Ultimate tensile strength, often shortened to tensile strength or ultimate strength,
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 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 it can be reported
just as a force or as a force per unit width. In the SI system, the unit is the pascal,
using the mega- prefix); or, equivalently to pascals, newtons per square metre. A United
States customary unit is pounds-force per square inch, or kilo-pounds per square inch,
which is equal to 1000 psi; kilo-pounds per square inch are commonly used when measuring
tensile strengths.
Concept Ductile materials
Many material display linear elastic behavior, defined by a linear stress-strain relationship,
as shown in the left 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 elastic region, for ductile materials, such as steel, deformations are plastic. A
plastically deformed specimen will 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 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; 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 ultimate tensile 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.
The UTS is a common engineering parameter when designing brittle members, because there
is no yield point. 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. the high carbon steel content carbon o.8%
and has high ultimate strength 640-690 mpa. It should be noted that, while most metal
forms, such as 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. 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, still well below their theoretical
limit of 300 GPa. The first nanotube ropes whose tensile strength was published had a
strength of 3.6 GPa. The density depends on the manufacturing method, and the lowest value
is 0.037 or 0.55. ^c The strength of spider silk is highly variable.
It depends on many factors including kind of silk, species, age of silk, temperature,
humidity, swiftness at which stress is applied during testing, length stress is applied,
and way the silk is gathered. 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. ^d Human hair strength varies by ethnicity
and chemical treatments. See also
Flexural strength Strength of materials
Tensile structure Toughness
Failure Tension
References
Further reading Giancoli, Douglas, Physics for Scientists
& Engineers Third Edition. Upper Saddle River: Prentice Hall.
Köhler, T., Vollrath, F.. "Thread biomechanics in the two orb-weaving spiders Araneus diadematus
and Uloboris walckenaerius". 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. "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes
Under Tensile Load". Science 287: 637–640. Bibcode:2000Sci...287..637Y. doi:10.1126/science.287.5453.637.
PMID 10649994. George E. Dieter, Mechanical Metallurgy. McGraw-Hill,
UK