Damping Sources
There are many sources of damping in a system. Some of them are:
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Dissipation in the material. This dissipation can be for example be proportional to the amplitude of the strain rate (viscous damping) or to the amplitude of the strain itself (hysteretic damping).
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Thermoelastic damping, which is a thermodynamic effect which is related not only to the state in a point, but also to the gradients of the temperature field.
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Damping caused by the surrounding medium, often air or water.
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Friction between joined parts.
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Components intended to supply damping like a dashpot in a car suspension.
It is often difficult to separate and quantify these effects, so damping modeling is one of the biggest challenges in structural dynamics.