Most commercial specifications for metals have requirements for physical properties as determined by the tensile-strength test. The properties determined include ultimate tensile strength, yield strength or yield point, elongation, character of fracture, and reduction of area. In order to obtain complete information concerning the tensile properties of a metal, a stress-strain curve should be determined experimentally. Strains corresponding to definite stresses imposed upon the specimen are measured by me ans of an extensometer. The extensometer is attached to the specimen so as to measure elongations between two gauge points. The procedure may consist of taking a series of load readings on the testing machine with corresponding readings of the extensomete r, or the stress-strain diagram may be drawn directly by and graphic attachment to the testing machine.
For metals having no well-defined yield point, the yield strength is ordinarily determined, as explained in previously. Ductile carbon steel has a well-defined yield point. Owing to the importance of steel, the methods of testing steel in tension are desc ribed in some detail in the following articles.
The General Behavior of Steel under Tensile Stress
The behavior of steel under tensile stress is best studied with the aid of the stress-strain diagram. Curve one of Figure 1 represents a typical stress-strain diagram for a mild steel. Curve 2 shows the portion of curve 1 between O and B to an enlarged ho rizontal scale. This second curve to such a scale is necessary in order that the limit of proportionality or the point of departure of the curve from the straight line at A can be accurately located. For steel the elastic limit and proportional limit are usually considered to be identical. The yield point B is easily detected either by making a test or from the stress-strain diagram were a more or less sharp break appears in the curve when the yield point is reached. The ultimate strength is represented b y the maximum ordinate to the stress-strain curve at C.
The stress-strain diagram for steel in compression is essentially similar to the curve as described for tension, and the elastic limit is approximately the same in both cases.
The modules of elasticity can be determined from the slope of the first portion of the stress-strain diagram but is usually computed from the observed values of stress and strain. The modules of elasticity is on unaffected by the factors which influence o ther properties of steels and is about 29,000,000 pounds per square inch for practically any class of carbon steel.
