Hooke, Hooke's Law
Index
Robert Hooke studied the uniaxial force required to extend springs and long straight wires and found it to be proportional to the extension of the device under test. He stated his observation as "Ut tensio, sic vis" - - As the extension, so is the force.

This linear elastic response has been formalized into the stress/strain relationship known as Hooke's Law: 
s = E e, where E is Young's modulus, an elastic constant. Materials obeying this relationship are known as Hookian materials.

For a generalized three-dimensional stress field a tensor form of the above relation is appropriate to characterize the linear elastic response of a Hookian material: sij = Cijkl ekl. Here, sij is the stress tensor, Cijkl is the elastic constant matrix, and ekl is the strain tensor.
Robert Hooke, UK 
(1635 - 1703)