Modulus of Elasticity
It may be noted that in most materials which may be ductile or brittle, the elastic behaviour is marked by linear relationship between stress (σ) and strain (ε). This is seen for A and B type ductile materials in Figure and for type A brittle material in Figure. Such linear relationship was first identified through Hooke's law which states that "stress is directly proportional to strain within elastic limit." This means that
σ ∝ε or
σ = E ε i.e. E = σ ε
The constant of proportionality E is called Modulus of Elasticity or Young's Modulus. Apparently the slope of elastic part of stress-strain diagram is the modulus of elasticity. Modulus of elasticity is a constant for a given class of material and variation in small alloying contents does not cause change in E. For example, wide range of carbon steel has E = 210 × 103 MPa. If stress-strain curve is established accurately the slope of elastic part is E of the material. However, it may be noted that this is not a reliable method because tension tests require such loads that may cause small deformations in loading system, part of which may look like deformation of the specimen.