Standard methods for testing fracture toughness of homogeneous materials are currently being applied to welds. A weld is, by nature, a heterogeneous structure containing zones with different microstructures and mechanical properties. The validity of using fracture parameters developed for homogeneous materials to characterize fracture of welds has not been established. Tests were conducted to investigate whether the extension of a crack along an interface is governed by the same fracture parameters used to predict fracture in homogeneous materials. The tests involved measuring the displacement fields around a crack tip in the interface between two materials with matched elastic properties and differing yield strengths. The form of the displacement distribution was then compared with the theoretical displacements for linear elasticity and nonlinear elasticity with power-law hardening.