The elastic moduli express in mathematical form the ability of a material to temporarily deform under stress.

**Hooke’s Law** shows the relationship between stress and linear strain (usually represented by ε) through a constant which depends on the body under strain (σ = Eε). This constant is one of the elastic moduli, the **Young’s Modulus (E)**, defined as the ratio of the stress σ along an axis on the deformation (ε) along the same axis:

**Poisson’s Ratio (ν) **is the radial strain to axial strain ratio (when a material is compressed along a direction it usually tends to expand in another direction):

The** Rigidity Modulus (μ)**, or shear modulus, is the ratio between shear stress τand shear strain (it is also known as Lamé’s 2nd constant):

The C**ompressibility Modulus L** (or Bulk Modulus) expresses the volume change following a deformation or strain:

**Lamé’s 1st constant**** λ **relates the Bulk Modulus K with the shear modulus μ (Lamé’s 2nd constant):

hence:

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