The Specification and Testing of Motorcar Steels.
Page 12
Page 13
If you've noticed an error in this article please click here to report it so we can fix it.
The remarkable development of the motor-vehicle industry has opened up for general study many subjects; the majority of these at one time received attention from the exponents of only a few highly-skilled trades and professions. The motor vehicle has made familiar objects many things which previously were considered instruments for the special and exclusive use of scientists. To-day, the skilled mechanic manipulates a micrometer with the same ease and confidence with which his old-time shopmate handled a pair of callipers– certainly with far greater accuracy. The standard of intelligence among workmen has been raised to a degree of which old-time shopmen did not even dream ; as motoring engineering has advanced, its demands for increased efficiency and high-grade materials have been felt by allied trades, and by none more so than steel makers. Some steel works, which for a great many years had satisfactorily met the most-exacting requirements of their time, were for a brief period ill at ease when it was demanded of them that they should produce materials capable of sustaining heavy and continuous stress and vibration. Steel makers, however, are men of metal in more senses than one, and, once they are convinced that a particular duty is required of a piece of material, they spare no effort, mental or physical, until they have produced that which is required.
Many steel users, unfortunately, do not know what they require. Developments in connection with the manufacturing side of the industry have been so rapid, that these men have been unable to devote any time to the study of metals, and, as many of them previously had no need for such special knowledge, their paucity of information on the subject leads them to issue to steel makers specifications which betray a lamentable lack of acquaintance with even the elements of metallurgy. Some of the specifications issued are impossible of attainment, and, even if possible, steel, so produced would not give the desired result when incorporated in the anatomy of a motor vehicle. Few motor manufacturers or their departmental managers will ;idittit this lack of knowledge, but some of their communications to and conversations with steel makers belie their protestations of competence to prepare specifications of materials. There is a large amount of misconception, amongst these men, as to the meanings of the various terms which are to be found in the metallurgist's vocabulary, and one frequently hears such expressions as stress, strain, tensile strength, ductility, elastic limit, yield point, etc.. misapplied by individuals who should really know their true meanings. It is not inopportune, therefore, that we proceed to give, in simple language, some explanations of the various terms used in regard to the testing of metals and alloys. The careful testing of materials is absolutely necessary, in order to determine their suitability for any given purpose, and it is equally necessary that every engineer who has to deal with such matters should possess a thorough understanding of the commonplace expressions of the test house. Of the terms quoted, the second one is misapplied most frequently, the word strain being frequently used when stress is meant.
Stress and Strain.
Stress is the term reiferring to the state produced in a specimen of material by the application of a load, which load may ,produce tensile stress (pulling), compression stress (crushing), torsional stress (twisting), transverse stress (bending), or a combination of two or more of these methods of loading. Strain, however, is a measure of the amount of distortion of the material produced by the application of the load, or stress. So long as the stress does not exceed the limit of elasticity of the material, the amount of distortion, or strain, is proportional to the intensity of the load, or stress, but directly the load, or
stress, is increased beyond the elastic limit, the strain increases at a greater rate ; this characteristic, however, is made clear in the paragraph on " Elastic Limit and Yield Point."
Ultimate Tensile Strength and Breaking Stress.
The tenacity, or tensile strength, of a metal is the measure of the ability of its particles to hold together and to resist separation, or fracture, by forces acting in opposite directions. The usual method adopted by engineers to ascertain the tenacity, or tensile strength, of a particular grade of metal is to begin by preparing a test bar of known length and cross section ; the ends of such a bar are then gripped in the special jaws of a testing machine, which machine is so constructed that the two ends may be pulled apart until the liar breaks into two portions. The load thus applied is, of course, a dead load : it does not fluetuate in its intensity, from zero to maximum, and vice rersc but is gradually applied and increased, slowly, from zero to maximum, and then, as will be made clear later, the load decreases very slowly, until the test piece is finally broken. This is known as the static tensile test, and, whilst it gives a true indication of the metal's twatcity, it cannot he accepted as the only test of a metal's suitability for a given purpose, especially if it is expected to sustain severe shocks.
