View source for Young's Modulus

[[File:StressStrainGraph1.png|350px|right]]
As material samples are stretched, initially, the '''[[Stress]]''' is directly proportional to the '''[[Strain]]''' - producing the straight line as shown on the graph. The material is behaving in much the same way as a spring would under tension and, if the force is removed, the material returns to its original length. The material is obeying [http://en.wikipedia.org/wiki/Hookes_law '''Hooke's Law'''] and this phase is known as the '''Elastic Stage'''.


The '''[[Stress]]''' divided by the '''[[Strain]]''' during this '''Elastic Stage''' is a measure of the [http://en.wikipedia.org/wiki/Stiffness '''stiffness'''] of a material and is known as its [http://en.wikipedia.org/wiki/Young's_modulus '''Young's Modulus'''] or [http://en.wikipedia.org/wiki/Elastic_modulus '''Modulus of Elasticity''']


If a force is continued to be applied beyond this phase, the material reaches a point at which it stays permanently stretched. This is the '''Yield Point''' or '''Elastic Limit''' and beyond this, during its '''Plastic Stage''', the material will continue to remain permanently deformed until eventually it breaks.
[[File:StressStrainGraph2.png|300px|right]]


Much can be determined about a material from its '''Stress/Strain Curve''' - for example, '''[[Ductility|Ductile]]''' materials will exhibit an extended '''Plastic Stage''' whereas '''[[Brittleness|Brittle]]''' materials would have little or none at all. In short:
*a stiffer material will exhibit a steeper slope
*a brittle material will lack a plastic region
*a stronger material will show a higher '''[[Tensile Strength|U.T.S.]]'''


The '''Young's Modulus''' is a fundamental property of the material and is unchanged by '''[[Heat Treatment]]''' processes such as '''[[Heat Treatment|Annealing]]''' or '''[[Crystalline_Structures#Work_Hardening|Work Hardening]]''' - you simply change degrees of '''[[Elasticity]]''' and '''[[Plasticity]]'''. The shape of the '''Stress/Strain Curve''' therefore changes but the area under it remains the same because the work done to fracture is constant.

[[Category:Terminology]]