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Colony and empire, colonialism and imperialism, are often treated as synonyms. This can be acceptable for many purposes. But there may be also good reasons to distinguish between them. This article considers in detail one important attempt in that direction by the classicist Moses Finley. It argues that there is considerable strength in that approach, putting the stress as it does on the distinctiveness of the settler community. It is also valuable in suggesting that early-modern Western colonialism marked a new departure in an older history of imperialism, thus once again suggesting the need for a conceptual separation of the two. But the article concludes that ultimately more may be lost than gained by insisting on the distinction. In particular, it inhibits wide-ranging comparisons between ancient and modern, and Western and non-Western, empires, which can often suggest illuminating connections and parallels. The field of empire studies gains by drawing on the rich store of examples provided by the whole history of empire, from the earliest times to now. Western colonialism is part of that story; to separate it out is to impoverish the field.
After a period of neglect, civilization as a concept seems once more to have regained popularity among a number of historians and social scientists. Why? What is the appeal of civilization today? And might the return of civilization also herald a return to the work of Arnold Toynbee, once regarded as the towering figure of civilizational analysis? This paper considers the history of the concept of civilization, and argues for the continuing importance and relevance of Toynbee's multi-volume A Study of History within that tradition. The claim is that, whatever the weaknesses of Toynbee's general approach, the civilizational perspective he adopts allows him to cast an illuminating light on many important historical questions. Moreover his belief in the “philosophical contemporaneity” and equal value of all civilizations should make him peculiarly attractive to those many today who reject Eurocentrism and who are increasingly persuaded of the need to consider the total human experience from earliest times up to the present.
A pilot study was undertaken to find significance of vascular endothelial growth factor (VEGF) and cancer antigen (CA 15.3) in breast cancer patients.
Materials and methods
Total 70 patients with breast cancer were divided into triple negative breast cancer (TNBC) and non-TNBC depending on oestrogen receptors, progesterone receptors or HER-2/neu receptors status. Serum CA 15.3 and VEGF levels were evaluated with enzyme-linked immunosorbent assay at the time of diagnosis and were correlated with age, tumour size and stage of the disease in both the groups. Spearman's test was used to find the correlation.
VEGF levels were found to be >400 pg/ml in 27 patients, 19 (54·33%) of them were TNBC and only 8 (22·87%) non-TNBC. Mean values of the VEGF were, 784·34 pg/ml in TNBC and 334·60 pg/ml non-TNBC patients, respectively. CA 15.3 level was found to be higher in non-TNBC group (60·72 U/ml) than in TNBC group (45·24 U/ml). In all patients significant correlation was found between serum CA 15.3 level and tumour size and stage of the disease. In non-TNBC patients significant correlation was seen between CA 15.3 values and stage of the disease, but VEGF had no correlation with any of the disease parameters. In TNBC patients, there was no correlation between CA 15.3 level and any of the disease parameters but VEGF showed a significant correlation with both tumour size and stage of the disease.
Expression profile of VEGF was high in TNBC than non-TNBC patients. VEGF serves to be a better biomarker as compared with CA 15.3 in TNBC patients.
In Chapter 4, we dealt with point and line defects. There is another class of defects called interfacial, or planar, defects. These imperfections, as the name signifies, occupy an area or surface and so are two-dimensional, as well as being of great importance. Examples of such defects are free surfaces of a material, grain boundaries, twin boundaries, domain boundaries, and antiphase boundaries. Of all these, grain boundaries are the most important from the point of view of the mechanical properties of the material. In what follows, we consider in detail the structure of grain and twin boundaries and their importance in various deformation processes, and, very briefly, the structure of other interfacial defects. Details regarding the strengthening of a material by grain boundaries are given in Section 5.3. Volumetric defects, such as voids, also play a major role in the mechanical properties of materials, affecting the strength and elastic properties of the material significantly. Volumetric defects are briefly described in Section 5.7. In Section 5.8, we present the defects occurring in polymers.
Crystalline solids generally consist of a large number of grains separated by boundaries. Most industrial metals and ceramics are polycrystalline aggregates, and the mechanical properties of these polycrystals can be radically different from those of the monocrystals that form the individual grains. Figure 5.1 illustrates a polycrystalline aggregate, in which each grain has a distinct crystallographic orientation.
