Difference between revisions of "Tensile strength"

Revision as of 15:31, 10 December 2011

Entry by Emily Redston, AP 225, Fall 2011

Figure 1 Stress vs. Strain curve typical of aluminum 1. Ultimate strength 2. Yield strength 3. Proportional limit stress 4. Fracture 5. Offset strain (typically 0.2%) (http://en.wikipedia.org/wiki/Ultimate_tensile_strength)

The tensile strength (also called ultimate tensile strength) is the maximum stress that can be sustained by a structure in tension; if this stress is applied and maintained, the sample will fracture. It is important to note, however, that tensile strength is not necessarily the same as fracture strength. Let's consider the typical stress-strain curve of a ductile material, seen in Figure 1. You obtain such a curve by doing a tensile test, one of the most common mechanical tests. Tensile tests can be used to ascertain several important mechanical properties, such as the tensile strength of a material. For a ductile material, after yielding, the stress necessary to continue plastic deformation increases to a maximum, and then decreases to the eventual fracture point. The highest point of the stress-strain curve is the tensile strength, corresponding to point 1. All deformation up to this point is uniform through the tensile specimen. However, at this maximum stress, a small constriction, or neck, begins to form (typically near the middle of the sample), and all subsequent deformation is confined to this neck. This phenomenon is termed "necking", and fracture will ultimately occur at the neck. Fracture strength corresponds to the stress at fracture. It is clear that the tensile strength is not the same as the fracture strength in this case. However, for brittle materials, these will tend to be more similar. In brittle materials, the UTS will at the end of the linear-elastic portion of the stress-strain curve or close to the elastic limit. In ductile materials, the UTS will be well outside of the elastic portion into the plastic portion of the stress-strain curve.

Tensile strength is defined as a stress, which is measured as force per unit area. It is typically reported in units Pa or psi. The tensile strength is an intensive property, meaning that its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.

The UTS may not be completely representative of the highest level of stress that a material can support, but the value is not typically used in the design of components anyway. For ductile metals the current design practice is to use the yield strength for sizing static components. However, since the UTS is easy to determine and quite reproducible, it is useful for the purposes of specifying a material and for quality control purposes. On the other hand, for brittle materials the design of a component may be based on the tensile strength of the material.

Typical tensile strengths

References

[1] Callister, William D. Materials Science and Engineering: an Introduction. New York: John Wiley & Sons, 2007.

[2] http://en.wikipedia.org/wiki/Ultimate_tensile_strength

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