Maximum stress withstood by stretched material before breaking
Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fractures. The maximum stress it withstands before fracturing is its ultimate tensile strength.
Ultimate tensile strength (also calledUTS,tensile strength,TS,ultimate strength or in notation)[1] is the maximumstress that a material can withstand while being stretched or pulled before breaking. Inbrittle materials, the ultimate tensile strength is close to theyield point, whereas inductile materials, the ultimate tensile strength can be higher.
The ultimate tensile strength is usually found by performing atensile test and recording theengineering stress versusstrain. The highest point of thestress–strain curve is the ultimate tensile strength and has units of stress. The equivalent point for the case of compression, instead of tension, is called thecompressive strength.
Tensile strengths are rarely of any consequence in the design ofductile members, but they are important with brittle members. They are tabulated for common materials such asalloys,composite materials,ceramics, plastics, and wood.
The ultimate tensile strength of a material is anintensive property; therefore its value does not depend on the size of the test specimen. However, depending on the material, it may be 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.
Some materials break very sharply, withoutplastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possiblynecking before fracture.
Tensile strength is defined as a stress, which is measured asforce per unit area. In theInternational System of Units (SI), the unit is thepascal (Pa) which is 1 N/m2 or for tensile strength more often a multiple thereof, like megapascals (MPa) or gigapascals (GPa). For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. AUnited States customary unit ispounds per square inch (lb/in2 or psi). Kilopounds per square inch (ksi, or sometimes kpsi) is equal to 1000 psi, and is commonly used in the United States, when measuring tensile strengths.
Many materials can display linearelastic behavior, defined by a linearstress–strain relationship, as shown in figure 1 up to point 3. The elastic behavior of materials often extends into a non-linear region, represented in figure 1 by point 2 (the "yield strength"), up to whichdeformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically intension will elongate, but will return to its original shape and size when unloaded. Beyond this elastic region, forductile materials, such as steel, deformations areplastic. A plastically deformed specimen does not completely return to its original size and shape when unloaded. For many applications, plastic deformation is unacceptable, and is used as the design limitation.
After the yield point, ductile metals undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin toneck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress–strain curve (curve A, figure 2); this is because theengineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress–strain curve, and the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1.
Ultimate tensile strength is not used in the design of ductilestatic members because design practices dictate the use of theyield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples.[2]
The ultimate tensile strength is a common engineering parameter to design members made of brittle material because such materials have noyield point.[2]
Typically, the testing involves taking a small sample with a fixed cross-sectional area, and then pulling it with atensometer at a constant strain (change in gauge length divided by initial gauge length) rate until the sample breaks. In metals and especially in polymers, the ultimate strength can depend significantly on the strain rate selected for the test.[3]
When testing some metals,indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-heldRockwell hardness testers.[4] This practical correlation helpsquality assurance in metalworking industries to extend well beyond the laboratory anduniversal testing machines.
^a Many of the values depend on manufacturing process and purity or composition.
^b Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with one measurement of 63 GPa, still well below one theoretical value of 100 GPa.[41] The first nanotube ropes (20 mm in length) whose tensile strength was published (in 2000) had a strength of 3.6 GPa.[42] The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).[43]
^c The strength of spider silk is highly variable. It depends on many factors including kind of silk (Every spider can produce several for sundry purposes.), species, age of silk, temperature, humidity, swiftness at which stress is applied during testing, length stress is applied, and way the silk is gathered (forced silking or natural spinning).[44] The value shown in the table, 1,000 MPa, is roughly representative of the results from a few studies involving several different species of spider however specific results varied greatly.[45]
^d Human hair strength varies bygenetics, environmental factors, and chemical treatments.
^strictly speaking this figure is theflexural strength (ormodulus of rupture), which is a more appropriate measure for brittle materials than "ultimate strength."
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