It is, of course, necessary that, engineers should have some common basis of comparison for the tenacity of steel, and, for that reason, in those countries in which the inch and the pound or ton are units of length and weight respectively, a material is said to have a tensile strength of so many pounds or tons per square inch, whilst, in those countries which have adopted the metric system of measurement, the tenacity is expressed as so many kilograms per square millimetre. This may better be understood, if we take a simple example in which the inch-pound notation is employed. We say that a piece of steel has a tensile strength of 60,0(1(1 lb. per sq. in., if the steady application of a load of 60,000 lb. breaks into two pieces a bar which had an original section of 1 sq. in. at the point of fracture. For obvious reasons, test pieces of 1 sq. in. in section are not usually employed, but, no matter what size or shape of piece be submitted to the tensile test, if the breaking load in pounds be divided by the area of the test bar in square inches, the resultant represents the tensile strength of the material in pounds per square inch. If a load of 15,000 lb. be just sufficient to break a bar having a cross-sectional area of sq. in., then 15,000 lb. divided by sq. in. gives us 60,000 lb. per sq. in., just as in the previous example. The breaking stress is rather less than the ultimate tensile strength of the material, but this is only because the specimen becomes stretched by the application of the test load, and the section is thus reduced. Consequently, the test specimen actually breaks under a load smaller than it has previously sustained. This known fact will be made clear in a later section, which will deal with the construction and functions of a tensile-testing machine.
The bars may be of any shape and section, but, for convenience and ease of comparison, the British Engineering Standards Committee has recommended that definite sizes be used—not necessarily all alike, but having the gauged length on which the elongation is measured proportional to the area of the cross section ; for all materials except steel plates and castings, the test pieces are generally given the form of an accurately-turned bar, the central portion of which is exactly .7979 in. in diameter, or exactly sq. in in area, with two enlarged ends, each of which is 1i in in diameter, in order that they may be gripped in the jaws of the testing machine. For the testing of steel plates, flat or rectangular test pieces are prepared, and their sectional area may easily be computed. Elastic Limit and Yield Point.
Elasticity is the property or ability of a metal to resume its original form after it has been distorted by the application of a load. All bodies are distorted to a greater or lesser extent, when they are stressed by loading, and all bodies will more or less recover their normal shape when the stress is withdrawn. An ideally-elastic body will instantly fly back to its normal shape, as soon as the distorting load is withdrawn, but metals are not perfectly elastic: they require periods of rest between the withdrawing of one load and the application of another, in order that they may recover their normal condition. Repeated application of a load results in the development of fatigue in the metal. Up to a certain stress, the metal will give a definite elongation, or strain, for every increment of the load applied, the strain, as we have already stated, being proportional to the load, or stress. Beyond that point, however, the strain ceases to be proportional to the stress—it increases at a more-rapid rate: this point is the elastic limit, and up to that stage the specimen will return to its original shape and length when the load is removed. If the load be increased beyond the elastic limit, the distortion becomes permanent—the material does not return to its original length and shape when the stress is removed. The load under which permanent distortion, or permanent set, occurs is known as the yield point, and is usually very-slightly higher than the elastic limit. The determination of the latter is the most-important factor in the testing of steel, as it is a valuable guide to the designer, and permits him to decide what factor of safety he may allow. It is very difficult accurately to determine the elastic limit of a metal, but the yield point, which approximates very closely to the elastic limit, may readily he ascertained. After the elastic limit has been passed, a test bar will become appreciably longer, without any corresponding increase of the test load. The yield point is reached when extension of the test specimen becomes apparent. The difference between these two terms should be appreciated by all users.
Ductility, Elongation and Contraction.
When stress is applied to the two ends of a piece of steel, as we have stated, the length of the specimen is increased, and its cross section diminished; the extent to which this change of shape continues is determined by the ductility of the metal. Ductility is that property of a metal which allows of the free " flow " of its particles under lateral pressure. If we may coin a term, ductility may he expressed as the " workability " of a material. If accurate measurements of the distance between two points of a test bar be taken before the load is applied, and their distance apart be again measured after fracture has taken place, the amount of elongation can be ascertained, and the ductility of the material can then be calculated as a percentage of the length under stress, and expressed as the elongation per cent. Similarly, the area of fractured surface may be measured, and, by comparison and computation with the original sectional area, the reduction of area per cent, may he derived.