Specific strength

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The specific strength is a material strength divided by its density. It is expressed in newton metres per kilogram, and is used for tensile strength as for compressive strength.

Another common expression is the breaking length in km when subjected to the earth gravity, which makes this notion palpable. The structure most affected by this characteristic is without any doubt the space elevator. It needs a material capable of sustaining 4960 kilometers of its own weight to become feasible, which carbon nanotube recently achieved. This "escape length" calculated by Arthur C. Clarke in 1979 ^[1] is much inferior than a geostationary satellite's altitude of 36,000 km because of the diminution of gravity with altitude by the means of greater distance to the earth center of gravity, centrifugal forces and the possibility to vary the section of the cable and thus its weight and resistance.

Specific tensile strength of various materials

Material	Strength	Density	Specific Strength	Breaking length	source
Unit	MPa	kg/dm³	kN·m/kg	km
Concrete	10	2.30	4.34	0.44	[2]
Rubber	15	0.92	16.3	1.66	[2]
Brass	580	8.55	67.8	6.91	[3]
Oak	60	0.69	86.95	8.86	[4]
Polypropylene	80	0.90	88.88	9.06	[5]
Nylon	75	1.15	97.3	9.92	[6]
Magnesium	275	1.74	158	16.11	[7]
Aluminium	600	2.70	222	22.65	[8]
Steel	2000	7.86	254	25.93	[8]
Titanium	1300	4.51	288	29.38	[8]
Silicon carbide	3440	3.16	1088	110	[9]
Glass fiber	3400	2.60	1307	133	[8]
Vectran	2900	1.40	2071	211	[8]
Graphite	4300	1.75	2457	250	[8]
Kevlar	3620	1.44	2514	256	[10]
Carbon nanotube	62000	1.34	46268	4716	[2]

The data of this table is from best cases, and has been established for giving a rough figure.

[edit] External links

1. ^ The space elevator: 'thought experiment', or key to the universe?, Arthur C. Clarke, 20 September 1979
2. ^ ^{a b c} tensile strength
3. ^ RoyMech: Copper Alloys
4. ^ Delft University of technology: Oak wood
5. ^ Goodfellow: Polypropylene
6. ^ Goodfellow: Polyamide - Nylon 6
7. ^ eFunda: Magnesium Alloys
8. ^ ^{a b c d e f} Vectran fiber: specific strength
9. ^ Specialty Materials, Inc SCS Silicon Carbide Fibers
10. ^ Network Group for Composites in Construction: Introduction to Fibre Reinforced Polymer Composites

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