EN224: Linear Elasticity


Homework 2: Potential representations, Fourier Transforms, Singular Solutions
Due Friday March 4, 2005





Division of Engineering

Brown University





1. The simplest possible derivation of the Kelvin State. Assume that the solution is to be generated from Papkovich-Neuber potentials  satisfying



Here, we have chosen the body force to be a Dirac delta sequence centered at . By taking Fourier transforms of the governing equations for  above, find the Papkovich-Neuber potentials that generate the required solution. 




2. Papkovich-Neuber potentials for the Doublet states.  Let   denote the Papkovich-Neuber potentials for the normalized Kelvin state, i.e. a point force of unit magnitude acting in the   direction at the origin.  Let

denote the doublet states, i..e.

Show that  may be generated from Papkovich-Neuber potentials



Hence, verify that the Papkovich Neuber potentials for the doublet states centered at the origin are




3. Center of Compression.  Using the results of the preceding section, find the displacement, strain and stress fields associated with a center of compression at the origin, i.e., find





4. Center of compression in a sphere.  Using superposition and the results of problem (3), find the displacement fields induced by a center of compression at the center of a sphere of radius a.  Assume that the surface of the sphere is free of traction. 





5. Dilatation at the center of a sphere due to arbitrary surface traction. Using the result of problem (4), show that the dilatation at the center of a sphere of radius a due to

a self-equilibrating distribution of traction t acting on its surface is



where r is the position vector of a point on the sphere’s surface relative to the origin, and

B denotes the surface of the sphere.  Verify the predictions that were made in our proof of Saint-Venants principle.  What happens if the tractions act tangent to the surface of the sphere?