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Young's modulus, also known as the tensile modulus or elastic modulus, is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the stress along an axis over the strain along that axis in the range of stress in which Hooke's law holds. In solid mechanics, the slope of the stress-strain curve at any point is called the tangent modulus. The tangent modulus of the initial, linear portion of a stress-strain curve is called Young's modulus. It can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material. In anisotropic materials, Young's modulus may have different values depending on the direction of the applied force with respect to the material's structure.
It is named after Thomas Young, the 19th century British scientist. However, the concept was developed in 1727 by Leonhard Euler, and the first experiments that used the concept of Young's modulus in its current form were performed by the Italian scientist Giordano Riccati in 1782, predating Young's work by 25 years.
Young's modulus is the ratio of stress (which has units of pressure) to strain (which is dimensionless), and so Young's modulus has units of pressure. Its SI unit is therefore the pascal (Pa or N/m2 or m−1·kg·s−2). The practical units used are megapascals (MPa or N/mm2) or gigapascals (GPa or kN/mm2). In United States customary units, it is expressed as pounds (force) per square inch (psi). The abbreviation ksi refers to thousands of psi.
Young's modulus is frequently denoted by E, and less commonly by Y.
The Young's modulus enables the calculation of the change in the dimension of a bar made of an isotropic elastic material under tensile or compressive loads. For instance, it predicts how much a material sample extends under tension or shortens under compression. Young's modulus is used in order to predict the deflection that will occur in a statically determinate beam when a load is applied at a point in between the beam's supports. Some calculations also require the use of other material properties, such as the shear modulus, density, or Poisson's ratio.
The Young's modulus represents the factor of proportionality in Hooke's law, relating the stress and the strain ; but this law is only valid under the assumption of an elastic or linear response. Any real material will eventually fail and break when stretched over a very large distance or with a very large force ; however, all materials exhibit Hookean behavior for small enough strains or stresses. If the range over which Hooke's law is valid is large enough compared to the typical stress that one expects to apply to the material, the material is said to be linear ; if the typical stress one would apply is outside the linear range, then the material is said to be non-linear.
Steel, carbon fiber and glass among others are usually considered linear materials, while other materials such as rubber and soils are non-linear. However, this is not an absolute classification : if very small stresses or strains are applied to a non-linear material, the response will be linear, but if very high stress or strain is applied to a linear material, the linear theory will not be enough. For example, as the linear theory implies reversibility, it would be absurd to use the linear theory to describe the failure of a steel bridge under a high load ; although steel is a linear material for most applications, it is not for this one.
Young's modulus is not always the same in all orientations of a material. Most metals and ceramics, along with many other materials, are isotropic, and their mechanical properties are the same in all orientations. However, metals and ceramics can be treated with certain impurities, and metals can be mechanically worked to make their grain structures directional. These materials then become anisotropic, and Young's modulus will change depending on the direction of the force vector. Anisotropy can be seen in many composites as well. For example, carbon fiber has much higher Young's modulus (is much stiffer) when force is loaded parallel to the fibers (along the grain). Other such materials include wood and reinforced concrete. Engineers can use this directional phenomenon to their advantage in creating structures.
The Young's modulus of a material can be used to calculate the force it exerts under specific strain.
where F is the force exerted by the material when compressed or stretched by ΔL.
Hooke's law can be derived from this formula, which describes the stiffness of an ideal spring:
where it comes in saturation
The elastic potential energy stored is given by the integral of this expression with respect to L:
where Ue is the elastic potential energy.
The elastic potential energy per unit volume is given by:
This formula can also be expressed as the integral of Hooke's law:
For homogeneous isotropic materials simple relations exist between elastic constants (Young's modulus E, shear modulus G, bulk modulus K, and Poisson's ratio ν) that allow calculating them all as long as two are known:
Young's modulus can vary somewhat due to differences in sample composition and test method. The rate of deformation has the greatest impact on the data collected, especially in polymers. The values here are approximate and only meant for relative comparison.
|Rubber (small strain)||0.01–0.1||1,450-14,503|
|PTFE (Teflon)||0.5||75,000|
|Low density polyethylene||0.11 - 0.45||16,000-65,000|
|Polyethylene terephthalate (PET)||2-2.7|
|Diatom frustules (largely silicic acid)||0.35–2.77||50,000–400,000|
|Medium-density fiberboard (MDF)||4||580,000|
|Pine wood (along grain)||9||1,300,000|
|Oak wood (along grain)||11|
|Human Cortical Bone||14||2,030,000|
|Aromatic peptide nanotubes,||19-27|
|Hemp fiber ||35|
|Magnesium metal (Mg)||45|
|Flax fiber ||58|
|Stinging nettle fiber ||87|
|Glass (see chart)||50–90|
|Mother-of-pearl (nacre, largely calcium carbonate) ||70||10,000,000|
|Tooth enamel (largely calcium phosphate)||83||12,000,000|
|Glass-reinforced plastic (70/30 by weight fibre/matrix, unidirectional, along grain)||40–45||5,800,000–6,500,000|
|Glass-reinforced polyester matrix ||17.2||2,500,000|
|Carbon fiber reinforced plastic (50/50 fibre/matrix, biaxial fabric)||30-50|
|Carbon fiber reinforced plastic (70/30 fibre/matrix, unidirectional, along grain)||181||26,300,000|
|Silicon Single crystal, different directions ||130-185|
|polycrystalline Yttrium iron garnet (YIG)||193||28,000,000|
|single-crystal Yttrium iron garnet (YIG)||200||30,000,000|
|Aromatic peptide nanospheres ||230-275|
|Sapphire (Al2O3) along C-axis||435||63,000,000|
|Silicon carbide (SiC)||450|
|Tungsten carbide (WC)||450–650|
|Single-walled carbon nanotube||1,000+||145,000,000+|
|Diamond (C)||1,220||150E6 – 175E6|
|Carbyne (C)||37,200||5,388E6 – 5,402E6|
|Homogeneous isotropic linear elastic materials have their elastic properties uniquely determined by any two moduli among these, thus given any two, any other of the elastic moduli can be calculated according to these formulas.|