![]() Polymers, polymer solutions and dispersions, metals and amorphous materials (organic and inorganic) are examples of materials that exhibit viscoelastic behaviour. Thus, for most of the materials there is usually an intermediate phase lag between stress and strain (i.e., they are out of phase) and a certain amount of frictional energy loss (see Fig. In reality, viscoelastic behaviour is more common. For an ideal viscous material, stress and strain are 90ยบ out of phase. For an ideal elastic material, stress and strain are in phase, and the energy lost as heat per cycle is null. Similarly, the second derivative of the velocity is proportional to the velocity divided by the square of the length scale. The Reynolds number Re then becomes: Re (r V dV/dx) / (mu d2V/dx2) The gradient of the velocity is proportional to the velocity divided by a length scale L. For instance, they are often strained or stressed sinusoidally with a Dynamic-Mechanical Analyser (DMA). The viscous forces are characterized by the dynamic viscosity coefficient mu times the second gradient of the velocity d2V/dx2. To characterize the response of a material, samples are usually excited dynamically under controlled temperature, frequency and amplitude of the loading, and the strain-stress behaviour is recorded. Sutherland provides an equation for the dependence on temperature T: mu mu0 ((T / T0)1.5) ((T0 + 198.72) / (T + 198. ![]() Whether the behaviour of a material is closer to purely elastic or purely viscous depends mainly on temperature and the excitation frequency (strain rate), but it may also depend significantly on test and environmental conditions such as the pre-load, dynamic load, environmental humidity, etc. The value of the dynamic viscosity coefficient is found to be a constant with pressure but the value depends on the temperature of the gas.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |