When choosing iron or steel for any particular work, we should consider tests made on material undergoing similar stresses to those in actual practice, and on test pieces as near as possible to the actual size of the pieces actually used. For example, the usual physical tests recorded are : the tenacity or tensile strength, which is the force with which a body resists being pulled asunder, stated in pounds or tons per square inch; the limit of elasticity, or the point at which a body is permanently deformed when subjected to a stretching strain, reckoned in pounds or tons per square inch, sometimes confused with the yield point, which is near to, but not the same as the elastic limit ; the ductility, which is the extent to which a body elongates before breaking, measured as a percentage of the length; the reduction in area, or the amount a test piece is reduced in size at the point of fracture; the com pressive strength, which is the pressure required to produce fracture; and the hardness, which is measured by the amount of indentation produced by a weighted steel ball 10 mm. diameter with a pressure of 3,000 kg. The hardness number is obtained by dividing the area of the spherical indentation into the pressure. Fig. 68 shows the hardnesses of some alloy steels. These are also hot and cold bending tests required for some material. For example, in specifying for wrought iron it is general to require the specimen to be bent hot and cold through an angle of 180 without signs of breaking.

These tests are evidently useful only for choosing a material that has to withstand static stresses, and are useless when we have to consider the effect of dynamic forces, such as forces due to impact, rotary vibrations or combinations of many dynamic forces, acting in union or intermittently. The point is to test the proposed material under as nearly the practical conditions as possible, for while we may have a small test piece with high static properties, we have no indication of how large forgings will act under dynamic stresses, as the difficulties encountered in the heat treatment of large pieces of metal are absent when small pieces are similarly treated.

Some authoritative figures will illustrate the point of view. First is a table showing a comparison between the physical qualities of mild steel and wrought iron.

Suppose that we require a material in which ductility is the most important quality necessary. We see that the mild steel is superior to the wrought iron in tensile strength and elastic limit, and that the elongation is greater on a 12-in. test piece, but on an 18-ft. test, the reverse is true. We also find the reduction of area to be greater on mild steel. Now suppose we are considering the material for a camshaft which is going to be 14 ft. long; evidently we will then favor the test of a length somewhat similar, and dynamic tests on the material proving equal, we may with good judgment select the wrought iron. The elongation and reduction of area are the effect of that quality of wrought iron, by which it elongates over a much greater length, owing to its fibrous nature, than does steel.