Ultimate tensile strength

Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or Ftu within equations,[1][2][3] is the capacity of a material or structure to withstand loads tending to elongate, as opposed to compressive strength, which withstands loads tending to reduce size. In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together). Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.

Some materials break very sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, 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 can be defined for liquids as well as solids under certain conditions. For example, when a tree[4] draws water from its roots to its upper leaves by transpiration, the column of water is pulled upwards from the top by the cohesion of the water in the xylem, and this force is transmitted down the column by its tensile strength. Air pressure, osmotic pressure, and capillary tension also plays a small part in a tree's ability to draw up water, but this alone would only be sufficient to push the column of water to a height of less than ten metres, and trees can grow much higher than that (over 100 m).

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 International System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix mega); or, equivalently to pascals, newtons per square metre (N/m²). A United States customary unit is pounds per square inch (lb/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 in one country (US), when measuring tensile strengths.

Concept

Ductile materials

Many materials can display linear elastic behavior, defined by a linear stress–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 point"), 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 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 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, figure 2); 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.

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.[5]

The UTS is a common engineering parameter to design members made of brittle material because such materials have no yield point.[5]

Testing

Typically, the testing involves taking a small sample with a fixed cross-sectional area, and then pulling it with a tensometer at a constant strain (change in gauge length divided by initial gauge length) rate until the sample breaks.

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-held Rockwell hardness testers.[6] This practical correlation helps quality assurance in metalworking industries to extend well beyond the laboratory and universal testing machines.

Typical tensile strengths

Typical tensile strengths of some materials
MaterialYield strength
(MPa)
Ultimate tensile strength
(MPa)
Density
(g/cm³)
Steel, structural ASTM A36 steel250400–5507.8
Steel, 1090 mild2478417.58
Chromium-vanadium steel AISI 61506209407.8
Human skin15202
Steel, 2800 Maraging steel[7]261726938.00
Steel, AerMet 340[8]216024307.86
Steel, Sandvik Sanicro 36Mo logging cable precision wire[9]175820708.00
Steel, AISI 4130, water quenched 855 °C (1570 °F), 480 °C (900 °F) temper[10]95111107.85
Steel, API 5L X65[11]4485317.8
Steel, high strength alloy ASTM A5146907607.8
Acrylic, clear cast sheet (PMMA)[12]7287[13]1.16
High-density polyethylene (HDPE)26–33370.85
Polypropylene12–4319.7–800.91
Steel, stainless AISI 302 – cold-rolled520 8608.19
Cast iron 4.5% C, ASTM A-481302007.3
"Liquidmetal" alloy1723550–16006.1
Beryllium[14] 99.9% Be3454481.84
Aluminium alloy[15] 2014-T64144832.8
Polyester resin (unreinforced)[16]5555 
Polyester and chopped strand mat laminate 30% E-glass[16]100100 
S-Glass epoxy composite[17]23582358 
Aluminium alloy 6061-T62413002.7
Copper 99.9% Cu702208.92
Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu1303508.94
Brass200 +5008.73
Tungsten941151019.25
Glass 33[18]2.53
E-GlassN/A1500 for laminates,
3450 for fibers alone
2.57
S-GlassN/A47102.48
Basalt fiber[19]N/A48402.7
MarbleN/A152.6
ConcreteN/A2–52.7
Carbon fiberN/A1600 for laminates,
4137 for fibers alone
1.75
Carbon fiber (Toray T1100G)[20] (the strongest man-made fibres) 7000 fibre alone1.79
Human hair140–160200–250[21] 
Bamboo 350–5000.4
Spider silk (see note below)10001.3
Spider silk, Darwin's bark spider[22]1652
Silkworm silk500 1.3
Aramid (Kevlar or Twaron)362037571.44
UHMWPE[23]24520.97
UHMWPE fibers[24][25] (Dyneema or Spectra)2300–35000.97
Vectran 2850–3340 
Polybenzoxazole (Zylon)[26]270058001.56
Wood, pine (parallel to grain) 40 
Bone (limb)104–1211301.6
Nylon, molded, type 6/64507501.15
Nylon fiber, drawn[27]900[28]1.13
Epoxy adhesive12–30[29]
Rubber16 
BoronN/A31002.46
Silicon, monocrystalline (m-Si)N/A70002.33
Ultra-pure silica glass fiber-optic strands[30]4100
Sapphire (Al2O3)400 at 25 °C, 275 at 500 °C, 345 at 1000 °C19003.9–4.1
Boron nitride nanotubeN/A330002.62[31]
Diamond160028003.5
GrapheneN/A130000[32]1.0
First carbon nanotube ropes?36001.3
Carbon nanotube (see note below)N/A11000–630000.037–1.34
Carbon nanotube compositesN/A1200[33]N/A
High-strength carbon nanotube filmN/A9600[34]N/A
Iron (pure mono-crystal)37.874
Limpet Patella vulgata teeth (Goethite) 4900
3000–6500[35]
^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 300 GPa.[36] The first nanotube ropes (20 mm in length) whose tensile strength was published (in 2000) had a strength of 3.6 GPa.[37] The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).[38]
^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).[39] 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.[40]
^d Human hair strength varies by ethnicity and chemical treatments.
Typical properties for annealed elements[41]
ElementYoung's
modulus
(GPa)
Offset or
yield strength
(MPa)
Ultimate
strength
(MPa)
silicon1075000–9000
tungsten411550550–620
iron21180–100350
titanium120100–225246–370
copper130117210
tantalum186180200
tin479–1415–200
zinc alloy85–105200–400200–400
nickel170140–350140–195
silver83170
gold79100
aluminium7015–2040–50
lead1612

See also

References

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Further reading

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