We can define a composite material as a material consisting of two or more physically and/or chemically distinct phases, suitably arranged or distributed. A composite material usually has characteristics that are not depicted by any of its components in isolation. Generally, the continuous phase is referred to as the matrix, while the distributed phase is called the reinforcement. Three items determine the characteristics of a composite: the reinforcement, the matrix, and the interface between them. In this chapter, we provide a brief survey of different types of composite materials, highlight some of their important features, and indicate their various applications.
Types of Composites
We may classify composites on the basis of the type of matrix employed in them – for example, polymer matrix composites (PMCs), metal matrix composites (MMCs), and ceramic matrix composites (CMCs). We may also classify composites on the basis of the type of reinforcement they employ (see Figure 15.1):
Particle reinforced composites.
Short fiber, or whisker reinforced, composites.
Continuous fiber, or sheet reinforced, MMCs.
Figure 15.2 shows typical microstructures of some composites: boron fiber/Al (Figure 15.2(a)), short alumina fiber/Al (Figure 15.2(b)), and NbC/Ni–Cr, an in situ (eutectic) composite (Figure 15.2(c)). Examples of microstructure of a silicon carbide particle (three different volume fractions) reinforced aluminium matrix are given in Figure 15.3. These were made by hot pressing of powders followed by hot extrusion. Note the preferential alignment of SiC particles in the extrusion direction.
An intermetallic is a compound phase of two or more normal metals (ordered or disordered). Interest in intermetallics waned in the 1960s and 1970s. However, the demand for materials that are strong, stiff, and ductile at high temperatures has led to a resurgence of interest in intermetallics, especially silicides and ordered intermetallics such as aluminides. A testimony to this resurgence was the appearance in 1994 on the subject of a two-volume set by J. H. Westbrook and R. L. Fleischer, Intermetallic Compounds: Principles and Practice (New York: John Wiley). Intermetallic aluminides and silicides can be very oxidation and corrosion resistant, because they form strongly adherent surface oxide films. Also, intermetallics span a wide range of unusual properties. An important example outside the field of high-temperature materials involves the exploitation of martensitic transformations, exotic colors, and the phenomenon of shape memory in gold-based intermetallics in jewelry making. In what follows, we first describe the silicides, then the ordered intermetallics, and finally the basic structure and properties of foams.
About 300 intermetallic compounds melt at temperatures above 1,500 °C. A survey of some silicide intermetallics for high-temperature applications showed that, based on criteria such as availability, phase changes in the temperature range of interest, and oxidation resistance, Ti5S3 and MoSi2 seem to be the most promising materials: Ti5Si3 has the lowest density of all intermetallics, and MoSi2 has a superior oxidation resistance.
A solution can be defined as a homogeneous mixture of two or more substances. Generally, one thinks of a solution as liquid, but gaseous or solid forms are possible as well. Indeed, we can have solutions of gases in a gas, gases in a liquid, liquids in a liquid, solids in a liquid, and solids in a solid. A solution can have one or more solutes dissolved in a solvent. The solute is the substance that is dissolved; the solvent is the substance in which the solute is dissolved. In a solution, there is always less solute than solvent. There are two kinds of solid solutions: substitutional and interstitial. Figure 10.1 shows examples of each in a schematic manner. Figure 10.1(a) is of brass, which is a substitutional solid solution of zinc (the solute) in copper (the solvent). We call such an alloy substitutional because the solute atoms merely substitute for the solvent atoms in their normal positions. In a substitutional solution, the atomic sizes of the solute and solvent atoms are fairly close. The maximum size difference is approximately 15%. When the atomic sizes of the solute and solvent are very different, as in the case of carbon or nitrogen in iron, we get an interstitial solid solution. Figure 10.1(b) shows such a solid solution of carbon in iron. We call these solutions interstitial solid solutions because the solute atoms occupy interstitial positions in the solvent lattice.
