What is metallurgy ?
Metallurgy is the study of metals: their composition and properties. By understanding the basic science which governs the properties of a metal it is possible to modify these to greater or lesser extent and to tailor them towards specific uses. It is useful to understand the meaning of some terms commonly used when describing metals.
Load
Load is the overall force to which a material, component or structure is subjected.
Stress
Stress is the load divided by the area upon which that load is imposed, and is therefore measured in pounds per square inch. In partibus infidelis stress is usually measured in metric units, for which we care little.
Strain
Strain is the deformation of a material brought about by the application of stress, to which it is related by Young's Modulus. Strain can be either elastic or plastic. It is our generally held belief, as true for life as it is for metals, that the stress to which one is subject is irrelevant until one begins to show strain.
Elasticity
If a small load is applied to a metal, for example the ends of a bar are pulled apart by a tensile force, the atoms of the metal are also pulled apart to a certain degree. When the force is relaxed, the atoms return to their original positions and the bar regains it initial length. This is elastic behaviour and is a property we make use of in structural steels and springs.
Plasticity
If a larger load is applied to our metal bar it will be seen that above a certain point - the elastic limit - the bar will be permanently stretched and no longer returns to its original dimensions when the load is released. It is said to have undergone plastic deformation. Plastic deformation is the property we make use of when forging a knife blade for example, hammering a billet of metal into a new shape that we wish it to retain.
Hooke's Law and Young's Modulus, E
Robert Hooke, the English physicist, conducted a number of experiments to investigate elasticity and concluded around 1660 that "Ut tensio sic vis" or, in other words, "as the tension, so the extension." That of course means that the stretching of a material is directly proportional to the force causing it up until the point that the elastic limit is exceeded and the material begins to deform plastically. Mathematically, Stress / Strain = E where E is the contant of proportionality - named after another English physicist, Thomas Young. For many materials, Young's modulus is a constant over a range of strains. Such materials are called linear, and are said to obey Hooke's Law. Examples of linear materials include steel, carbon fibre, and glass. Rubber, however, is a non-linear material.
Created on 11/09/2006 11:03 PM by admin
Updated on 11/09/2006 11:48 PM by admin
|
|
