Rate Dependency of Plastics

Rate Dependency of Plastics- Plastics, like most materials are affected by how fast they are loaded. The faster you apply load to a material the higher the yield stress in the material. In some cases the faster you load the material the stiffer it behaves, the modulus goes up. This can cause the material to go from ductile behavior to a more brittle-like behavior. Many material models assume a constant modulus for all the different strain rates. This can be ignored in some cases when the dependency is not that great it is not of interest for the simulation. Make sure the material model that is selected can handle the rate dependent modulus should the need arise.

Rate dependent material models are based on the relationship between the stress strain curves and the strain rate they were tested at. Scaling is based on the stress at yield versus the strain rate that produced the stress. The relationship between yield stress and strain rate is most time linear on a log scale. Caution should be used when the relationship is highly non-linear. Many simulations cannot handle non-linear relationships of stress to strain rates. It is important to remember that just because you are trying to simulate something being impacted at twenty miles per hour it is not always necessary to test at that speed. One must look at what kind of strain rates are being observed by the model. And even then it is not always necessary to test at those high speeds, the rate dependent model is used for this very reason.

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