Fracture of any material (be it a recently acquired child's toy or a nuclear pressure vessel) is generally an undesirable happening, resulting in economic loss, an interruption in the availability of a desired service, and, possibly, damage to human beings. Besides, one has good, technical reasons to do fracture testing: to compare and select the toughest (and most economical material) for given service conditions; to compare a particular material's fracture characteristics against a specified standard; to predict the effects of service conditions (e.g., corrosion, fatigue, stress corrosion) on the material toughness; and to study the effects of microstructural changes on material toughness. One or more of these reasons for fracture testing may apply during the design, selection, construction, and/or operation of material structures. There are two broad categories of fracture tests; qualitative and quantitative. The Charpy impact test exemplifies the former, and the plane-strain fracture toughness (KIc) test illustrates the latter. We describe briefly important tests in both of these categories.
We saw in Chapter 7 that stress concentrations, like cracks and notches, are sites where failure of a material starts. It has been long appreciated that the failure of a given material in the presence of a notch is controlled by the material's fracture toughness. Many tests have been developed and standardized to measure this “notch toughness” of a material. Almost all are qualitative and comparative in nature.
Everything that surrounds us is matter. The origin of the word matter is mater (Latin) or matri (Sanskrit), for mother. In this sense, human beings anthropomorphized that which made them possible – that which gave them nourishment. Every scientific discipline concerns itself with matter. Of all matter surrounding us, a portion comprises materials. What are materials? They have been variously defined. One acceptable definition is “matter that human beings use and/or process.” Another definition is “all matter used to produce manufactured or consumer goods.” In this sense, a rock is not a material, intrinsically; however, if it is used in aggregate (concrete) by humans, it becomes a material. The same applies to all matter found on earth: a tree becomes a material when it is processed and used by people, and a skin becomes a material once it is removed from its host and shaped into an artifact.
The successful utilization of materials requires that they satisfy a set of properties. These properties can be classified into thermal, optical, mechanical, physical, chemical, and nuclear, and they are intimately connected to the structure of materials. The structure, in its turn, is the result of synthesis and processing. A schematic framework that explains the complex relationships in the field of the mechanical behavior of materials, shown in Figure 1.1, is Thomas's iterative tetrahedron, which contains four principal elements: mechanical properties, characterization, theory, and processing. These elements are related, and changes in one are inseparably linked to changes in the others.
Elasticity deals with elastic stresses and strains, their relationship, and the external forces that cause them. An elastic strain is defined as a strain that disappears instantaneously once the forces that cause it are removed. The theory of elasticity for Hookean solids – in which stress is proportional to strain – is rather complex in its more rigorous treatment. However, it is essential to the understanding of micro- and macromechanical problems. Examples of the former are stress fields around dislocations, incompatibilities of stresses at the interface between grains, and dislocation interactions in work hardening; examples of the latter are the stresses developed in drawing, and rolling wire, and the analysis of specimen–machine interactions in testing for tensile strength. This chapter is structured in such a way as to satisfy the needs of both the undergraduate and the graduate student. A simplified treatment of elasticity is presented, in a manner so as to treat problems in an undergraduate course. Stresses and strains are calculated for a few simplified cases; the tridimensional treatment is kept at a minimum. A graphical method for the solution of two-dimensional stress problems (the Mohr circle) is described. On the other hand, the graduate student needs more powerful tools to handle problems that are somewhat more involved. In most cases, the stress and strain systems in tridimensional bodies can be better treated as tensors, with the indicial notation.
The relaxation times for the molecular processes in gases and in a majority of liquids are so short, that molecules/atoms are almost always in a well-defined state of complete equilibrium. Consequently, the structure of a gas or liquid does not depend on its past history. In contrast, the relaxation times for some of the significant atomic processes in crystals are so long, that a state of equilibrium is rarely, if ever, achieved. It is for this reason that metals in general (and ceramics and polymers, under special conditions) show the usually desirable characteristic of work-hardening with straining, or strain-hardening. In other words, plastic deformation distorts the atoms from their equilibrium positions, and this manifests itself subsequently in hardening.
