A three-dimensional lattice spring model of a heterogeneous material is pre
sented. For small deformations, the model is shown to recover the governing
equations for an isotropic elastic medium. The model gives reasonable agre
ement with theoretical predictions for the elastic fields generated by a sp
herical inclusion, although for small particle sizes the discretization of
the underlying lattice causes some departures from the predicted values. Pl
asticity is introduced by decreasing the elastic moduli locally whilst main
taining stress continuity. Results are presented for a spherical inclusion
in a plastic matrix and are found to be in good agreement with the predicti
ons of Wilner.