The specific strength is a material's strength (force per unit area at failure) divided by its density. It is also known as the strength-to-weight ratio or strength/weight ratio. In fiber or textile applications, tenacity is the usual measure of specific strength. The SI unit for specific strength is (N/m2)/(kg/m3) or more commonly N·m/kg.
Another way to describe specific strength is breaking length, also known as self support length: the maximum length of a vertical column of the material (assuming a fixed cross-section) that could suspend its own weight when supported only at the top. For this measurement, the definition of weight is the force of gravity at the Earth's surface applying to the entire length of the material, not diminishing with height. This usage is more common with certain specialty fiber or textile applications.
The materials with the highest specific strengths are typically fibers such as carbon fiber, glass fiber and various polymers, and these are frequently used to make composite materials (e.g. carbon fiber-epoxy). These materials and others such as titanium, aluminium, magnesium and high strength steel alloys are widely used in aerospace and other applications where weight savings are worth the higher material cost.
Note that strength and stiffness are distinct. Both are important in design of efficient and safe structures.
|Balsa (axial load)||73||0.14||521||53.2||8|
|Scifer steel wire||5500||7.87||706||71.2||7|
|Silicon carbide fiber||3440||3.16||1088||110||10|
|1 μm iron whiskers||14000||7.87||1800||183||7|
|Carbon fiber (AS4)||4300||1.75||2457||250||6|
|Carbon nanotube (see note below)||62000||.037-1.34||46268-N/A||4716-N/A||1516|
|Colossal carbon tube||6900||.116||59483||6066||17|
The data of this table is from best cases, and has been established for giving a rough figure.
- Note: Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with labs producing them at a tensile strength of 63 GPa,15 still well below their theoretical limit of 300 GPa. The first nanotube ropes (20 mm long) whose tensile strength was published (in 2000) had a strength of 3.6 GPa, still well below their theoretical limit.18 The density is different depending on the manufacturing method, and the lowest value is 0.037 or 0.55(solid).16
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- "Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes" by F. Li, H. M. Cheng, S. Bai, G. Su, and M. S. Dresselhaus. doi:10.1063/1.1324984
Since N·m/kg (the unit of specific strength) expands to (kg·m/s2)·m/kg or (m/s)2, a specific strength can also be understood as the square of a speed, hence represented by it square root, a speed. Further, the structural analysis for whether a particular material is strong enough for some purpose can sometimes be expressed in terms of comparing this speed to the speed of a relevant part of the structure. For example, a spinning cylindrical shell (as in an idealized flywheel or an idealized cylinder space ship) is only feasible if built of a material whose square root of specific strength is at least the speed at which the spinning shell moves.
The International Space Elevator Consortium has proposed the Yuri (unit) as a unit of specific strength useful for describing space tether designs. 1 Yuri = 1 (m/s)2 = N/(kg/m)1 and 1 MYuri (capital M denoting mega- and capital "Y" being typical for this non-SI unit) is equal to 1 N/Tex = 1 GPa-cc/g.2 A functional space elevator would require a tether of 30-80 MYuri.3
- Specific stiffness - Specific strength chart, University of Cambridge, Department of Engineering