Handbook of testing materials for the constructor

My book on Testing Materials for the Constructor is designed to be a counsellor to the constructor in all questions relating to the properties of his materials of construction. Therefore the book is divided into two volumes, each independent and complete in itself.

This first volume relates to the general properties of materials of construction, and especially to the art and science of testing materials as applied to machinery and superstructure.

To the description of the customary methods of testing I have added a presentation and discussion of the most important types of testing-machines and auxiliary apparatus, dwelling mainly upon the underlying principles of design, sources of errors, and on their calibration.

As this volume contains the manifold experiences of the laboratories under my direction, and as I have availed myself of the liberal arrangements granted by the publishers to fully illustrate, by figures and plates, the most important machines and instruments of all countries, I hope to produce a lasting benefit, not alone to my students, but also to manufacturers of apparatus, by my frank and candid criticism.

I do not wish to let this volume go forth without thanking those who, jointly with me and under my direction, have helped to increase our stock of knowledge to the best of their ability, and have hence directly assisted in this work.

How much the publishers have done to perfect this book, the book will show for itself.

The bed, pedestal, frame of a machine may change its shape under stress - e.g. in a steam-engine by the piston pressure; in a press by the plunger ; in the lathe while taking a cut - only to such a small degree that noticeable relative displacements of individual parts do not occur. It must be ascertained of the material of which the separate parts consist that it answers these requirements, that it behaves as nearly as possible like a rigid body and can safely bear the various loads to which it is subjected in the steam-engine, the press, the lathe. The movable parts of machines must, however, answer their purpose in a similar manner, being intended to transmit motion and loads from one part of a machine to another, while they are constrained to travel in prescribed paths, by the shape, of the rigid parts. The piston, the plunger, the connecting-rod, the crank of the steam-engine are examples of this case. But these parts as well, considering each for itself, must not change their shape to a noticeable degree; they should be constructed of materials as rigid as possible.

Physics teaches us that there are no strictly rigid bodies, that in so called rigid bodies all particles are in motion, and that a body is called rigid when it does not change its shape, of itself, under the influence of gravity, but which opposes a resistance to every attempt to change its external shape. Accordingly absolute rigidity must not be expected of materials used for the constructional parts above mentioned, but a definite resistance for the purposes intended under a determined change of shape must be satisfactory. In other words, a definite degree of rigidity must be demanded of them.

The material of construction must be resistant.

Other parts of machines must do a certain work, undergoing material changes of shape under the effect of external forces, and again returning to their original shapes when released. Such details are found in the suspension springs of locomotives; buffer springs, resisting the shock between cars; the bow which launches the arrow; etc., etc. Materials possessing these properties are called elastic.

The property of elasticity is requisite in all materials of construction in addition to resistance.

Bodies which slide upon each other like steam-engine crossheads, pistons in cylinders, shafting journals in boxes, produce friction and abrasion, which are both accompanied by considerable absorption of work, and as work costs money, must be avoided by the careful constructor and must be reduced to a minimum. Experience teaches that hard materials are abraded less than soft ones. The hard body resists penetration by a foreign body more than a soft one. Hardness and softness cannot, however, be considered as opposites, but the latter must be considered merely as a lesser degree of the former.

Materials of construction should therefore possess the property of hardness as well.

It happens in our machines occasionally that the forces acting on some parts are applied suddenly, even by impact, and experience teaches us that some bodies do not withstand such loads. They break like glass, while others withstand even heavy impact, sometimes undergoing considerable change of shape and retaining it. Materials of the first variety are called brittle, while those of the second are commonly called tough (ductile).

Toughness and brittleness are therefore properties of materials which must be subject to examination.

Thus far properties of materials were considered which were required of them for constructive purposes, and we must now consider those which they must develop in order that they may be readily given the forms required for the desired detail. To keep these two groups of properties easily separated and readily identified hereafter, arbitrary names shall be adopted for them, never forgetting, however, that such divisions into groups are by no means precise or definite.

The properties first treated of we shall call the mechanical properties of materials, because these are predominantly required in the mechanical application of loads in construction.

The properties now to be considered we shall call technological properties, because they are in predominant evidence in the fabrication of materials into constructive detail.


B. Technological Properties.

The transformation of materials into constructional detail is carried on in many ways. The materials must therefore develop manifold properties; they must be workable, i.e. they must be in such condition or capable of being put in a condition that they can be transformed into the desired final shape. This can be attained by

1. Division, i.e. separation into individual parts of the mass;

2. Transformation of the mass without separation; and

3. Agglomeration, i.e. uniting of various parts into one mass.

Among the processes of the first kind, viz. division, are working by edge-tools, such as wedges, shears, saws, chisels, gouges, drills, etc., etc. To make these tools serviceable the material must possess the properties previously described, to a more or less pronounced degree. Here the properties of. resistance, elasticity, hardness, toughness, and brittleness come into question.

In the second group, in which fabrication is carried on by transformation, the preceding properties must be considered as well, but in addition there are other properties not previously discussed.

The nomenclature of these properties in particular is derived from the methods of fabrication to which the material was subjected.

Other substances which are but slightly malleable or formable in their ordinary condition may, by certain processes, be transformed into bodies readily malleable and formable. When this becomes possible by the application of heat, and they are wrought by hammers, rolls, etc., the material is said to be forgeable, rollable, etc.

Sometimes it becomes possible to soften a body with- out application of heat, by adding another material which permits of its being moulded, and after this transformation removing the added material, as is done in moulding clay and porcelain, by addition of water. In this condition the material is called plastic.


C. Physical Properties.

In addition to the technological properties above considered which must be possessed by materials suitable for structural detail or which it must develop during conversion into such, there is a series of properties inherent in materials in every shape, and which, in addition to the first mentioned, go to make up their physical and chemical character. These form two additional groups, the physical and chemical properties, to be distinguished.

While these properties of course do not concern the constructor as directly as the above-considered technological properties, yet he must at times know their degree most accurately if he desires to calculate and construct reliably and correctly.

In the first place, as a physical property of materials, specific gravity and density must be considered. Hereafter the term specific gravity shall be used to designate the true specific gravity of the material itself, which must be determined from pieces free from interstices.