Elastic modulus

Since a deforming force can change a body's length, its volume, or its shape — and each of these three kinds of change has its own associated stress and its own associated strain (section 6.3) — it follows that there must be three separate moduli of elasticity, one for each type of stress-strain pair, rather than a single universal "elastic modulus" for a material. Each modulus is defined, exactly as in Hooke's law (section 6.4), as the ratio of the relevant stress to its corresponding strain within the elastic limit, and each captures a different aspect of how strongly a material resists being deformed. The three moduli — Young's modulus (for change in length), bulk modulus (for change in volume), and modulus of rigidi …