A stress intensity (K) function describing the extension of a crack through bounding compressive layers has been experimentally verified using two methods. For one method the crack length was measured using a replicating technique; in the other, the crack length was continuously measured in situ with an optical microscope during loading. In both, cracks were observed to propagate in a stable, nearly straight path across the compressive layer with increasing applied stress. Catastrophic failure was observed when the crack fully extended across the compressive layers at an applied stress predicted and described as the threshold stress. This behavior and agreement were valid only when either the compressive stress or the thickness of the compressive layer was small.