Static strength is the ability of a material to resist the externally applied forces without breaking or yielding. Ideally, in designing any machine element, the engineer should have available the results of a great many strength tests of the particular material chosen. These tests should be made on specimens having the same heat treatment, surface finish, and size as the element the engineer proposes to design; and the tests should be made under exactly the same loading conditions as the part will experience in service. This means that if the part is to experience a bending load, it should be tested with a bending load. If it is to be subjected to combined bending and torsion, it should be tested under combined bending and torsion. If it is made of heat-treated AISI 1040 steel drawn at 500ºC with a ground finish, the specimens tested should be of the same material prepared in the same manner. Such tests will provide very useful and precise information. Whenever such data are available for design purposes, the engineer can be assured of doing the best possible job of engineering. Static strength
The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life or if the part is manufactured in sufficiently large quantities. Refrigerators and other appliances, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divided by the total number of parts manufactured.
In analysis and design, the mechanical behavior of materials under load is of primary importance. Experiments, mainly in tension or compression tests, provide basic information about overall response of specimens to the applied loads in the form of stress–strain diagrams. These curves are used to explain a number of mechanical properties of materials. Data for a stress–strain diagram are usually obtained from a tensile test. In such a test, a specimen of the material, usually in the form of a round bar, is mounted in the grips of a testing machine and subjected to tensile loading, applied slowly and steadily or statically at room temperature. The ASTM specifies precisely the dimensions and construction of standard tension specimens.
The tensile test procedure consists of applying successive increments of load while taking corresponding electronic extensometer readings of the elongation between the two gage marks (gage length) on the specimen. During an experiment, the change in gage length is noted as a function of the applied load. The specimen is loaded until it finally ruptures. The force necessary to cause rupture is called the ultimate load. Below figure illustrates a steel specimen that has fractured under load and the extensometer attached at the right by two arms to it. Based on the test data, the stress in the specimen is found by dividing the force by the cross-sectional area, and the strain is found by dividing the elongation by the gage length. In this manner, a complete stress–strain diagram, a plot of strain as abscissa and stress as the ordinate, can be obtained for the material. The stress–strain diagrams differ widely for different materials.
