Tensile strength

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Entry by Emily Redston, AP 225, Fall 2011

Figure 1 Stress vs. Strain curve typical of aluminum 1. Ultimate strength 2. Yield strength 3. Proportional limit stress 4. Fracture 5. Offset strain (typically 0.2%) (http://en.wikipedia.org/wiki/Ultimate_tensile_strength)

The tensile strength (also called ultimate tensile strength) is the maximum stress that can be sustained by a structure in tension; if this stress is applied and maintained, the sample will fracture. It is important to note, however, that tensile strength is not necessarily the same as fracture strength. Let's consider the typical stress-strain curve of a ductile material, seen in Figure 1. You obtain such a curve by doing a tensile test, one of the most common mechanical tests. Tensile tests can be used to ascertain several important mechanical properties, such as the tensile strength of a material. For a ductile material, after yielding, the stress necessary to continue plastic deformation increases to a maximum, and then decreases to the eventual fracture point. The highest point of the stress-strain curve is the tensile strength, corresponding to point 1. All deformation up to this point is uniform through the tensile specimen. However, at this maximum stress, a small constriction, or neck, begins to form (typically near the middle of the sample), and all subsequent deformation is confined to this neck. This phenomenon is termed "necking", and fracture will ultimately occur at the neck. Fracture strength corresponds to the stress at fracture. It is clear that the tensile strength is not the same as the fracture strength in this case. However, for brittle materials, these will tend to be more similar because brittle materials will fracture near the end of the linear-elastic portion of the stress-strain curve.

Tensile strength is defined as a stress, which is measured as force per unit area. It is typically reported in units of Pa or psi. The tensile strength is an intensive property, meaning that its value does not depend on the size of the test specimen. However, it is dependent on other factors such as (1) the preparation of the specimen, (2) temperature, and (3) the presence of surface defects.



Typical tensile strengths

Typical tensile strengths of some materials (from [2])
Material Yield strength
(MPa)
Ultimate strength
(MPa)
Density
(g/cm³)
Structural steel ASTM A36 steel 250 400 7.8
Carbon steel 1090 250 841 7.58
Human skin 15
Steel, high strength alloy ASTM A514 690 760 7.8
High density polyethylene (HDPE) 26-33 37 0.95
Polypropylene 12-43 19.7-80 0.91
Stainless steel AISI 302 - Cold-rolled 520 860 8.19
Cast iron 4.5% C, ASTM A-48 130 200  
Aluminium alloy 6063-T6   248 2.63
Copper 99.9% Cu 70 220 8.92
Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu 130 350 8.94
Brass 200 + 550 5.3
Tungsten   1510 19.25
E-Glass N/A 1500 for laminates,
3450 for fibers alone
2.57
S-Glass N/A 4710 2.48
Marble N/A 15  
Concrete N/A 3 2.7
Carbon fiber N/A 1600 for Laminate,
4137 for fiber alone
1.75
Human hair   380  
Bamboo   350-500 0.4
Spider silk 1000 1.3
Silkworm silk 500   1.3
Aramid (Kevlar]] or Twaron) 3620 2757 1.44
23 46 0.97
Vectran   2850-3340  
Pine wood (parallel to grain)   40  
Bone (limb) 104-121 130 1.6
Nylon, type 6/6 45 75 1.15
Rubber - 15  
Boron N/A 3100 2.46
Silicon, monocrystalline (m-Si) N/A 7000 2.33
Silicon carbide (SiC) N/A 3440  
Sapphire (Al2O3) N/A 1900 3.9-4.1
Boron Nitride Nanotube N/A 33000  ?
Diamond N/A 2800 3.5
First carbon nanotube ropes  ? 3600 1.3
Colossal carbon tube N/A 7000 0.116
Carbon nanotube N/A 11000-63000 0.037-1.34


References

[1] Callister, William D. Materials Science and Engineering: an Introduction. New York: John Wiley & Sons, 2007.

[2] http://en.wikipedia.org/wiki/Ultimate_tensile_strength

Keyword in references:

Electronic skin: architecture and components