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Technical Documents - Documentos Técnicos: Identification of Physical and Mechanical Properties of Various Metals

Identification of Physical and Mechanical Properties of Various Metals

a. General.

The distinguishing characteristics or qualities that are used to describe a substance such as metal are known as its physical properties. Those physical properties which describe the behavior of a metal when it is subjected to particular types of mechanical usage are called mechanical properties.

Subsequent paragraphs describe the physical and mechanical properties of metals. The mechanical properties are of chief concern and will therefore receive greater coverage.

b. Definition of Metal and Alloy.

(1) Before going into a discussion of the properties of metals, first let us define the terms "metal" and "alloy". The basic chemical elements are divided into metals and nonmetals; however, there is no sharp dividing line between the two. A metal may be defined as a chemical element that possesses metallic luster and which, in electrolysis, carries a positive charge that is liberated at the cathode. Most nonmetallic elements do not possess metallic luster, and in electrolysis the nonmetals carry negative charges that are liberated at the anode. Of all the natural chemical elements, about 70 are metals and, of these, about 39 are used commercially.

(2) An alloy is a metallic substance, but it is not a single chemical element. An alloy is formed by the union or mixture of two or more metals; in some cases, it may consist of one or more metals and a nonmetal.

Examples of alloys are iron and carbon, forming steel, and the great variety of copper alloys, such as brass and bronze.

c. Physical Properties.

These properties are related to the atomic structure and density of the material, as described in the following paragraphs.

(1) Co-efficient of Linear Expansion. The co-efficient of linear expansion is the increase in length of a body for a given rise in temperature. The increase is the changed length of a rod for each degree that the temperature is increased. Metal expands when heated and contracts when cooled. It increases not only in length, but also in breath and thickness. The increase in unit length when a solid is heated one degree is called the co-efficient of linear expansion.

(2) Heat and Electrical Conductivity. Heat and electrical conductivity is the ability of a material to conduct or transfer heat or electricity.

(3) Magnetic Susceptibility. Magnetic susceptibility is the ability of a material to hold a magnetic field when it is magnetized.

(4) Reflectivity. Reflectivity is the ability of a material to reflect light or heat.

(5) Specific Gravity. Specific gravity is the ratio of weights between two objects of equal volume, one of which is water.

(6) Melting Point. The melting point is the temperature at which a substance passes from a solid state to a liquid state.

d. Mechanical Properties.

(1) Strength. The strength of a material is the property of resistance to external loads or stresses while not causing structural damage. Ultimate strength is the unit stress, measured in pounds per square inch, developed in the material by the maximum slowly applied load that the material can resist without rupturing in a tensile test. The strength of metals and alloys depends upon two factors: the strength of the crystals of which the metals are constructed, and the tenacity of adherence between these crystals. The strongest substance known is tungsten-molybdenum; titanium and nickel follow in order of strength of commercially pure metals. Pure iron is much weaker, but, when alloyed with the chemical element known as “carbon" to make steel, it may then become stronger than any of the pure metals except tungsten. Strength and plasticity are considered the two most important properties that a metal can possess.

(a) Tensile Strength. Tensile strength is the ability of a metal to resist being pulled apart by opposing forces acting in a straight line. It is expressed as the number of pounds of force required to pull apart a bar of material 1 inch wide and 1 inch thick. The tensile test is the one most often used to measure the strength of metals. Pure molybdenum has a high tensile strength and is very resistant to heat. It is used principally as an alloying agent in steel to increase strength, hardenability, and resistance to heat.

(b) Shear Strength. Shear strength is the ability of a material to resist being fractured by opposing forces acting in a straight line but not in the same plane.

(c) Compressive Strength. Compressive strength is the ability of a material to withstand pressures acting on a given plane.

(2) Elasticity. Elasticity is the ability of material to return to its original size, shape, and dimensions after being deformed . Any material that is subjected to an external load is distorted or strained. Elastically stressed materials return to their original dimensions when the load is released, provided that the load is not too great. Distortion or deformation is in proportion to the amount of the load, up to a certain point. If the load is too great, the material is permanently deformed, and, when the load is further increased, the material will break. The property of regaining the original dimensions upon removal of the external load is known as elasticity.

(a) The elastic limit is the point at which permanent deformation begins.

(b) The yield point is the point at which a definite deformation occurs with little or no increase in load.

(c) The yield strength is the number of pounds per square inch required to produce deformation to the yield point.

