Materials of Engineering Volume 3 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1889 Excerpt: ...and trustworthy; as our knowledge of the effect of prolonged and of intermittent stress becomes more certain and complete; as our formulas are improved and rationalized, and as their empirically determined constants are more exactly obtained, the factor of safety is gradually reduced, and will finally become a minimum when the engineer acquires the ability to assume with confidence the conditions to be estimated upon, and to say with precision how his materials will continuously carry their loads. A characteristic distinction between the ductile non-ferrous metals and ductile iron or steel, is that the former have usually, as purchased, no true elastic limit, but yield to small stresses without recovery of form and their permanent set equals their maximum distortion. Where brittle, they are often very elastic, however, and recover fully. In such cases, the elastic limit coincides with their ultimate resistance to fracture, as is the case with glass, hard cast iron, and often with hardened steel. In the table above it is assumed that an elastic limit occurs at the point at which the elongation becomes 0.0010 of the total length of the piece stretched. In some cases it is advisable to design some minor part, or element, of a train with a lower factor of safety, to insure that when a breakdown does occur it shall be certain to take place where it will do least harm. 149. The Measure of Resistance to strain is determined, in form, by the character of the stress. By stress is here understood the force exerted, and by strain the change of form produced by it. Tenacity is resistance to a pulling stress, and is measured by the resistance of a section, one unit in area, as in pounds or tons on the square inch, or in kilogrammes per square centimetre or square millim...