A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, through‐thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda‐lime glass and monolithic Al2O3for lengths in excess of 20 mm without uncontrollable catastrophic fail