# Elastic modulus

The tendency of a material to deform when an external force is applied: <math> \frac{\lambda}{\epsilon} </math> , where <math>\lambda</math> and <math>\epsilon</math> are the stress and the strain, respectively. The elastic modulus is defined only over the elastic deformation region in which the strain which the material undergoes is fully reversible.

A modulus is further defined by the precise experimental conditions used to arrive at the stress-strain strain curve used in its computation. Some common moduli include Young's modulus (E), which involves tensile forces; shear modulus (G or <math>\mu</math>), which is defined under shear-inducing forces; and the bulk modulus (K), which is defined as the volumetric stress over volumetric strain, making it effectively a 3D Young's modulus. Note that these moduli are further defined by the types of materials being tested, with most people relying on ISO standards for clarity.