In this test the ultimate tensile strength, the yield point, the percent elongation, and the character of the fracture are determined. Sometimes the percent reduction of area is also determined. The Standard tensile-test specimen, shown in Figure 2, has a diameter of 0.505 inch and a cross-sectional area of 0.2 square inch.
All these values can be obtained by a single operator. It is not customary to determine deformations of the specimen for plotting the stress-trading curve in the routine commercial tension testing of ductile carbon steel since a great deal of additional labor and cost would be required.
Ultimate Tensile Strength. This value is determined by dividing the maximum load registered on the testing machine by the nominal cross-sectional area of the specimen. Maximum load occurs simultaneously with the beginning of the "necking down" in ductile steel and with rupture in brittle materials.
Yield Point. Yield point may be obtained on ductile carbon steel by knowing were a more or less sharp break appears in the stress-strain diagram.
Elongation. Elongation of the specimen after fracture may be determined by placing the parts of the broken specimen closely together and holding them in place by means of a vice. The distance between gauge marks may be measured to the nearest 0.01 inch b means of dividers.
Character of the Fracture. Valuable information concerning the characteristics and composition of metals may be obtained by observing the character of the fracture of the test specimens. Necking down of the cross-section of the specimen near the fracture accompanied by a fracture in the form of a cup and cone indicates difficulty and is typical OF low-carbon steel. A square break normal to the longitudinal axis with little or no necking down indicates a non-ductile metal such as high-carbon steel or gray cast iron.
The texture of the metal at the fracture is commonly recorded. The texture of low-carbon steel is usually "silky"; of high-carbon steel, finely crystalline; of cast iron, finely or coarsely crystalline. By observing the extent of necking down and the type and texture of the fracture, the carbon content of steels can be estimated approximately.
A or separated fracture is typical of heat-treated high-strength alloy steel. Many not-ferrous metals exhibit considerable elongation over the length of the specimen without appreciable necking down. Wrought iron may be distinguished by the fiberous struc ture of its fracture.
Reduction of Area. Reduction of area can usually be determined if the specimen is circular in cross-section. The area of the smallest cross-section after fracture may be measured by means of thin pointed calipers to the nearest 0.01 inch. If the section s slightly irregular, the maximum and minimum diameter may be measured and the cross-sectional area computed as a circle, taking the mean of the observed values as diameter. Such an irregular section may be computer as an ellipse.
It is difficult to measure the cross-sectional area of rectangular sections when fractured since necking down tends to produce a section composed of four concave surfaces. The cross-sectional area of deformed reinforcing bars is also difficult to determin e. Reduction of area is ordinarily not reported for such systems.
Distribution of Elongation. The distribution of elongation of a steel specimen in tension is illustrated by Figure 3. If uninfluenced by any local cause, a bar of homogeneous steel subjected to axial tension should fail near the center of the distance be ween the points of application of the load, because there the flow of metal will be least impeded. The maximum ordinate to the curve of figure 3 represents the point where necking down occurred since the point of maximum reduction in area must necessarily coincide with the point of maximum elongation. The fact that steel test specimens are reduced in cross-section as they are elongated under stress, and finally necking now, accounts for the fact that the breaking load, according to the stress-strain diagr am, is below the ultimate strength. If the stress at all loads was computed upon the basis of the actual section then existing, instead of being computed (as it usually is) upon the basis of the original section, the stress-strain curve would follow.
