Ultimate tensile strength

Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength,^[1][2] is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. Tensile strength is not the same as compressive strength and the values can be quite different.

Some materials will break sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, will experience some plastic deformation and possibly necking before fracture.

The UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress-strain curve (see point 1 on the engineering stress/strain diagrams below) 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.

Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood.

Tensile strength is defined as a stress, which is measured as force per unit area. For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In the SI system, the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the mega- prefix); or, equivalently to pascals, newtons per square metre (N/m²). A customary unit is pounds-force per square inch (lbf/in² or psi), or kilo-pounds per square inch (ksi, or sometimes kpsi), which is equal to 1000 psi; kilo-pounds per square inch are commonly used for convenience when measuring tensile strengths.

Concept

Ductile materials

"Engineering" 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%)

"Engineering" (red) and "true" (blue) stress vs. strain curve typical of structural steel
1. Ultimate strength
2. Yield strength
3. Fracture
4. Strain hardening region
5. Necking region
A: Engineering stress
B: True stress

Many materials display linear elastic behavior, defined by a linear stress-strain relationship, as shown in the figure up to point 3. The elastic behavior of materials often extends into a non-linear region, represented in the figure by point 2, up to which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this linear region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen will not return to its original size and shape when unloaded. Note that there will be elastic recovery of a portion of the deformation. For many applications, plastic deformation is unacceptable, and is used as the design limitation.

After the yield point, ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, 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); this is because the engineering 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 tensile ultimate strength, given by point 1.

The UTS is not used in the design of ductile static members because design practices dictate the use of the yield 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.^[3]

The UTS is a common engineering parameter when designing brittle members, because there is no yield point.^[3]

Testing

Round bar tensile specimen after testing

Typically, the testing involves taking a small sample with a fixed cross-section area, and then pulling it with a tensometer, gradually increasing force until the sample breaks.

When testing 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-held Rockwell hardness testers.^[4]

It should be noted that while most metal forms, like sheet, bar, tube and wire can exhibit the test UTS, fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, must be made into composites to create useful real-world forms. As the datasheet on T1000G below indicates, while the UTS of the fiber is very high at 6,370MPa, the UTS of a derived composite is 3,040MPa - less than half the strength of the fiber.^[5]

Typical tensile strengths

^a Many of the values depend on manufacturing process and purity/composition.

^b Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with labs producing them at a tensile strength of 63 GPa,^[31] still well below their theoretical limit of 300 GPa.^{[citation needed]} The first nanotube ropes (20mm in length) whose tensile strength was published (in 2000) had a strength of 3.6 GPa.^[32] The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).^[33]

^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).^[34] The value shown in the table, 1000 MPa, is roughly representative of the results from a few studies involving several different species of spider however specific results varied greatly.^[35]

^d Human hair strength varies by ethnicity and chemical treatments.

See also

• Flexural strength
• Strength of materials
• Tensile structure
• Toughness

References

Further reading

• Giancoli, Douglas, Physics for Scientists & Engineers Third Edition (2000). Upper Saddle River: Prentice Hall.
• Köhler, T., Vollrath, F. (1995). "Thread biomechanics in the two orb-weaving spiders Araneus diadematus (Araneae, Araneidae) and Uloboris walckenaerius (Araneae, Uloboridae)". Journal of Experimental Zoology 271: 1–17. doi:10.1002/jez.1402710102. 
• T Follett, Life without metals
• Min-Feng, Yu, Lourie, O, Dyer, MJ, Moloni, K, Kelly, TF, Ruoff, RS (2000). "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load". Science 287 (5453): 637–640. Bibcode:2000Sci...287..637Y. doi:10.1126/science.287.5453.637. PMID 10649994. 
• George E. Dieter, Mechanical Metallurgy (1988). McGraw-Hill, UK