Elastic Fault

Kevlar fibers are temperature-resistant special fibers used, for instance, in bulletproof vests. The main application of Kevlar is, however, as a reinforcing fiber in composites. One drawback here is the lack of compression strength of the fibers, presumably due at least in part to the mantle-core structures produced by spinning from sulfuric acid solutions. Helical kink band defects appear when Kevlar is compressed. Composites with Kevlar as a reinforcing fiber therefore do not fail due to rupturing of the fibers or matrix, but rather due to an elastic fault caused by shearing forces. 

If an elastic or insufficiently hard primer or paint has been applied under a less elastic top coat, or if the first coat (or set of coats) of oil-base paint has been second-coated before it is completely dry, not only will the paintwork remain soft for an unduly fong period, but cracking will also follow, as the upper layer cannot follow the movement. If the last coat is very thick this fault will frequently manifest itself in the form of alligatoring, i.e. the formation of cracks which do not penetrate all the films down to the substrate, and which may be present in the top layer only. 

Mention should be made in this connection of the physics and chemistry involved in faulting as well as in jointing and minor movements of the solid rocks. These phenomena have often been treated under the principles of elastic theory as applied to homogeneous bodies, yet there can be no question that the elastic properties and conditions of rupture of aggregates must differ in many essential particulars from those of homogeneous bodies. Here is a considerable field for experimentation. 

Two examples may be used to illustrate the complexity of problems of this kind. When film is made by extrusion followed by casting on chill rolls there can be a tendency for the extruded web to shrink inwards towards the centre of the rolls—the phenomenon known as neck-in . The edge of film concerned becomes thicker than the rest. It has been found that more elastic melts, capable of keeping a tension in the direction of extrusion, are less liable to exhibit this fault. 

The simplest defect in a semiconductor is a substitutional impurity, such as was discussed in Section 6-E. There are also structural defects even in pure materials, such as vacant lattice sites, interstitial atoms, stacking faults (which were introduced at the end of Section 3-A) and dislocations (see, for example, Kittel, 1971, p. 669). They are always in small concentration but can be important in modifying conduction properties (doping is an example of this) or elastic properties (dislocations arc an example of this). 

Yin A. and Kelly T. K. (2000) An elastic wedge model for the development of coeval normal and thrust faulting in the Manna Loa-Kilauea rift system in Hawaii. J. Geophys. Res. Solid Earth 105, 25909-25925. 

Another kind of fault block mountains comes from stretching of Earth s crust. A model of this kind of mountains could be made by compacting a 6-in (15-cm) thick layer of moist sand on top of a rubber (not rubberized) sheet. When the sheet stretches, mimicking the elastic properties of the lower crust, the sand will crack along lines perpendicular to the direction the sheet is being pulled. Some of the surface will remain the same height, and some blocks will slide down the sides of the blocks which remain stable. This is particularly noticeable if the top surface of the compacted sand has been dusted with powder. This is a model of the process that formed the mountains in the Basin and Range province. 

Anderson/ As elastic strain can be calculated by classical elasticity theory in a relatively straightforward way, it was natural to put some effort into such theoretical attempts to understand the microstructures found in, for example, the CS phases. The first paper on this topic was by Stoneham and Durham. This was followed by several reports by Iguchi and Tilley, Iguchi, and Iguchi and Shimizu, while recently Bursill, Netherway, and Grey have used a different approach to evaluate effect of elastic strain on the microstructures of CS phases in rutile-derived oxides. We will not describe these in chronological order, but rather consider the results in terms of isolated faults and then arrays of faults. Unfortunately, the calculations have all been restricted to CS phases with but one exception (Iguchi, unpublished results) and so we can only consider these materials here. 

Arrays of Fault Planes. When we consider arrays of CS planes we can use the Fourier Transform method of evaluating the elastic-strain energy of the array, as well as the classical theory. This allows us to evaluate not only the elastic strain in the matrix between CS planes, but also to obtain some measure of the relaxation energy of the ions in the CS planes themselves. In this Section we will initially discuss the matrix strain, and then consider calculations which include relaxation. 

We can finally conclude that the number of chemical systems which appear to reject point-defect populations as a mode of accommodating their non-stoicheiometric behaviour is large and varied and here we have touched upon only a few which make use of planar faults or parallel lamellar or foliar intergrowth structures. The results presented show that physical terms, such as elastic strain, are of importance in controlling the microstructures of such phases, but whether they form or whether they coexist with some form of point-defect clusters may well depend in a sensitive way to the anion-cation bonding within the individual co-ordination polyhedra which made up the structure. The continuing research in this area is certain to produce new and unexpected results before complete answers to the problems posed here are found. 

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