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.
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.
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Just as there are several different properties that can be determined by testing a single property, there are also several useful and accepted methods for testing specific properties of a material. Two common tests that are utilized for testing the ductility and the fracture resistance of a material are the Charpy impact test and the direct measure of fracture toughness. There are advantages and disadvantages to both approaches, but the primary advantages to the Charpy impact test are in the practicalities of the test itself -- it is well established and, given proper equipment that is well-calibrated, the results are incredibly easy to obtain. At the same time, these results are really only useful as comparisons, with more qualitative than purely quantitative results, and the quantitative data that this test can lead to -- while still highly reliable -- is all achieved through inference.
Direct testing of fracture toughness, however, provides direct quantitative measurements of certain aspects of a material's strength, and does not simply provide a qualitative analysis of this strength based on the comparative responses of the material and other materials. This is why it is more beneficial in design decisions and engineering applications; the information it provides is more concrete and more useful, even though the test itself is more difficult to conduct.
Engineers Toolbox. (2010). Young midlus for metals and…