Engineering materials fundamentals and properties

¶ … engineering application is often a more complex feature of an engineering endeavor than might be expected from external observations. There are many different considerations that must be taken into account when selecting materials for use in engineering, including the various strengths of the material, its weight in relation to its strength, its cost, issues involved with its extraction and transportation, and a host of other considerations. An understanding of these basic classes of materials and the properties inherent to each of the classes can be immensely useful in the initial winnowing down of the multitude of materials available for any given project, either by suggesting material classes that have especially desirable properties for a given project, or by demonstrating with equal clarity that a specific class of materials would not be suitable for a specific engineering application being designed, developed, and implemented.

One of the most recognizable classes of materials even to non-engineers is the metal class, though there are specific definitions and properties to the materials in this class that might not be known by the average individual. Metals, which have specific chemical properties that can be typified and catalogued in the periodic table of elements, tend to be quite dense (high weight to volume ratio), ductile (i.e. malleable without leading to fracture), and electrically conductive. Ceramics are other nonorganic, nonmetallic solid compounds, and though hard like many metals they are more brittle. They are also less electrically conductive and are more able to withstand extreme heats without warping or deforming.

The other two material classes are somewhat less easily understood, because they are generally developed through more complex and artificial chemical properties. Polymers are constructed of long chains of a relatively small set of elements, have very low electrical and thermal conductivity, are very lightweight, and are also resistant to corrosion from strong acids and bases, yet are relatively weak in terms of tensile strength. Smei-conductors are typically only used in electrical applications as they have moderate conductive properties and certain resistivity and optical properties that make them ideally suited to certain electrical applications. Semi-conductors also have a low ductility combined with a fair amount of strength and low thermal conductivity, allowing them to maintain their shape in these electrical applications despite the currents they are subjected to.

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Many substances have a great deal of elasticity whether or not they appear to upon initial examination. Steel, for example, though a very hard and a very strong metal substance, also has a fair amount of elasticity -- it will stretch when a load is put on it, rather than simply breaking when the load bearing limits of its specific size and shape are met. This leads to an important distinction between what is known as the yield and the tensile or ultimate tensile strength of a given substance; though both of these terms apply to the strength of the substance in terms of stretching based on loads, the yield strength is the amount of force win which the elastic distortions can be recovered after removal of the load, while ultimate tactile strength refers to the amount of stress a material can withstand before necking and breaking occur.

Yield strength is far more important in most design applications than ultimate tensile strength because the maintenance of a specific shape and size for all material in the given structure is generally quite important for the proper functioning of the structure. In a steel bridge, for example, the elasticity of the steel would be highly important, but the recovery of any stretching due to stress is also highly important, otherwise the bridge would become malformed due to stress and would change the relationship between various other materials used in its construction. Building structures to the yield strengths of the materials used rather than their tensile strengths avoids warping due to stress and strain and is thus the truly necessary design consideration in a wide variety of engineering applications.

In this diagram, where the curve becomes extreme is the yield strength of the substance; the end of the line represents the tensile strength.

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Though "steel" is a commonly used word to refer to certain metal alloys, there are actually significant differences in the different alloys of steel that affect chemical and physical properties of each specific type of steel and thus the manner in which they can be used in engineering applications. One of the ways in which steels are affected by their chemical composition is in their elastic modulus, which varies due to the level of ferromagnetism exhibited by the various iron alloys that make up the family of steel metals. The varying degrees of ferromagnetism and the different temperatures at which this trait is exhibited are themselves due to changes in electron patterns in the individual molecules of the alloy; these same changes in the bonds that are thus formed have concurrent changes in the elasticity of the different steel alloys. Determining the correct type of alloy for any application in which steel is preferred must account for temperature and a variety of other factors in order to arrive at an accurate knowledge of the elastic propensities of any given type of steel.

Data concerning the different elasticities of varying steels has been well-established in the research literature. Chromium and molybdenum alloy steels (Cr-Mo steels) have the highest elasticity, with an e-modulus of elasticity of thirty-one million psi; carbon steels are a close second with just under thirty million psi (Engineering Toolbox 2010). Nickel steels have an e-modulus of just over twenty six million, and other than leaded nickel-bronze alloys these steels far surpass other common metals in terms of their elastic strength (Engineering Toolbox 2010). The degree of closeness in the observed varying elasticities of steel is a result of the high similarities in their ferromagnetic properties, just as the subtle differences result from differences there in these same ferromagnetic properties in the different steel alloys.

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There are many different properties defined for the various materials used in engineering projects and in building applications generally, but not all of these different identified properties are actually fully separable qualities. Due to the high degree of overlap in many of these properties, such that the identified "properties" are in many instances simply different facets or aspects of the same property, extensive knowledge about certain of a metal's properties can be obtained through testing another property. This is the case with hardness and tensile strength; the two are actually different facets of the same basic property of metal, namely its resistance to deformation by various applications of stress. Hardness and tensile strength are not exactly the same, of course; hardness can also refer to resistance to scratching from abrasion and an ability to withstand impact without demonstrating plasticity, but these are also manners in which the material resists deforming.

There is a known general relationship between tensile strength and hardness that allows for the approximation of one if the other is known, and for materials with which research has been conducted more accurate comparisons can be made. Hardness is generally much easier to test for than is tensile strength, and because of this relationship engineers are able to use hardness tests to determine tensile strength and thus the suitability of a given material for a specific application. Though the tensile strength (or more specifically, the yield strength) of a material is often the necessary design consideration, indirectly determining this strength based on the easier-tested hardness is often a preferred method of progressing.