Specific strength

For the stiffness to weight ratio, see specific modulus.

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 (N/m2)/(kg/m3) or more commonly N·m/kg.

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 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.

Contents

Examples

Specific tensile strength of various materials
Material Strength
(MPa)		 Density
(g/cm³)		 Specific Strength
(kN·m/kg)		 Breaking length
(km)		 source
Scifer steel wire 5,500 7.87 706 71.2 [1]
Bainite 2,500 7.87 321 32.4 [1]
1 μm iron whiskers 14,000 7.87 1800 183 [1]
Concrete 10 2.30 4.35 0.44
Rubber 15 0.92 16.3 1.66
Brass 580 8.55 67.8 6.91 [2]
Oak 60 0.69 86.95 8.86 [3]
Balsa (axial load) 73 0.14 521 53.2 [4]
Polypropylene 80 0.90 88.88 9.06 [5]
Nylon 78 1.13 69.0 7.04 [6]
Magnesium 275 1.74 158 16.11 [7]
Aluminium alloy (3003-H14) 600 2.70 222 22.65 [8]
Steel alloy 2,000 7.86 254 25.93 [8]
Titanium alloy 1,300 4.51 288 29.38 [8]
carbon-epoxy composite 1240 1.58 785 80 [9]
Silicon carbide 3,440 3.16 1,088 110 [10]
Glass fiber 3,400 2.60 1,307 133 [8]
Basalt fiber 4,840 2.70 1,790 182.7 [11]
Vectran 2,900 1.40 2,071 211 [8]
Carbon fiber (AS4) 4,300 1.75 2,457 250 [8]
Kevlar 3,620 1.44 2,514 256 [12]
Spectra fiber 3,510 0.97 3,619 369 [13]
Zylon 5,800 1.54 3,766 384 [14]
Colossal carbon tube 6,900 .116 59,483 6,066 [15]
Carbon nanotube (see note below) 62,000 .037-1.34 46,268-N/A 4,716-N/A [16][17]
spider silk 1,400 1.31 1,069 109

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

Relation to velocity

Since N·m/kg (the unit of specific strength) expands to (kg·m/s2)·m/kg or (m/s)2, a specific strength can also be understood as the square of a speed, hence represented by it square root, a speed. Further, the structural analysis for whether a particular material is strong enough for some purpose can sometimes be expressed in terms of comparing this speed to the speed of a relevant part of the structure. For example, a spinning cylindrical shell (as in an idealized flywheel or an idealized cylinder space ship) is only feasible if built of a material whose square root of specific strength is at least the speed at which the spinning shell moves.

\sqrt{\tfrac{E}{\rho}}\ge v

See also

References

  1. ^ a b c 52nd Hatfield Memorial Lecture: "Large Chunks of Very Strong Steel" by H. K. D. H. Bhadeshia 2005
  2. ^ RoyMech: Copper Alloys
  3. ^ Delft University of technology: Oak wood
  4. ^ [1]
  5. ^ Goodfellow: Polypropylene
  6. ^ Goodfellow: Polyamide - Nylon 6
  7. ^ eFunda: Magnesium Alloys
  8. ^ a b c d e f Vectran fiber: specific strength
  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
  12. ^ Network Group for Composites in Construction: Introduction to Fibre Reinforced Polymer Composites
  13. ^ Spectra Fiber - Honeywell Advanced Fibers and Composites
  14. ^ Toyobo Co.,Ltd.. "ザイロン®(PBO 繊維)技術資料 (2005)" (free download PDF). http://www.toyobo.co.jp/seihin/kc/pbo/technical.pdf. 
  15. ^ 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. 
  16. ^ 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. 
  17. ^ a b K.Hata. "From Highly Efficient Impurity-Free CNT Synthesis to DWNT forests, CNTsolids and Super-Capacitors" (free download PDF). http://www.nanocarbon.jp/english/research/image/review.pdf. 
  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

External links