ratio at the governing thickness
Interpreting the result
- If the stiffness thickness is impractical, add ribs or place material away from the neutral axis instead.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 575–580. Topic reference; no source tables or recommended settings are reproduced.
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