Young's Modulus

Young's modulus (E) relates the STRESS and STRAIN in a solid (usually wire) using the following formula:

E = stress/strain = σ/ε

where σ = Force/Area and E = Change in Length/Length

Young's modulus has units of newtons per metre squared (Nm^-2) and is calculated only when the material is under elastic conditions, i.e., the force applied does not exceed the elastic limit and cause deformation.

Taken from Dictionary of Science

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In solid mechanics, Young's modulus (E) is a measure of the stiffness of an isotropic elastic material. It is also known as the Young modulus, modulus of elasticity, elastic modulus (though Young's modulus is actually one of several elastic moduli such as the bulk modulus and the shear modulus) or tensile modulus. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. This can be experimentally determined from the slope of a stress-strain curve created during tensile tests conducted on a sample of the material.

Young's modulus 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.

Taken from Wikipedia

Yield Point

HOOKE'S LAW states that for a material such as steel, in wire form, the extension is proportional to the tension, up to what is called the elastic limit. An increase in tension beyond this limit takes the material to the yield point, where a sudden increase in elongation occurs with only a small further increase in tension.

Taken from Dictionary of Science

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The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permanent and non-reversible.

In the three-dimensional space of the principal stresses (σ₁,σ₂,σ₃), an infinite number of yield points form together a yield surface.

Knowledge of the yield point is vital when designing a component since it generally represents an upper limit to the load that can be applied. It is also important for the control of many materials production techniques such as forging, rolling, or pressing. In structural engineering, this is a soft failure mode which does not normally cause catastrophic failure or ultimate failure unless it accelerates buckling.

Taken from Wikipedia