Pseudoelasticity

Another property pecuHar to SMAs is the abiUty under certain conditions to exhibit superelastic behavior, also given the name linear superelasticity. This is distinguished from the pseudoelastic behavior, SIM. Many of the martensitic alloys, when deformed well beyond the point where the initial single coalesced martensite has formed, exhibit a stress-induced martensite-to-martensite transformation. In this mode of deformation, strain recovery occurs through the release of stress, not by a temperature-induced phase change, and recoverable strains in excess of 15% have been observed. This behavior has been exploited for medical devices. 

Stage at which liquid begins to show pseudoelastic properties. 

Industrial force generators, superelastic and pseudoelastic SMA devices for, 22 352... 

Orthoarsenic acid, 3 264—265 Orthoboric acid, 4 242t, 249—255 Orthoclase, hardness in various scales, l 3t Orthocortex, in wool fibers, 11 173 Orthodontics, superelastic and pseudoelastic SMA devices in, 22 350-351 Orthoesters, 10 498 Orthoferrites, 11 56t, 57 Orthogonal matrices, 6 27 ort/io-hydrogen, 13 759, 760—761 Orthokinetic flocculation, 11 631 22 56 Orthokinetic flocculator, 22 59 Orthomyxoviruses, 3 136—137... 

Near the transition temperature, SMAs also exhibit the curious effect of pseudoelasticity, in which the metal recovers (apparently in the usual manner) from an isothermal bending deformation when the stress is removed. However, the elasticity is not due to the usual elastic modulus of a fixed crystalline form, but instead results from strain-induced solid-solid phase transition to a more deformable crystalline structure, which yields to the stress, then spontaneously returns to the original equilibrium crystal structure (restoring the original macroscopic shape) when the stress is removed. 

It is also possible to apply a stress to the material in its high-temperature austenitic phase. However, since the temperature is above Af, the original shape will be reformed immediately after the load is removed. Such an immediate shape change is referred to as pseudoelasticity (or superelasticity), and is the active principle underlying cellular phone antennae that may be greatly distorted only to immediately return to their original shapes. 

Here, the overall flow q is dependent on variables such as the pressure applied across the tube (AP), the radius (r), length (L) of the tube, and the viscosity ( u.) of the solution. Unfortunately, this equation must be modified somewhat because blood is a non-Newtonian fluid having some pseudoelasticity. Therefore, at similar applied pressures, blood will flow faster than a typical Newtonian fluid. Furthermore, in vivo, the vessel diameter (due to dilation and constriction of the resistance vessels) is dynamic and constantly changing. Moreover, the viscosity of blood is very difficult to measure because the hematocrit of blood changes depending on the diameter of the vessel in which it flows. Because of this, blood viscosity in vivo is often reported as an apparent viscosity. Finally, in the bloodstream, blood flow is somewhat pulsatile, especially in the venous side of the circulation. In sum, it is difficult to know the exact linear rate of the blood flow or a pressure drop at any one point in the microcirculation. 

Blood vessels are able to maintain their structural stability and contain steady oscillating internal pressures. This property suggests a strong elastic component, which has been called the pseudoelasticity [4]. This... 

These assumptions are not always justifiable when applied to plastics unless modification has occurred. The classical equations cannot be used indiscriminately. Each case must be considered on its own merits, with account being taken of such factors as the mode of deformation, the service temperature and environment, the fabrication method, and so on. In particular, it should be noted that the past traditional equations that have been developed for other materials, principally steel, use the relationship that stress equals the modulus times strain, where the modulus is constant. Except for thermoset reinforced plastics and certain engineering plastics, many plastics do not generally have a constant modulus. Different approaches have been used for the nonconstant situation some are quite accurate. The drawback is that most of these methods are quite complex, involving numerical techniques that are not attractive to designers. One method that has been widely accepted is this so-called pseudoelastic design method. In this method appropriate values of such time-dependent properties as the modulus are selected and substituted into the standard equations. 

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