Specific strength

The specific strength is a material's (or muscle'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 or strength-to-mass ratio. In fiber or textile applications, tenacity is the usual measure of specific strength. The SI unit for specific strength is Pa⋅m3/kg, or N⋅m/kg, which is dimensionally equivalent to m2/s2, though the latter form is rarely used. Specific strength has the same units as specific energy, and is related to the maximum specific energy of rotation that an object can have without flying apart due to centrifugal force. 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.