Deformation, reversible recovery

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

The reversible recovery of a deformed elastomer to its original (undeformed) state is due to an entropic driving force. The entropy of polymer chains is minimum in the extended conformation and maximum in the random coil conformation. Cross-linking of an elastomer to form a network structure (IX) is... 

An ideal elastic body (also called Hooke s body) is defined as a material that deforms reversibly and for which the strain is proportional to the stress, with recovery to the original volume and shape occurring immediately upon release of the stress. In a Hooke body, stress is directly proportional to strain, as illustrated in Fig. 3. The relationship is known as Hooke s law, and the behavior is referred to as Hookean behavior. 

Another type of experiment often done in conjunction with creep is creep recovery, the recoil of strain after the stress is removed, as illustrated in Figure 3.3.2. After the stress has been removed from a viscoelastic material, the deformation reverses itself. We can define a recoverable creep function... 

For those gels in which shear thinning occurs (ALSP 1-3%), the process can be reversed by either the removal or the reduction of the shem-strain rate. That is, the breakdown process is reversible. Each point on the non-linear zone of the flow curve corresponds to a state of dynamic equilibrium between processes of deformation and recovery of structure of the matrix. The situation can be represented by the following equation ... 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Influence of Solvents. The stress-strain curves of untreated and ether-extracted corneum in water show marked differences (81). Untreated corneum, extended 5% and relaxed, shows hysteresis similar to that observed for other keratinaceous structures (Figure 35). The deformation mechanism is completely reversible, and hydrogen-bond breakdown and slow reformation may be the major factors determining the stress-strain relationships. With ether-extracted samples, complete recovery is observed from 5% extension but with little or no hysteresis. The more rapid swelling and lack of hysteresis of ether-extracted corneum in water may be related to the breakdown of hydrogen bonds normally shielded from the eflFects of water by the lipid-like materials removed by ether. 

Microhardness, therefore, appears to be an elastic-plastic rather elusive parameter (Marsh, 1964). Microhardness as a property is, in fact, a complex combination of other properties elastic modulus, yield strength and strain hardening capacity. One way to differentiate between the reversible and irreversible components of contact deformation is to measure the elastic recovery during unloading of the indenter (Stilwell Tabor, 1961). Extreme cases of depth recovery are best described by soft metals, where it is negligible, and fully elastic rubber, where it is complete. 

The preform expands due to relaxation and recovery. The pressure placed on the molded resin exerts three types of changes in the particles of resin. Resin particles undergo plastic deformation and are intermeshed together leading to the development of cohesive or green strength. Particles also deform elastically and experience cold flow underpressure. The air trapped in the space between the resin particles is compressed. Removal of pressure allows the recovery of elastic deformation, which creates a quick snap back of the preform. Overtime, stress relaxation partly reverses the cold flow, and the preform expands. 

Figure 8.17 Parts (a)-(e) show the progressive recovery of the original shape of a deformed rod by reversion to the original crystal structure, which also removes twins...

The Burger model provides a correct graphic description of the elongation-time behavior of most plastics in a first approximation. The spring 1 results in spontaneous elastic load application and relaxation elongation, 1 + 2 in parallel cause creep during load application and creep recovery (delayed viscoelastic reverse deformation) after relaxation, damper 2 results in residual elongatimi. 

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