The relation between breaking stress and extension in tensile tests of steel

A large number of the tensile tests of steel are now made with test-pieces, which are only a few diameters long (fig. 1). When such a test-piece is broken by tension, it has a profile, as shown in fig. 2. The usual records, made when the tests are carried out, include, among other things, “breaking stress” and “extension per cent.” “Breaking stress” here means the maximum tension applied divided by the original area of the test-piece; and extension per cent, is taken as the percentage increase due to the strain, in the distance between two marks, one at either end of the test-piece, whose unstrained distance is known. The use of the term “breaking stress” in the above sense is convenient, from an engineer’s point of view, as showing what force a bar, etc., of given sectional area will stand before giving way. The true breaking stress of a material, however, is the actual intensity of the stress at the broken surface, and is, of course, greater than the nominal breaking stress, because of the reduced area of the broken surface. To avoid con­fusion, I will call the true breaking stress the “intrinsic strength” of the material.