The behavior of nonlinear, viscoelastic materials
Stress relaxation describes how viscoelastic materials relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion, where the nonlinearity is described by both stress relaxation and a phenomenon known as creep.
Viscoelastic materials have the properties of both viscous and elastic materials, and are modeled by combining elements that represent both characteristics. There are several models of interest to quantify the behavior. The Maxwell model predicts behavior using a spring (elastic element) in series with a dashpot (viscous element). While the Maxwell model is good at predicting stress relaxation, it is a poor predictor of creep. The Voigt model places the spring and dashpot in parallel, and is the opposite of the Maxwell model in predicting the creep and stress relaxation. The Standard Linear Solid model combines the characteristics of both the Maxwell and Voigt models to display both creep and stress relaxation, and is generally accepted as the most accurate of the various viscoelastic models.
The following non-material parameters all affect stress relaxation in polymers:
- Magnitude of initial loading
- Speed of loading
- Temperature (isothermal versus non-isothermal conditions)
- Loading medium
- Friction and wear
- Age related degradation (shelf life - long term storage) due to UV radiation or O2 penetration
Bose® ElectroForce® test instruments can play an important role in evaluating the viscoelastic behavior of a variety of materials, including biomaterials and engineered materials.
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