Returning to Shapes

Every refractory has a limited service temperature, or maximum temperature. Above these temperatures brick linings are used. The cutoff of refractory in lieu of brick is about 1800°F. Bricks have the capacity to seal and return to shape after heating without spalling. Sohd monolithic linings do not. In addition, bricks can be made in thicker sections than can be cast from refractory. 

It is difficult to think of another material which would perform in just this way. Other thermoplastics would not be sufficiently elastic, when softened, to return to shape after stretching to admit other parts. Vulcanized rubbers would not show the necessary thermoplastic behaviour. This material behaviour is explored in more detail in PST 1. 

Returning to equilibrium shapes, these have been determined both experimentally and by solution of the Young-Laplace equation for a variety of situations. Examples... 

Elastomers or rubbers are almost-linear polymers with occasional cross-links in which, at room temperature, the secondary bonds have already melted. The cross-links provide the "memory" of the material so that it returns to its original shape on unloading. The common rubbers are all based on the single structure... 

As with thermoplastics melt processes, the setting is achieved by cooling. It will be appreciated that such cooling is carried out while the polymer is under stress so that there is considerable frozen-in orientation. This can be maintained throughout the life of the article. It is possible with the higher molecular weight materials to heat shapes made from blanks many years previously and see them return to the original shape of the blank. 

Traditional rubbers are shaped in a manner akin to that of common thermoplastics. Subsequent to the shaping operations chemical reactions are brought about that lead to the formation of a polymeric network structure. Whilst the polymer molecular segments between the network junction points are mobile and can thus deform considerably, on application of a stress irreversible flow is prevented by the network structure and on release of the stress the molecules return to a random coiled configuration with no net change in the mean position of the Junction points. The polymer is thus rubbery. With all the major rubbers the... 

When a shearing stress is imposed on a solid, deformation occurs, until a point is reached when the internal stresses produced balance the shearing stresses. Provided the elastic limit for the material is not exceeded the solid will return to its original shape when the load is removed. 

In the surrounding atmosphere, a blast wave is experienced as a transient change in gas-dynamic-state parameters pressure, density, and particle velocity. Generally, these parameters increase rapidly, then decrease less rapidly to sub-ambient values (i.e., develop a negative phase). Subsequently, parameters slowly return to atmospheric values (Figure 3.7). The shape of a blast wave is highly dependent on the nature of the explosion process. 

In one of the two cells placed back to back, the solvent, as mentioned above, was pure water in each case. When the mixed solvent in the other cell contains only a small percentage of methanol, the resultant e.m.f. will obviously be small, and it should progressively increase with increasing difference between the solvents. In Fig. 61 abscissas are values of 1/e for the mixed solvent, running from 0.0126 for pure water to 0.0301 for pure methanol. Ordinates give the unitary part of the e.m.f. extrapolated to infinite dilution. It will be seen that for KC1, NaCl, and LiCl the curves differ only slightly from straight lines, but the curve for HC1 has quite a different shape. From the experimental results on the electrical conductivity depicted in Fig. 31 we expect the curve for HC1 to take this form. In Sec. 115 we shall discuss this result for HC1, and in Sec. 116 we shall return to the interpretation of the results obtained with the alkali chlorides. 

Thermoplastics can be bent, pulled, or squeezed into various useful shapes. But eventually when heat is added, they return to their original form. This behavior, known as plastic memory, can be annoying. If property applied, however, plastic memory offers interesting design possibilities for all types of fabricated products. 

A further result of Sadler s 2D-simulation was a relation between the step density and growth rate on the one hand and the inclination of the surface with respect to the principal axes on the other. From this relation crystal shapes were derived which show considerable curvature. This result of an exact treatment stands in contrast to Frank s mean-field curvature expression which gives essentially flat profiles. We will return to the discussion of curved edges in Sect. 3.6.3. 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

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