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Ebook modern physical metallurgy and materials engineering (6th edition): Part 2

(BQ) Part 2 book "Modern physical metallurgy and materials engineering" has contents: Mechanical behaviour of materials, strengthening and toughening, modern alloy developments, plastics and composites, corrosion and surface engineering,. and other contents. | Chapter 7 Mechanical behaviour of materials 7.1 Mechanical testing procedures 7.1.1 Introduction Real crystals, however carefully prepared, contain lattice imperfections which profoundly affect those properties sensitive to structure. Careful examination of the mechanical behaviour of materials can give information on the nature of these atomic defects. In some branches of industry the common mechanical tests, such as tensile, hardness, impact, creep and fatigue tests, may be used, not to study the ‘defect state’ but to check the quality of the product produced against a standard specification. Whatever its purpose, the mechanical test is of importance in the development of both materials science and engineering properties. It is inevitable that a large number of different machines for performing the tests are in general use. This is because it is often necessary to know the effect of temperature and strain rate at different levels of stress depending on the material being tested. Consequently, no attempt is made here to describe the details of the various testing machines. The elements of the various tests are outlined below. 7.1.2 The tensile test In a tensile test the ends of a test piece are fixed into grips, one of which is attached to the load-measuring device on the tensile machine and the other to the straining device. The strain is usually applied by means of a motor-driven crosshead and the elongation of the specimen is indicated by its relative movement. The load necessary to cause this elongation may be obtained from the elastic deflection of either a beam or proving ring, which may be measured by using hydraulic, optical or electromechanical methods. The last method (where there is a change in the resistance of strain gauges attached to the beam) is, of course, easily adapted into a system for autographically recording the load–elongation curve. The load–elongation curves for both polycrystalline mild steel and copper are shown in Figures 7.1a .