Abstract: An apparatus and method of proof testing a planar, brittle materials such as those used for valve leaflets in the manufacture of mechanical heart valve prostheses, in which the material to be tested is placed between two interfitting, spherically-shaped shoes, one convex and one concave, and the shoes are compressed together, causing a biaxial stress force to be applied across the surface of the material. The use of a biaxial force created by the spherical shoe surfaces simplifies the testing procedure over the prior art, allowing a single application of force to test the material for flexure in any direction. The magnitude of the stress which is applied to the material is determined by the radii of the interfitting test shoes relative to the thickness of the material sample being tested. Sufficient force is applied to conform the material to the shoes such that any flaws of a predetermined minimum critical size will be revealed by acoustical monitoring during the test or visual inspection thereafter.