Anelastic deformation

Deformation, anelastic It is any portion of the total deformation of a body that occurs as a function of time when load is applied, and which disappears completely after a period of time when the load is removed. In practice the term also describes viscous deformation. Deformation, elastic A deformation in which a material returns to its original dimensions on release of the deforming load/stress. 

We also compared two sets of specimens that had been drawn to the same extent. Both were allowed to relax for 48 h after stretching, one set clamped at its specified elongation, the other allowed to relax freely, undamped. The dichroism results show a large difference between the recoverable (elastic+anelastic) deformation and the total deformation for both XL and NXL phases (see Ligures 3 and 4 of Davidson and Gounder ). 

In such cases W will be a function of position as well as of temperature and the coordinates of the deformation gradient tensor. Finally, most materials, in particular polymers, are anelastic. Energy is dissipated in them during a deformation and the stored energy function W cannot be defined. It is still of value, however, to consider ideal materials in which W does exist and to seek its form since such ideal materials may approximate quite closely to the real ones. 

Viscoelastic materials are those which exhibit both viscous and elastic characterists. Viscoelasticity is also known as anelasticity, which is present in systems when undergoing deformation. Viscous materials, like honey, polymer melt etc, resist shear flow (shear flow is in a solid body, the gradient of a shear stress force through the body) and strain, i.e. the deformation of materials caused by stress, is linearly with time when a stress is applied [1-4]. Shear stress is a stress state where the stress is parallel or tangencial to a face of the material, as opposed to normal stress when the stress is perpendicular to the face. The variable used to denote shear stress is r which is defined as ... 

Fig. 6.17. Map of polycluster mechanical states. Region I elastic and anelastic (shaded area) deformations Region II inhomogeneous plastic deformation Region III homogeneous diffusional-viscous flow. Curves 1-3 show the temperature dependence of the stress at different constant strain rates...

On the map of mechanical states in region I, elastic and anelastic (shaded areas) deformations take place. In the region II, the inhomogeneous plastic deformation with the formation of shear bands takes place. The horizontal broken line corresponds to the theoretical yield stress of LRC. In the region III, the homogeneous diffusional-viscous flow takes place and, in the region IV, the mixed viscous flow is realized. Curves 1,2, 3 show the temperature dependence of the stress at different constant strain rates. The continuations of these curves in regions IV and II correspond to the mixed nonuniform plastic deformation. 

Rheology - The study of the flow of liquids and deformation of solids. Rheology addresses such phenomena as creep, stress relaxation, anelasticity, nonlinear stress deformation, and viscosity. 

Because rubber is viscoelastic, or more generally anelastic, to varying extents and because the mechanical properties depend on rate of deformation and temperature, it is not surprising to find that the strength is also dependent on the rate at which stresses are applied and on the temperature of measurement. These effects are discussed in Sections 10.4.2 and 10.5.1. Other effects of the environment, notably the destructive action of ozone, are discussed in Section 10.8. Finally, a brief survey is given of abrasive wear. 

Time-dependent hysteresis effects can also occur in crystalline materials and these lead to mechanical damping. Models, such as the SLS and the generalized Voigt model, have been used extensively to describe anelastic behavior of ceramics. It is, thus, useful to describe the sources of internal friction in these materials that lead to anelasticity. The models discussed in the last section are also capable of describing permanent deformation processes produced by creep or densification in crystalline materials. For polycrystalline ceramics, creep is usually considered from a different perspective and this will be discussed further in Chapter 7. 

Anelasticity in a material gives rise to permanent deformation. True or False ... 

The experiment we introduced at the beginning of the previous subsection is also called the creep experiment. A small stress of Gq is imposed on a solid sample for a time period of to at a constant temperature after the stop of stress, the strain of changing with the time period of t monitors the relaxatirMi curve. There are four typical responses separately corresponding to viscous, elastic, anelastic and viscoelastic responses, as illustrated in Fig. 6.8. The creep curve of polymer viscoelasticity exhibits both instant and retarded elastic responses upon imposing and removal of the stress, and eventually reaches the permanent deformation. 

As illustrated in Fig. 6.10, a —> b represents the instant elastic response, b c represents the anelastic response and permanent deformation made by the viscous fluid, c —> d represents the instant elastic recovery, d —> e represents the gradual recovery from the anelastic deformation in the viscous fluid, and the height of e represents the permanent deformation of the viscous fluid that could not be recovered. Here, the two springs are not necessary to be identical, and neither are the two dashpots. 

A small hysteresis between tensile loading and unloading curves was detected. The total strain is regained. Those processes are defined as anelastic ones. A temperature change occurs during deformation (adiabatic process). Thus, anelastic processes should depend on the strain rate, e, but this was not found in the experiment. This point cannot be explained. 

For test temperatures Tflow stress increases markedly. Borderlines between elastic/anelastic strain (cta) and microstrain/macrostrain deformation ranges can be deduced, subdivided into athermal ([Pg.316]

When a material is loaded under a fixed stress, after a certain time the strain continues to increase at a rate depending on the type of material. This slow continuing deformation of a material when subjected to a constant stress is called the creep mechanism, which is a typical anelastic behavior. The rate at which the strain change occurs is called the strain rate and is denoted ds/dt and expressed in s . For each material loaded under a constant stress it is... 

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