Tensile strength
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Template:Mergeto Tensile strength measures the force required to pull something such as rope, wire, or a structural beam to the point where it breaks.
Specifically, the tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. This is an important concept in engineering, especially in the fields of material science, mechanical engineering and structural engineering.
There are three typical definitions of tensile strength:
- Yield Strength - The stress a material can withstand without permanent deformation.
- Ultimate Strength - The maximum stress a material can withstand.
- Breaking Strength - The stress coordinate on the Stress-strain curve at the point of rupture.
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Concept
The various definitions of tensile strength are shown in the following stress-strain graph for low-carbon steel:
Image:Stress v strain A36 2.png
Steel has a very linear stress-strain relationship up to a sharply defined yield point, as shown in the figure. For stresses below this yield strength all deformation is recoverable, and the material will relax into its initial shape when the load is removed. For stresses above the yield point, a portion of the deformation is not recoverable, and the material will not relax into its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.
After the yield point, steel and many other ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain up to the ultimate strength. If the material is unloaded at this point, the stress-strain curve will be parallel to that portion of the curve between the origin and the yield point. If it is re-loaded it will follow the unloading curve up again to the ultimate strength, which has become the new yield strength.
After steel has been loaded to its ultimate strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. Necking is accompanied by a region of decreasing stress with increasing strain on the stress-strain curve. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the breaking stress. Note that if the graph is plotted in terms of true stress and true strain necking will not be observed on the curve as true stress is corrected for the decrease in cross-sectional area. Necking is also not observed for materials loaded in compression.
Ductile metals other than steel typically do not have a well defined yield point. For these materials the yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. A stress-strain curve typical of aluminum along with the 0.2% offset line is shown in the figure below.
Image:Stress v strain Aluminum 2.png
Brittle materials such as concrete and carbon fiber do not have a yield point, and do not strain-harden which means that the ultimate strength and breaking strength are the same. A stress-strain curve for a typical brittle material is shown in the figure below.
Image:Stress v strain brittle 2.png
Tensile strength is measured in units of force per unit area. In the SI system, the units are newtons per square metre (N/m²) or pascals (Pa), with prefixes as appropriate. The non-metric units are pounds-force per square inch (lbf/in² or PSI).
The breaking strength of a rope is specified in units of force, such as newtons, without specifying the cross-sectional area of the rope. This is often loosely called tensile strength, but this not a strictly correct use of the term.
In brittle materials such as rock, concrete, cast iron, or soil, tensile strength is negligible compared to the compressive strength and it is assumed zero for most engineering applications. Glass fibers have very high tensile strength, but bulk glass usually does not.
Tensile strength can be measured for liquids as well as solids. For example, when a tree draws water from its roots to its upper leaves by transpiration, the column of water is pulled upwards from the top by capillary action, and this force is transmitted down the column by its tensile strength. Air pressure from below 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 about ten metres, and trees can grow much higher than that. (See also cavitation, which can be thought of as the consequence of water being "pulled too hard".)
Typical tensile strengths
Some typical tensile strengths of some materials:
| Material | Yield strength (MPa) | Ultimate strength (MPa) | Density (g/cm3) |
|---|---|---|---|
| Structural steel ASTM-A36 | 400 | 650 | |
| Steel, high strength alloy ASTM A-514 | 690 | 760 | |
| Steel, high tensile | 1650 | 1860 | |
| Steel, Piano wire | 2000 | ||
| Polypropylene | 12-43 | 19.7-80 | |
| Stainless steel AISI 302 - Cold-rolled | 520 | 860 | |
| Cast iron 4.5% C, ASTM A-48 | - | 200 | |
| Titanium Alloy (6% Al, 4% V) | 830 | 900 | 4.51 |
| Aluminum Alloy 2014-T6 | 180 | 200 | 2.7 |
| Copper 99.9% Cu | 70 | 220 | 8.92 |
| Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu | 130 | 350 | 8.94 |
| Brass | 250 | ||
| Glass (St Gobain "R") | 4400 (3600 in composite) | 2.53 | |
| Bamboo | |||
| Marble | - | 15 | |
| Concrete | - | 3 | |
| Spider silk | 1150 (??) | 1200 | |
| Silkworm silk | 500 | ||
| Kevlar | 3620 | 1.44 | |
| Vectran | 2850-3340 | ||
| Pine Wood (parallel to grain) | 40 | ||
| Bone (limb) | 130 | ||
| Nylon, type 6/6 | 45 | 75 | |
| Rubber | - | 15 | |
| Boron | 3100 | 2.46 | |
| Silicon carbide (SiC) | 3440 | ||
| Sapphire (Al2O3) | 1900 | 3.9-4.1 |
| Metallic elements in the annealed state | Young Modulus (GPa) | Proof or yield stress (MPa) | Ultimate strength (MPa) |
|---|---|---|---|
| Aluminium | 70 | 15-20 | 40-50 |
| Copper | 130 | 33 | 210 |
| Gold | 79 | 100 | |
| Iron | 211 | 80-100 | 350 |
| Lead | 16 | 12 | |
| Nickel | 170 | 14-35 | 140-195 |
| Silicon | 107 | 5000-9000 | |
| Silver | 83 | 170 | |
| Tantalum | 186 | 180 | 200 |
| Tin | 47 | 9-14 | 15-200 |
| Titanium | 120 | 100-225 | 240-370 |
| Tungsten | 411 | 550 | 550-620 |
| Zinc (wrought) | 105 | 110-200 |
(Source: A.M. Howatson, P.G. Lund and J.D. Todd, "Engineering Tables and Data" p41)
Single-walled carbon nanotubes made in academic labs have the highest tensile strength of any material yet measured, with labs producing carbon nanotubes with a tensile strength of 63 GPa (63,000 MPa)well below its theoretical tensile strength of 300 GPa (300,000 MPa). As of 2004, however, no macroscopic object constructed using a nanotube-based material has had a tensile strength remotely approaching this figure, or substantially exceeding that of high-strength materials like Kevlar.
Sources
- A.M. Howatson, P.G. Lund and J.D. Todd, "Engineering Tables and Data"
- Giancoli, Douglas. Physics for Scientists & Engineers Third Edition. Upper Saddle River: Prentice Hall, 2000.
- Köhler, T. and F. Vollrath. 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.
- Edwards, Bradly C. "The Space Elevator: A Brief Overview" http://www.liftport.com/files/521Edwards.pdf
See also
External links
- Tensile Strength Test
- January 2003 sci.physics thread on water tensile strength and trees
- Theory re the discrepancy in static vs dynamic measurements of water's tensile strength
- Engineering Stress-strain Curve
- Analytical Process Laboratories
- Tensile Strength Testing
- Free database of engineering properties for over 50,000 materialsde:Zugfestigkeit