HYSTEREISIS
Let's examine the compression-deflection curve for a given compound at a given shape factor and record the stress-strain relationship. As the piece is loaded we arrive at one reproducible curve after three or four cycles. Now, if we record the curve generated during the unloading we see that not all the initial energy is returned. The area under the loading curve can be called the input energy. The area under the return curve can be considered the return of stored energy and the area between the two curves is the energy which is not returned but is converted to heat. This heat conversion is considered by hysteresis loss. This phenomenon is characteristic of all types of rubber, because rubber compounds are viscoelastic systems. See the stress-strain cycling of two 95A durometer urethanes chart.
The urethane elastomers consist of an elastic portion which stores energy and returns it, and a viscous portion which captures energy and converts it to heat. It might help to consider the spring and dash pot experiments of Physics class. The spring represents the elastic response and dash pot the viscous response.
To some extent, the ratio of the elastic component to the viscous component can be altered by chemical manipulation during compounding, but both components are always present in a urethane compound.
A high elastic viscous ratio is common to highly resilient compounds. A low ratio is typical of a "dead" or low resilience compound. In design, the viscoelastic nature of urethanes must be taken into account. A low ratio compound converts more input energy to heat than a high ratio compound on each deflection cycle. By the same token, if the frequency of deflection is very low, the low resilience compound will absorb more impact. This is an important consideration because excess heat is the Achilles Heel of polyurethane. The range of the coefficient of thermal conductivity (K) for polyurethanes is quite low, as you can see.
Since urethanes do not lose heat rapidly, the designer must address ways to generate less heat under cyclic conditions. This can often be done by reducing the deflection (strain) per cycle, by increasing the compression modulus of the elastomer, or increasing the shape factor. Reducing the stress per unit area is another possible approach.
The greater the hysteresis per cycle coupled with low heat conductivity, the faster heat builds up in the compound. This consideration is particularly important in high cyclic applications. But selection of compounds and stoichiometry coupled with design can alleviate many potential heat build-up problems.
NEXT: Vibration Isolation
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Spring and dash pot model for a viscoelastic material.
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