Analysis tipsTips-Tricks

Creep Analysis of Plastics in SolidWorks


In this Tip & Trick we take a look at the creep analysis of plastics using the nonlinear analysis module in Simulation.

As with any analysis, the accuracy of the input information is absolutely key to meaningful results. In the case of creep analysis it's all about getting accurate material properties - which is sometimes easier said than done. We share some methods to help you decide when a creep analysis may be needed, and how to derive material data to use in the simulation. Firstly, a brief description about what creep is:

Creep is the tendency of a material to deform and stretch under the influence of stresses and strains, and can occur at stresses well below the yield strength of the material. When a plastic part is loaded and unloaded, there can also be other mechanisms occurring such as recovery and stress relaxation. The following article has some useful info on these topics, however for this tip we will be focussing on the case of creep under constant stress (shown in the diagram below).

Creep-contant-stress

Creep under constant stress

So when is it important to consider creep in the design of plastic components?
The driving factors are high temperature and long time periods. If deformation of the part will result in failure or prevent the part working correctly then creep should be considered as a long-term failure mode.

This creep behaviour is described by the Classical Power Law for Creep in Solid Works Simulation (power law for nonlinear analysis)

Creep-power-law

Creep Power Law

This formula represents the primary and secondary creep regiemes in one equation. Tertiary creep is not considered
- Primary Creep (Transient creep): some recovery
- Secondary Creep (Steady state creep): permanent deformation (no recovery)
- Tertiary Range: not considered

This means that to simulate the creep response, the material constants C0, C1, C2 and CT are needed, where:
- T = Temperature (Kelvin)
- CT = A material constant defining the creep temperature-dependancy
- Co = The Creep Constant 1 you enter in the Properties tab of the Material dialog box
- C1 = The Creep Constant 2 you enter in the Properties tab of the Material dialog box
- C2 = The Creep Constant 3 you enter in the Properties tab of the Material dialog box

Step 1: Gather material data

There are heaps of sources of material information. Some of the best we have found are:
CAMPUS Plastics
Matweb
Material Data Centre
If these resources fail to provide the info required, physical testing will then be needed.
Step 2: Derive material constants for the Power Law Equation
Check out the following example from the SolidWorks Knowledge Base for deriving the constants:
>>Deriving creep constants from creep curve data

Step 3: Set up the Non-Linear Analysis

Set up the time curve for the applied load(s). For a constant load it will look something similar to the curve below

creep-time-curve
Time Curve

Set up the solver parameters: The initial time-step and the time curve itself is important. A good rule of thumb is to set the initial time-step equal to the first step in the time-curve

Other settings to adjust are:

Singularity elimination factor
Max. incremental strain – make sure that there are no local areas which are highly strained and causing the solution to fail
Key Tip:
Start with a very simple test case to get solution parameters correct before spending hours running the full part model with complex geometry. For example, a simple bar with a hole in tension will help you validate the material and solver parameters quickly while you trouble-shoot the solver parameters.

creep-test

test case

Finally, when you have run the analysis, check the results by plotting the strains and compare the total, elastic and creep strains in the part.

creep-plots

Strain Plots

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