Young's modulus is named after Thomas Young, the 18th 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; therefore Young's modulus itself has units of pressure. A purist might argue that the units are fine if the dimensionless ratio is left in (i.e. lbs/in^2 over in/in). Carrying the full expression can be helpful for canceling units, in dimensional analysis.
The SI unit of modulus of elasticity (E, or less commonly Y) is the pascal; the practical units are megapascals (MPa) or gigapascals (GPa or kN/mm²). In United States customary units, it is expressed as pounds (force) per square inch (psi).
The Young's modulus allows the behavior of a bar made of an isotropic elastic material to be calculated under tensile or compressive loads. For instance, it can be used to predict the amount a wire will extend under tension or buckle under compression. Some calculations also require the use of other material properties, such as the shear modulus, density, or Poisson's ratio.
For many materials, Young's modulus is essentially constant over a range of strains. Such materials are called linear, and are said to obey Hooke's law. Examples of linear materials include steel, carbon fiber, and glass. Rubber and soils (except at very small strains) are non-linear materials.
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: 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 which direction the force is applied from. Anisotropy can be seen in many composites as well. For example, carbon fiber has a much higher Young's modulus (is much stiffer) when force is loaded parallel to the fibers (along the grain), and is an example of a material with transverse isotropy. Other such materials include wood and reinforced concrete. Engineers can use this directional phenomenon to their advantage in creating various structures in our environment.
The Young's modulus of a material can be used to calculate the force it exerts under a specific strain.
From this formula can be derived Hooke's law, which describes the stiffness of an ideal spring:
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:
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 comparisons.
|Material||Young's modulus (E) in GPa||Young's modulus (E) in lbf/in² (psi)|
|Rubber (small strain)||0.01-0.1||1,500-15,000|
|Low density polyethylene||0.2||30,000|
|Polyethylene terephthalate||2-2.5 OR 2.8-3.1||290,000-360,000|
|MDF (wood composite)||3.654||530,000|
|Pine wood (along grain)||8.963||1,300,000|
|Oak wood (along grain)||11||1,600,000|
|High-strength concrete (under compression)||30-100||4,350,000|
|Magnesium metal (Mg)||45||6,500,000|
|Glass (see also diagram below table)||65-90||9,400,000-13,000,000|
|Brass and bronze||103-124||17,000,000|
|Carbon fiber reinforced plastic (50/50 fibre/matrix, unidirectional, along grain)||125-150||18,000,000 - 22,000,000|
|Wrought iron and steel||190-210||30,000,000|
|Silicon carbide (SiC)||450||65,000,000|
|Tungsten carbide (WC)||450-650||65,000,000-94,000,000|
|Single carbon nanotube||1,000+||145,000,000+|