Specific strength

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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[edit]

Specific tensile strength of various materials
Material Tensile strength
(MPa)
Density
(g/cm³)
Specific strength
(kN·m/kg)
Breaking length
(km)
Source
Concrete 12 2.30 4.35 0.44
Rubber 15 0.92 16.3 1.66
Copper 220 8.92 24.7 2.51
Polypropylene 25-40 0.90 28-44 2.8-4.5 [1]
Brass 580 8.55 67.8 6.91 [2]
Nylon 78 1.13 69.0 7.04 [3]
Oak 90 0.78-0.69 115-130 12-13 [4]
Magnesium 275 1.74 158 16.1 [5]
Aluminium 600 2.80 214 21.8 [6]
Stainless steel 2000 7.86 254 25.9 [6]
Titanium 1300 4.51 288 29.4 [6]
Bainite 2500 7.87 321 32.4 [7]
Balsa 73 0.14 521 53.2 [8]
carbon-epoxy composite 1240 1.58 785 80.0 [9]
spider silk 1400 1.31 1069 109
Silicon carbide fiber 3440 3.16 1088 110 [10]
Glass fiber 3400 2.60 1307 133 [6]
Basalt fiber 4840 2.70 1790 183 [11]
1 μm iron whiskers 14000 7.87 1800 183 [7]
Vectran 2900 1.40 2071 211 [6]
Carbon fiber (AS4) 4300 1.75 2457 250 [6]
Kevlar 3620 1.44 2514 256 [12]
Dyneema (UHMWPE) 3600 0.97 3711 378 [13]
Zylon 5800 1.54 3766 384 [14]
Carbon nanotube (see note below) 62000 .037-1.34 46268-N/A 4716-N/A [15][16]
Colossal carbon tube 6900 .116 59483 6066 [17]

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,[15] 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.[18] The density is different depending on the manufacturing method, and the lowest value is 0.037 or 0.55(solid).[16]
  1. ^ Go to WayBackMachine. Enter:[http://www.goodfellow.com/csp/active/STATIC/A/Polypropylene.HTML]. Choose 2 JUN 2008 for Goodfellow: Polypropylene
  2. ^ RoyMech: Copper Alloys
  3. ^ Go to WayBackMachine. Enter:[http://www.goodfellow.com/csp/active/static/E/Polyamide_-_Nylon__6.HTML]. Choose 9 JUN 2008 for Goodfellow: Polyamide Nylon 6
  4. ^ Go to WayBackMachine. Enter:[http://www.io.tudelft.nl/research/dfs/idemat/Onl_db/Id192p.htm]. Choose 9 OCT 2007 for Delft University of technology: Oak wood
  5. ^ eFunda: Magnesium Alloys
  6. ^ a b c d e f "Vectran fiber tensile Properties". Kuraray group. 
  7. ^ a b 52nd Hatfield Memorial Lecture: "Large Chunks of Very Strong Steel" by H. K. D. H. Bhadeshia 2005
  8. ^ http://www.matweb.com/search/DataSheet.aspx?MatGUID=368427cdadb34b10a66b55c264d49c23
  9. ^ McGRAW-HILL ENCYCLOPEDIA OF Science & Technology, 8th Edition, (c)1997, vol. 1 p 375
  10. ^ Specialty Materials, Inc SCS Silicon Carbide Fibers
  11. ^ http://www.albarrie.com/techfabrics/continuousfiber.aspx[dead link]
  12. ^ Network Group for Composites in Construction: Introduction to Fibre Reinforced Polymer Composites[dead link]
  13. ^ "Dyneema Fact sheet". DSM (Company). 1 January 2008. [dead link]
  14. ^ Toyobo Co.,Ltd. "ザイロン®(PBO 繊維)技術資料 (2005)" (free download PDF). 
  15. ^ a b Yu, Min-Feng; Lourie, O; Dyer, MJ; Moloni, K; Kelly, TF; Ruoff, RS (2000). "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load". Science 287 (5453): 637–640. Bibcode:2000Sci...287..637Y. doi:10.1126/science.287.5453.637. PMID 10649994. 
  16. ^ a b K.Hata. "From Highly Efficient Impurity-Free CNT Synthesis to DWNT forests, CNTsolids and Super-Capacitors" (free download PDF). 
  17. ^ Peng, H.; Chen, D.; et al., Huang J.Y. et al. (2008). "Strong and Ductile Colossal Carbon Tubes with Walls of Rectangular Macropores". Phys. Rev. Lett. 101 (14): 145501. Bibcode:2008PhRvL.101n5501P. doi:10.1103/PhysRevLett.101.145501. PMID 18851539. 
  18. ^ "Tensile strength of single-walled carbon nanotubes directly measured from their macroscopic ropes" by F. Li, H. M. Cheng, S. Bai, G. Su, and M. S. Dresselhaus. doi:10.1063/1.1324984

The Yuri and Space Tethers[edit]

The International Space Elevator Consortium has proposed the "Yuri" as a name for the SI units describing specific strength. Specific strength is of fundamental importantance 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 reduces to an equivalent of one (m/s)2, or one N/(kg/m).[1][2] A functional space elevator would require a tether of 30-80 MegaYuri.[3]

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