Specific strength

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 Pa/(kg/m³), or N·m/kg, which is dimensionally equivalent to m²/s², though the latter form is rarely used.

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 (standard gravity, 9.80665 m/s²) 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.

Examples

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,^[18] 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.^[22] The density is different depending on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).^[19]

The 'Yuri' and space tethers

The International Space Elevator Consortium has proposed the "Yuri" as a name for the SI units describing specific strength. Specific strength is of fundamental importance in the description of space elevator cable materials. One Yuri is conceived to be the SI unit for yield stress (or breaking stress) per unit of density of a material under tension. So, the units for one Yuri are Pa/(kg/m³). This unit is equivalent to one N/(kg/m), which is the breaking/yielding force per linear density of the cable under tension.^[23][24] A functional space elevator would require a tether of 30-80 MegaYuri.^[25]

Fundamental limit on specific strength

The null energy condition places a fundamental limit on the specific strength of any material.^[21] The specific strength is bounded to be no greater than c² ~ 7013900000000000000♠9×10¹³kN·m/kg, where c is the speed of light. This limit is achieved by electric and magnetic field lines, QCD flux tubes, and the fundamental strings hypothesized by String Theory.

See also

• Specific modulus
• Space elevator
• Space tether

References

External links

• Specific stiffness - Specific strength chart, University of Cambridge, Department of Engineering