Specific strength

Ashby diagram, which compares the strength of materials with their density (logarithmic plot). Materials in the upper left corner have a high specific strength.

The specific strength is a specific quantity that relates the strength to the density of a material : ${\displaystyle {\mathcal {}}R_{\mathrm {m} }}$ ${\displaystyle {\mathcal {}}\rho }$

${\displaystyle R_{\mathrm {spez} }={\frac {R_{\mathrm {m} }}{\rho }}\quad \left[\mathrm {{\frac {N}{mm^{2}}}\cdot {\frac {dm^{3}}{kg}}={\frac {N\cdot m}{g}}} \right]}$

If you divide the specific strength by the acceleration due to gravity , you get the breaking length . This is the length at which a freely hanging cross-section (for example a wire ) of a material tears under its own weight .

Since the specific strength parameter has a direct influence on the construction volume and the mass of constructions, material selection decisions and thus the preference for more compact or less compact construction methods can be compared with this scale.

For example, pure aluminum has a lower absolute tensile strength than pure titanium :

${\displaystyle {R_{\mathrm {m} }}_{\mathrm {Al} }\approx 45\,\mathrm {\frac {N}{{mm}^{2}}} <{R_{\mathrm {m} }}_{\mathrm {Ti} }\approx 235\,\mathrm {\frac {N}{{mm}^{2}}} }$

and also a lower density:

${\displaystyle \rho _{\mathrm {Al} }\approx 2{,}7\,\mathrm {\frac {kg}{{dm}^{3}}} <\rho _{\mathrm {Ti} }\approx 4{,}5\,\mathrm {\frac {kg}{{dm}^{3}}} }$

The higher specific strength results for titanium:

${\displaystyle \Rightarrow {R_{\mathrm {spez} }}_{\mathrm {Al} }\approx 16{,}7\,\mathrm {\frac {N\cdot m}{g}} <{R_{\mathrm {spez} }}_{\mathrm {Ti} }\approx 52{,}1\,\mathrm {\frac {N\cdot m}{g}} }$

Due to its higher specific strength, titanium is preferred to aluminum as a material in the aerospace industry (despite its higher price). The specific strength means that a structure with a certain strength in titanium has a lower mass than in aluminum; that is, it is lighter, which is the critical criterion in aerospace. Or the other way around: at a certain permissible mass, a structure made of titanium is stronger than one made of aluminum. Aluminum alloys , on the other hand, have significantly higher strengths and also have a higher specific strength than steel. They can therefore be used as a lightweight material compared to ordinary steel.

Individual evidence

1. ↑ Eberhard Roos and Karl Maile, Material Science for Engineers, Springer, Berlin 2008, ISBN 978-3-540-68403-9 , page 314