Specific modulus is a materials property consisting of the elastic modulus per mass density of a material. It is also known as the stiffness to weight ratio or specific stiffness. High specific modulus materials find wide application in aerospace applications where minimum structural weight is required. The dimensional analysis yields units of distance squared per time squared.
The utility of specific modulus is to find materials which will produce structures with minimum weight, when the primary design limitation is deflection or physical deformation, rather than load at breaking—this is also known as a "stiffness-driven" structure. Many common structures are stiffness-driven over much of their use, such as airplane wings, bridges, masts, and bicycle frames.
To emphasize the point, consider the issue of choosing a material for building an airplane. Aluminum seems obvious because it is "lighter" than steel, but steel is stronger than aluminum, so one could imagine using thinner steel components to save weight without sacrificing (tensile) strength. The problem with this idea is that there would be a significant sacrifice of stiffness, allowing, e.g., wings to flex unacceptably. Because it is stiffness, not tensile strength, that drives this kind of decision for airplanes, we say that they are stiffness-driven.
The connection details of such structures may be more sensitive to strength (rather than stiffness) issues due to effects of stress risers.
Specific modulus is not to be confused with specific strength, a term that compares strength to density.
Read more about Specific Modulus: Approximate Specific Stiffness For Various Materials
Famous quotes containing the word specific:
“Patriotism is proud of a country’s virtues and eager to correct its deficiencies; it also acknowledges the legitimate patriotism of other countries, with their own specific virtues. The pride of nationalism, however, trumpets its country’s virtues and denies its deficiencies, while it is contemptuous toward the virtues of other countries. It wants to be, and proclaims itself to be, “the greatest,” but greatness is not required of a country; only goodness is.”
—Sydney J. Harris (1917–1986)