MATERIALS, STRENGTH OF, is a branch of applied mechanics which deals with the effects produced by forces in the materials of architectural and engineering construction. Its aims (in so far as these are practical) are to discover rules whereby the strength of a given part may be assessed, and on these to base general principles of design, whereby each member of a structure or machine may be given the material and proportions best suited to its function.
Thus, to take a fairly simple example, the choice of suitable material and proportions for a locomotive coupling rod is a problem which falls within the province of our subject. The rod is required to transmit from one wheel to the next a force which will depend upon the speed and on the tractive effort of the loco motive, and which will fluctuate during each revolution of the wheels. In addition, it will be subjected to bending actions, due to its own up-and-down motion, which will depend not only upon the speed of the locomotive but also upon the mass of the rod itself. When the material and proportions have been (provision ally) determined, the magnitudes of these different forces can be estimated : the question then presents itself, whether the strength of the rod will be adequate to its task.
At the outset it is evident that two factors are involved. What ever be the material, a member will break or bend if its cross section is too small : whatever be its size, certain properties (such as hardness or rigidity) are required of the material. So, follow ing our subject, two investigations must be pursued. First, we must be able to predict the state of stress (that is, of internal action) which results when specified forces are applied to a body of specified form; secondly, we must be able to decide whether these internal actions can be brought into existence without detri ment to a specified material. The first line of investigation is the province of the theory of elasticity; the second, that of metallurgy and the testing of materials: our problem is to combine, for the guidance of the architect or engineer, the knowledge which these different sciences afford.