Stress and strain are deŸned in many elementary textbooks about the mechanics of deformable bodies [1,2]. œe terms stress and strain are used to describe loads on and deformations of solid materials. œe simplest types of solids to describe are homogeneous and isotropic. Homogeneous means the material properties are the same at di¥erent locations and isotropic means the material properties are independent of direction in the material. An annealed steel bar is homogeneous and isotropic, whereas a human femur is not homogeneous because the marrow has very di¥erent properties from the bone, and it is not isotropic because its properties are di¥erent along the length and along the cross section.