In fact, hardening by plastic deformation (rolling, drawing, etc.) is one of the most important methods of strengthening metals, in general. Figure 6.1 shows a few deformation-processing techniques in which metals are work-hardened. These industrial processes are used in the fabrication of parts and enable the shape of metals to be changed. The figure is self-explanatory. Rolling is used to produce flat products such as plates, sheets, and also more complicated shapes (with special rolling cylinders). In forging, the top hammer comes down, and the part is pushed into a die (closed-die forging) or is simply compressed. Extrusion uses a principle similar to that in the use of a tube of toothpaste. The material is squeezed through a die, and its diameter is reduced.
The technological developments wrought since the early twentieth century have required materials that resist higher and higher temperatures. Applications of these developments lie mainly in the following areas:
Gas turbines (stationary and on aircraft), whose blades operate at temperatures of 800–950 K. The burner and afterburner sections operate at even higher temperatures, viz. 1,300–1,400 K.
Nuclear reactors, where pressure vessels and piping operate at 650–750 K. Reactor skirts operate at 850–950 K.
Chemical and petrochemical industries.
All of these temperatures are in the range (0.4–0.65) Tm, where Tm is the melting point of the material in kelvin.
The degradation undergone by materials in these extreme conditions can be classified into two groups:
Mechanical degradation. In spite of initially resisting the applied loads, the material undergoes anelastic deformation; its dimensions change with time.
Chemical degradation. This is due to the reaction of the material with the chemical environment and to the diffusion of external elements into the materials. Chlorination (which affects the properties of superalloys used in jet turbines) and internal oxidation are examples of chemical degradation.
This chapter deals exclusively with mechanical degradation. The time-dependent deformation of a material is known as creep. A great number of high-temperature failures can be attributed either to creep or to a combination of creep and fatigue. Creep is characterized by a slow flow of the material, which behaves as if it were viscous.
Upon being mechanically stressed, a material will, in general, exhibit the following sequence of responses: elastic deformation, plastic deformation, and fracture. This chapter addresses the second response: plastic deformation. A sound knowledge of plasticity is of great importance for the following reasons.
Many projects are executed in which small plastic deformations of the structure are accepted. The “theory of limit design” is used in applications where the weight factor is critical, such as space vehicles and rockets. The rationale for accepting a limited plastic deformation is that the material will work-harden at that region, and plastic deformation will cease once the flow stress (due to work-hardening) reaches the applied stress.
It is very important to know the stresses and strains involved in deformation processing, such as rolling, forging, extrusion, drawing, and so on. All these processes involve substantial plastic deformation, and the response of the material will depend on its plastic behavior during the processes. The application of plasticity theory to such processes is presented later in this chapter.
The mechanism of fracture can involve plastic deformation at the tip of a crack. The way in which the high stresses that develop at the crack can be accommodated by the surrounding material is of utmost importance in the propagation of the crack. A material in which plastic deformation can take place at the crack is “tough,” while one in which there is no such deformation is “brittle.”
The separation or fragmentation of a solid body into two or more parts, under the action of stresses, is called fracture. The subject of fracture is vast and involves disciplines as diverse as solid-state physics, materials science, and continuum mechanics. Fracture of a material by cracking can occur in many ways, principally the following:
Slow application of external loads.
Rapid application of external loads (impact).
Cyclic or repeated loading (fatigue).
Time-dependent deformation (creep).
Internal stresses, such as thermal stresses caused by anistropy of the thermal expansion coefficient or temperature differences in a body.
The process of fracture can, in most cases, be subdivided into the following categories:
Nucleation of one or more cracks or voids.
Growth of cracks or voids. (This may involve a coalescence of the cracks or voids.)
Damage accumulation is associated with the properties of a material, such as its atomic structure, crystal lattice, grain boundaries, and prior loading history. When the local strength or ductility is exceeded, a crack (two free surfaces) is formed. On continued loading, the crack propagates through the section until complete rupture occurs. Linear elastic fracture mechanics (LEFM) applies the theory of linear elasticity to the phenomenon of fracture – mainly, the propagation of cracks. If we define the fracture toughness of a material as its resistance to crack propagation, then we can use LEFM to provide us with a quantitative measure of fracture toughness.