Entry by Emily Redston, AP 225, Fall 2011
The tensile strength is the maximum stress that can 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.
All derformation up to this point is uniform through the narrow region of the tensile specimen. However, at this maximum stress, a small constriction or neck begins to form at some point, and all subsequent deformation is confined at this neck. This phenomenon is termed "necking", and fracture ultimate occurs at the neck. Fracture strength corresponds to the stress at fracture.
The UTS is usually found by performing a tensile test and recording the stress versus strain; the highest point of the stress-strain curve is the UTS. It is an intensive property; therefore 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.
After yielding, the stress necessary to continue plastic deformation increases to a mximum, and then decreases to the eventual fracture point. The tensile strength is the stress at the maximum on the engineering 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 ultimate tensile strength (UTS) or, more simply, the tensile strength, is the maximum engineering stress level reached in a tension test. The strength of a material is its ability to withstand external forces without breaking. 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.
On the stress-strain curve above, the UTS is the highest point where the line is momentarily flat. Since the UTS is based on the engineering stress, it is often not the same as the breaking strength. In ductile materials strain hardening occurs and the stress will continue to increase until fracture occurs, but the engineering stress-strain curve may show a decline in the stress level before fracture occurs. This is the result of engineering stress being based on the original cross-section area and not accounting for the necking that commonly occurs in the test specimen. 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.
 Callister, William D. Materials Science and Engineering: an Introduction. New York: John Wiley & Sons, 2007.