The engineering tensile test is widely used to provide basic design information on the strength of materials. In a tension test, a test specimen is subjected to a continually increasing axial tensile force while simultaneous measurements of load on the specimen and amount of specimen elongation, stretch, or strain are recorded. An engineering stress-strain plot is derived from these measurements. The values used to describe a tensile stress-strain curve are generally ultimate tensile strength; yield strength, and percent elongation. The first two parameters address material strength whereas the last parameter indicates material ductility, or the amount of stretch a material will endure before breaking into two pieces. These three parameters are defined below:
ULTIMATE TENSILE STRENGTH
This property is defined as the maximum test load divided by the original cross-sectional area of the test specimen. This value is the highest amount of stress that a material can support without breaking. Ultimate strength is dependent on material atomic structure and alloying. For example: low carbon steels have relatively low ultimate strength values, which can range roughly from 40,000-60,000 pounds per square inch (psi). Other steels, however, with increased carbon levels and optimized alloying elements can exhibit strengths in the 260,000-psi range.
Ultimate tensile strength is an important engineering material property and is used to size machine components so that they will survive field loads without breaking. For example: an automotive rim that fractures during impact with a pothole may have insufficient ultimate tensile strength. Obviously a complete tensile failure can be catastrophic.
YIELD STRENGTH
Yield strength is calculated by dividing the test load required to produce a small but specific amount of plastic deformation in the test material. Plastic deformation in this case means stretching a material permanently. For example, bending a coat hanger to a degree where it will not spring back to its original shape requires plastic deformation to occur (picture stretching a piece of Silly Putty plastic). Bending a coat hanger lightly to a lesser degree where it will spring back to its original shape only requires elastic deformation to occur (picture a diving board bending).
The load used to calculate yield strength is chosen where the material transitions from the elastic region to the plastic region. The tensile test stress-strain plot is used to identify the test loads associated with this transition region.
Yield strength is also an important engineering material property and is used to size machine components so that they will not permanently change shape when submitted to service loads. For example, an automotive rim that bends out of shape during impact with a pothole may have insufficient yield strength. Such a failure in this instance is not catastrophic but yield failures can create as much damage as outright tensile, break-into-two-pieces, type fractures.
MATERIAL DUCTILITY
The common measure of material ductility is obtained from the tension test. Ductility is defined as the amount of permanent elongation or stretch endured by the test specimen before fracturing.
Ductility is also an important engineering property, which provides a gage for material damage tolerance. For example, an automotive rim manufactured from a highly ductile material would survive more yield causing impacts than a less ductile material. Generally, materials that exhibit low ductility are not used in harsh, unpredictable environments or when safety is a concern.