1 Continued on next page... MECHANICAL PROPERTIES OF METALS The mechanical properties of a material are those related to its ability to withstand external mechanical forces such as pulling, pushing, twisting, bending, and sudden impact. In general terms, we think of these properties as various kinds of ”strength”. However, the word ”strength”, used alone, doesn’t tell us very much. Steel, cast iron, rubber, and glass are each ”strong” in different ways. Tensile Strength, Elasticity, and Ductility In the field of metals, when the word ”strength” is used alone (as in ”high-strength steels”) it almost always refers to the ability of the metal to resist pulling force; specifically, to what is termed its tensile strength. If we start by considering what happens when a bar of steel is subjected to a steadily-increasing pull, we cannot only define tensile strength, but also yield strength, elasticity, and ductility. It’s obvious that it will take more pull to break a steel bar with a cross-sectional area of 10 square centimeters (10 cm2) than to break one with a cross-sectional area of 5 cm2, so we must start with a specimen having a precisely-determined cross-sectional area if the results are to yield useful data. This specimen is secured firmly in a tensile testing machine which is capable of applying all the pulling force needed to break it. The machine is equipped with gauges which will show both the force being applied and the increase in length of the specimen as force is applied. The force can be mathematically converted to stress by applying the known minimum cross- sectional area of the specimen. (Stress equals force divided by area.)* *In the ”English” system of measurement, force has been expressed in pounds, stress in pounds per square inch (psi). In the metric system, until recently, force was expressed in kilograms, stress in kilograms per square centimeter (kg/cm2). In the updated metric system (SI), force is expressed in newtons (N), stress in pascals (Pa). (One newton, acting across an area of one square meter, equals one pascal). Tensile strengths will usually be stated in megapascals (MPa) (millions of pascals).
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