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Linear vs. Non-linear

28-02-2019
Non-linear analysis with plastic designs
 
3D CAD software became a common design tool in the last decade.
 
Nowadays almost all have a simple FEM plug-in to make an early estimation if the design meets the required strength and displacements.
These plug-in tools are suitable for metal constructions but for plastics, it can lead you in the wrong way, even for a quick rough estimation.
 
The reason for this is that these plug-in tools calculate with a constant factor between stress and strain, also called linear elasticity (according to Hook’s law).
For metals, this constant factor is okay since this is valid until the yield strength.  
Plastics are elastic, but when the stress increases, the strain increases even more. This makes this material non-linear elastic according the stress-strain curve.
 
A quick calculation made with a linear elasticity factor shows too high stress with too small displacements.
A non-linear calculation however (even made with a low resolution 5-point stress strain curve) already shows lower stress levels and higher deformations being far more close to the reality.