Cracking resulting from thermal expansion mismatch generated residual stresses is investigated using a model system consisting of bonded layers of sapphire and borosilicate glass. Three planar geometries are employed; a bilayer configuration and two sandwich configurations. The bilayer configuration of a thin sapphire sheet bonded to a thick glass substrate models the cracking due to a thin film under residual tension. The cracks formed adopt a characteristic shape running parallel to the planar interface and at a depth consistent with recent predictions of Suo and Hutschinson for a KII = 0 steady-state propagation trajectory. The same result is obtained with a novel test sample consisting of a sapphire sheet bonded to a triangular shaped substrate. The other two configurations, of a glass block sandwiched between two sapphire sheets and of a sapphire sheet between two glass blocks, enable the residual stress cracking to be explored for conditions under which no net bending moment exists. © 1990.