Tensile Testing of Engineering Materials
Evaluating the Strength and Ductility of Materials
Tensile testing is an evaluation process used to characterize the mechanical and materials performance of a test specimen, particularly the strength and ductility of the material. Engineers use the results of tensile testing for material selection for a specific application, alloy development, and quality control.
Tensile Test Procedure
A tensile test consists of a test specimen that is grasped at each end by a tensile testing machine and subjected to a tensile axial load. For calibration purposes, a standard specimen is used for tensile testing. The specimen is generally a cylindrical rod with a precise diameter, although other specimen geometries can be used to evaluate certain situations. Gauge marks are scribed onto the surface of the specimen and separated by a known distance.
The specimen is loaded into a universal testing machine that can apply a load to the specimen at a specific rate. As the specimen is axially loaded in tension, the distance between the gauge marks is monitored. This measurement can be made mechanically with an extensometer or optically with digital video.
Because the cross-sectional area is known, the stress can be calculated based on the applied load. The change in the distance between the gauge marks can be converted to strain. These two factors can be plotted against each other to form a stress-strain diagram. A stress-strain curve illustrates the relationships between the applied load, the geometry of the part, the material properties of the part, and the reaction of the part to the applied load.
Virtually any material can be subjected to a tensile test. The most common materials evaluated in this way are metals, but anything from plastics, fibers, and even human hair can be subjected to tensile testing to understand the strength and ductility of the material.
Tensile Testing Standards
Because tensile testing requires precise specimens and measurements to ensure accuracy, specific standards have been created to mandate the creation of test specimens and the tensile testing procedure.
The American Society for Testing and Materials (ASTM) maintains a library of testing procedures, including procedures for tensile testing for different materials under varying circumstances. The standards are specific in many cases, so several standards may have to be referenced to cover all procedures for preparation and conduct of a tensile test. The primary standard for metallic materials is ASTM E8 / E8M - 08 Standard Test Methods for Tension Testing of Metallic Materials.
Tensile testing of materials allows engineers to gain a solid understanding of the materials they wish to incorporate into a design, and can also provide quality control of materials being used in existing designs.
Sources
ASTM Physical and Mechanical Testing Standards
Beer, F., Johnston, E.R., Mechanics of Materials, Second Edition, McGraw-Hill, 1992.
Universal Testing Machine
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Stress-strain diagram for brittle material.
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