The Young modulus is an elastic constant which, for a homogeneous isotropic material, relates stress to strain (or deformation). The S.I. unit is MPa.
This coefficient is a measure of the stiffness of a material. At the same stress, a material with a high modulus of elasticity will undergo a smaller deformation than a material with a low modulus of elasticity.
When referring to the modulus of elasticity, it is understood to be "under tension"; strictly, however, the type of loading used to measure the modulus should be specified each time, because depending on the material (and this is the case with composites) the modulus of elasticity is not the same for tension, compression or bending.
For an orthotropic composite (such as most laminates),there are three different elasticity moduli along the three axes, and the complete relation between the strain and the stress is then given in matrix form.
To determine the modulus of elasticity E of an isotropic material, a tensile test is carried out and the stress-strain curve is recorded. The slope of the linear part of the curve is the modulus of elasticity of the material.