  Recently, several algorithms for strain tomography from energy-resolved
neutron transmission measurements have been proposed. These methods assume that
the stress-free lattice spacing $d_0$ is a known constant limiting their
application to the study of stresses generated by manufacturing and loading
methods that do not alter this parameter. In this paper, we consider the more
general problem of jointly reconstructing the strain and $d_0$ fields. A method
for solving this inherently non-linear problem is presented that ensures the
estimated strain field satisfies equilibrium and can include knowledge of
boundary conditions. This method is tested on a simulated data set with
realistic noise levels, demonstrating that it is possible to jointly
reconstruct $d_0$ and the strain field.
