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MECHANICAL PROPERTIES OF METALS AND ALLOYS

The mechanical properties are of chief consideration in the larger industrial applications of metals so they require much attention in their study.


Strength. - The strength of a material is the property of resistance to external loads or stresses without incurring structural damage. The term ultimate strength refers to the unit stress (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 tensile test is the one most often applied to metals because it tells so much more about their properties than any other single test. In metallurgy, breaking is often spoken of as failure, or rupture, or fracture; the fracture of a metal is the name given to the surface across which the break has occured.

The strength of metals and alloys depends upon two factors, viz., the strength of the crystals of which the metals are constructed and the tenacity of adherence between these crystals. The strongest substance known is the tungsten wire of incandescent electric lights. Pure iron is weak, but when alloyed with carbon to make steel, the steel may be stronger than any of the pure metals except tungsten.


Stress and Strain. - A stress is the force within a body which resists deformation due to an externally applied load. If this load acts upon a surface of unit area, it is called a unit force and the stress resisting it a unit stress. Quantitatively, then, stress is a force per unit area; on the European continent it is expressed in kilograms per square millimeter, in the United States, pounds per square inch, while in England long tons per square inch is commonly used.

When an external force acts upon an elastic material, the material is deformed and the deformation is in proportion to the load. This distortion or deformation is strain, and unit strain is measured in the United States and in England in inches per inch, while in Europe it is measured in centimeters per centimeter. Unit strain, then is a ratio of distances or lengths.


Elasticity. - Any material subjected to an external load is distorted or strained. Elastically stressed materials return to their original dimen­sions when the load is released if the load is not too great. Such distortion or deformation is in proportion to the amount of the load up to a certain point, but when the load is too great the material is permanently deform­ed, and when the load is increased further to a certain point the material breaks. The property of regaining the original dimensions upon removal of the external load is known as elasticity.


Modulus of Elasticity. - Within the limit of elasticity the ratio of stress to strain is known as the modulus of elasticity (i. e. measure of elasticity).

The modulus of elasticity expresses the stiffness of a material. For steel and most metals this is a constant property very little affected by heat-treatment, hot or cold working, or the actual ultimate strength of the metal. Their moduli of elasticity show that, when equal-size bars of steel and aluminium are subjected to the same load, the resulting elastic deformation in the aluminium will be almost three times as great as in the steel bar.


Proportional Elastic Limit. - Metals generally are not elastic over the entire loading range. The limit of proportionality of stress to strain is known as the proportional limit. The elastic limit is the highest unit stress which the specimen under test will support and still return to its original dimensions when the load is removed. The proportional limit and the elastic limit in metals are very close together, so much so that they are often confused, and it is now common to combine them in the single term proportional elastic limit. This is an important property, a stress which must not -be exceeded in engineering design.


The Nature of Elasticity. - The elasticity of a metallic substance is a function of a resistance of its atoms to separation or compression or rotation about one another, and thus is a fundamental property of the material. So elasticity is demonstrated as a function of atomic forces. This explains why the modulus of elasticity of a strong and brittle heat-treated alloy steel is exactly the same as that of a comparatively weak and ductile annealed steel.


Yield Point. - This is a point on the stress - strain curve at which the stress levels off or actually decreases while strain continues. The term is strictly applicable only to mild steels, since the characteristic which defines it is not found in other metals or in alloy steels or even in cold-worked or normalized low-carbon steels.


Ultimate Strength. - The greatest load that the specimen has support­ed divided by the original cross-sectional area is called the ultimate tensile strength or the ultimate strength of the piece.


Ductility. - Ductility is the capacity of a metal to be permanently deformed in tension without breaking. Specifically, the term denotes the capacity to be drawn from a larger to a smaller diameter of wire. Such an operation obviously involves both elongation and reduction of area, and the values for these two characteristics of a metal determined in the tension test are usually taken as a measure of the ductility of the metal.


Toughness. - Toughness has been defined as the property of absorbing considerable energy before fracture. It 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. One of the com­monest tests for toughness is 'the impact test, in which the energy absorb­ed by breaking a specimen by a sudden blow is measured.


The Nature of Toughness. - The toughness of a metal is indicated by the amount of slip which may occur within the crystals without resulting in rupture of the metal. It is perhaps the result of alternate slippings and wedgings each wedged crystallographic plane holding until a greater stress is applied. A brittle metal or alloy either will not stop slipping after the elastic strain is reached, or else will stop only for a brief period before breaking. Obviously, the successive stopping and slipping will produce deformation; therefore, the tough metals and alloys are often the most malleable and ductile.

Sometimes the crystals of a metal may be tough, but the crystal boundaries may contain impurities so that the least deformation of the crystal mass may cause cracking through the brittle grain boundary material. This is true of steel containing considerable phosphorus and of copper containing bismuth.


Malleability. - Malleability is the property of a metal which permits permanent deformation by compression without rupture. 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: while lead and tin are relatively high in order of malleability, 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 at a bright red heat, (l000°C).


Brittleness. - Brittleness implies sudden failure. It is the property of breaking without warning, i.e., without visible permanent deformation. It is the reverse of toughness in the sense that a brittle body has little resistance to rupture after it reaches its elastic limit. Brittleness is the opposite of ductility in the sense that it involves rupture without much deformation. Often the hard metals are brittle, but the terms should not be confused or used synonymously.


Fatigue Failure. - If metal is subjected to frequent repetitions of a stress, it will ultimately rupture and fail.

Alternations of stress will produce failure more rapidly than repetition of stress. By "alternations of stress" is meant alternate tension and compression in any fiber. Failure of metals and alloys under repeated or alternating stresses, too small to produce even a permanent deforma­tion when applied statically, is called fatigue failure.


Corrosion Fatigue. - If a member exposed also to corrosive agencies such as damp atmosphere or oil not freed from acid, the stress necessary to cause failure is much lower. The strongest steels will fail under fatigue and corrosion with a unit fiber stress of not more than 24000 psi, even when their ultimate strengths might indicate that they could withstand a much higher stress. It is interesting to note that the unit stress of an extremely strong heat-treated alloy steel subject to corrosion fatigue will be not greater than that of a relatively weak structural steel. The importance of protecting the surfaces of fatigue members against corro­sion by galvanizing, plating, etc., if and when possible, is obvious.


Hardness. - The quality of hardness is a complex one which detailed study has shown to be a combination of a number of physical and mecha­nical properties. It is more often defined in terms of the method used for its measurements and usually means the resistance of a substance to indentation. Hardness may also be defined in terms of resistance to scratching, and thus is related to wear resistance. The term hardness is some­times used to refer to the stiffness or temper of wrought products because the indentation hardness of a metal is closely related to its tensile strength.

In engineering practice the resistance of a metal to penetration by a hard indenting tool is generally accepted as defining the hardness pro­perty. A number of standardized testing machines and penetrators have been devised, the most common of which are the Brinell, Rockwell, and Vickers machines.






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