Two-dimensional problems of plane stress and plane strain are considered. The plane stress problems are the problems of thin plates loaded over their lateral boundary by tractions which are uniform across the thickness of the plate, while its flat faces are traction free. The plane strain problems involve long cylindrical bodies, loaded by tractions which are orthogonal to the longitudinal axis of the body and which do not vary along this axis. The tractions over the bounding curve of each cross section are self-equilibrating. Two rigid smooth constraints at the ends of the body prevent its axial deformation. The stress components are expressed in terms of the Airy stress function such that the equilibrium equations are automatically satisfied. The Beltrami–Michell compatibility equations require that the Airy stress function is a biharmonic function. The Airy theory is applied to analyze pure bending of a thin beam, bending of a cantilever beam by a concentrated force, and bending of a simply supported beam by a distributed load. The approximate character of the plane stress solution is discussed, as well as the transition from the plane stress to the plane strain solution.

The analysis of normal and shear stresses over differently oriented surface elements through a considered material point is presented. The Cauchy relation for traction vectors is introduced, which leads to the concept of a stress tensor. The analysis is presented of one-, two-, and three-dimensional states of stress, the principal stresses (maximum and minimum normal stresses), the maximum shear stress, and the deviatoric and spherical parts of the stress tensor.The equations of equilibrium are derived and the corresponding boundary conditions are formulated.

In addition to rotation, non-circular cross sections of twisted rods undergo longitudinal displacement, which causes warping of the cross section. This warping is independent of the longitudinal z coordinate and is a harmonic function of the (x,y) coordinates within the cross section. The Prandtl stress function is introduced, in terms of which the shear stresses are given as its gradients. This automatically satisfies equilibrium equations, while the compatibility conditions require that the stress function is the solution to Poisson’s equation. From the boundary condition of a traction-free lateral surface, it follows that the stress function is constant along the boundary of the cross section. The applied torque is related to the angle of twist by the integral condition of moment equilibrium. This theory is applied to determine the stress and displacement components in twisted rods of elliptical, triangular, rectangular, semi-circular, grooved-circular, thin-walled open, thin-walled closed, and multicell cross sections. The expressions for the torsional stiffness are derived in each case. The maximum shear stress and the warping displacement are also evaluated and discussed.

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.