Just as the concept of stress gives us a measure of force distributions in a deformable body, the concept of strain describes the distribution of deformations locally at every point within the body. In this chapter we will define strains and describe how strains change with directions and with the choice of coordinates, as was done with stresses. Strains will also be related to the displacements of the deformable body. It will be shown that strains must satisfy a set of compatibility equations at every point in a body to ensure that they represent a well-behaved deformation. Since the strains often found in practice are quite small, this book will only consider problems for small strains.

Partial differential equations whose solution specifies the elastic response of a loaded body are summarized. If all boundary conditions are given in terms of tractions, the boundary-value problem can be specified entirely in terms of stresses. The governing differential equations are then the Cauchy equations of equilibrium and the Beltrami–Michell compatibility equations. If some of the boundary conditions are given in terms of the displacements, the boundary-value problem is formulated in terms of the displacement components through the Navier equations of equilibrium. The boundary conditions can be expressed in terms of displacements, or in terms of displacement gradients. Due to the linearity of all equations and boundary conditions, the principle of superposition applies in linear elasticity. The semi-inverse method of solution and the Saint-Venant principle are introduced and discussed. The solution procedure is illustrated in the analysis of the stretching of a prismatic bar by its own weight, thermal expansion of a compressed prismatic bar, pure bending of a prismatic bar, and torsion of a prismatic rod with a circular cross section.