ASTM C 158 (C-158) determines the modulus of rupture in bending of glass and glass-ceramics. When executed properly the modulus of rupture from this test method is considered a valid measure of tensile strength. The standard covers two test methods with one designed to include the condition of the surface of the specimen as a factor in the measured strength for flat glass and the other is applicable to glass specimens of rectangular or elliptical cross sections. TestResources provides machines, grips, equipment, and fixtures for testing in accordance with ASTM C158.