 

Fig. 1 - A: Tensile Strength, B:Shear Strength, C: Compressive Strength, D: Elasticity, E: Ductility, F: Malleability,

(3) Modulus of Elasticity. The modulus of elasticity is the ratio of the internal stress to the strain produced. It expresses the stiffness of a material. For steel and most metals, this is a constant property and is affected very little by heat treatment, hot or cold working, or the actual ultimate strength of the metal.

According to Hooke's Law: "The degree to which an elastic body bends or stretches out of shape is in direct proportion to the force (stress) acting upon it." But, this law only applies within a certain range.

(4) Ductility. Ductility is the capacity of a material, such as copper, to be drawn or stretched under tension loading and permanently deformed without rupture or fracture. Specifically, the term denotes the capacity to be drawn from a larger to a smaller diameter of wire. This operation involves both elongation and reduction of area.

(5) Malleability. Malleability is the property of a metal to be deformed or compressed permanently without rupture or fracture. Specifically, it means the capacity to be rolled or hammered into thin sheets. The property of malleability is similar to but not the same as that of ductility, and different metals do not possess the two properties in the same degree. Lead and tin are relatively high in order of malleability; however, they lack the necessary tensile strength to be drawn into fine wire. Most metals have increased malleability and ductility at higher temperatures. For example, iron and nickel are very malleable when heated bright-red.

(6) Plasticity. Plasticity is the ability of a metal, such as gold, silver, or lead, to be deformed extensively without rupture. This property, together with strength, are considered to be the two most important properties that a metal can possess.

(7) Toughness. Toughness is a combination of high strength and medium ductility. Toughness is the ability of a material or metal to resist fracture, plus the ability to resist failure after the damage has begun. In short, a tough metal, such as a cold chisel, is one that can withstand considerable stress, slowly or suddenly applied, and that will deform before failure. Toughness has been defined by some metallurgists as having the property of absorbing considerable energy before fracture and, therefore, involves both ductility and strength. Toughness is a measure of the total energy-absorbing capacity of the material, including the energy of both elastic and plastic deformation under a gradually applied load. Generally speaking, toughness applies to both strength and plasticity. Thus, a very easily deformed substance of low strength would not be considered tough, nor would a material of high strength, but with little plasticity, such as hardened tool steel. The true tough metal is one that will rapidly distribute within itself both the stress and resulting strain caused by a rapidly applied load.

(8) Brittleness. The term "brittleness" implies sudden failure. It is the property of breaking without warning; that is, without visible permanent deformation. It is the reverse of toughness in the sense that a brittle piece of metal has little resistance to rupture after it reaches its elastic limit. Brittleness can also be said to be the opposite of ductility, in the sense that it involves rupture with very little deformation. In many cases, hard metals are brittle; however, the terms should not be confused or used synonymously.

(9) Corrosive Resistance. Corrosive resistance is the resistance to eating away or wearing by the atmosphere, moisture, or other agents, such as acid.

(10) Abrasion Resistance. Abrasion resistance is the resistance to wearing by friction.

(11) Fatigue. When metal is subject to frequent repetitions of a stress, it will ultimately rupture and fail, even though the stress may not be sufficient to produce permanent deformation if continuously applied for a relatively brief time. Such a repetition of stress may occur, for example, in the shank of a rock drill. Alternation of stress will produce failure more rapidly than repetition of stress. Alternations of stress mean the alternate tension and compression on any material. The definition of fatigue is the failure of metals and alloys that have been subjected to repeated or alternating stresses too small to produce a permanent deformation when applied statically.

(12) Corrosion Fatigue. Failure by corrosion fatigue is a fatigue failure in which corrosion has lowered the endurance limit by the formation of pits which act as centers for the development of fatigue cracks. Moreover, when any protective film that has been placed on the metal is broken by fatigue stresses, corrosion spreads through the cracks in the film and produces pits which act as stress raisers. If a metal member exposed to fatigue is also exposed to corrosive agencies, such as a damp atmosphere or oil that has not been freed from acid, the stress necessary to cause failure is lowered. It is interesting to note that the unit stress of an extremely strong heat-treated alloy steel that is subjected to corrosion fatigue will be no greater than that of a relatively weak structural steel. The importance of protecting the surfaces of fatigue members against corrosion by galvanizing, plating, etc., is obvious.

(13) Machinability. Machinability is the ease or difficulty with which a material lends itself to being machined.

(14) Hardness. Hardness is the ability of a material to resist penetration and wear by another material. It takes a combination of hardness and toughness to withstand heavy pounding. The hardness of a metal is directly related to its machinability, since toughness decreases as hardness increases. Steel can be hardened by heat-treating it. The object of heat- treating steel is to make the steel better suited, structurally and physically, for some specific application.

 

 

 

 

 

 
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