Libro Mecanica

SECTION 1.3 Mechanical Properties of Materials

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1.3 MECHANICAL PROPERTIES OF MATERIALS
The design of machines and structures so that they will function properly requires that we understand the mechanical behavior of the materials being used. Ordinarily, the only way to determine how materials behave when they are subjected to loads is to perform experiments in the laboratory. The usualprocedure is to place small specimens of the material in testing machines, apply the loads, and then measure the resulting deformations (such as changes in length and changes in diameter). Most materials-testing laboratories are equipped with machines capable of loading specimens in a variety of ways, including both static and dynamic loading in tension and compression. A typical tensile-testmachine is shown in Fig. 1-7. The test specimen is installed between the two large grips of the testing machine and then loaded in tension. Measuring devices record the deformations, and the automatic control and data-processing systems (at the left in the photo) tabulate and graph the results. A more detailed view of a tensile-test specimen is shown in Fig. 1-8 on the next page. The ends of thecircular specimen are enlarged where they fit in the grips so that failure will not occur near the grips themselves. A failure at the ends would not produce the desired information about the material, because the stress distribution near the grips is not uniform, as explained in Section 1.2. In a properly designed specimen, failure will occur in the prismatic portion of the specimen where the stressdistribution is uniform and the bar is subjected only to pure tension. This situation is shown in Fig. 1-8, where the steel specimen has just fractured under load. The device at the left, which is attached by

FIG. 1-7 Tensile-test machine with

automatic data-processing system (Courtesy of MTS Systems Corporation)

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CHAPTER1 Tension, Compression, and Shear

FIG. 1-8 Typical tensile-test specimen

with extensometer attached; the specimen has just fractured in tension (Courtesy of MTS Systems Corporation)

two arms to the specimen, is an extensometer that measures the elongation during loading. In order that test results will be comparable, the dimensions of test specimens and the methods of applying loads mustbe standardized. One of the major standards organizations in the United States is the American Society for Testing and Materials (ASTM), a technical society that publishes specifications and standards for materials and testing. Other standardizing organizations are the American Standards Association (ASA) and the National Institute of Standards and Technology (NIST). Similar organizations exist inother countries. The ASTM standard tension specimen has a diameter of 0.505 in. and a gage length of 2.0 in. between the gage marks, which are the points where the extensometer arms are attached to the specimen (see Fig. 1-8). As the specimen is pulled, the axial load is measured and recorded, either automatically or by reading from a dial. The elongation over the gage length is measuredsimultaneously, either by mechanical

Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves...