Glass is difficult to grip so glass strength is measured in flexure or bending. Modulus of rupture, or MOR, is normally associated with glass strength and calculated to be the is maximum surface stress in a flexed beam at point of failure. In tensile tests, all flaws see the maximum stress so MOR or modulus of rupture is larger by 30% because the volume subjected to maximum stress is small, and the probability of a large flaw in the highly stressed region is also small. Glass strength testers are set up to test brittle materials. The most common test standard, or testing method or norma for glass strength is ASTM C158 which calculates modulus of rupture. Testing for glass strength and modulus of rupture typically involves using a large surface area and precluding edge effects which C158 attempts to do.
ASTM C158 uses flexure strength to calculate modulus of Rupture of glass as it minimizes breaks that originate at the edge. Other glass strength tests minimize edge effects by testing for equibiaxial strength such as ASTM F394 or ASTM C1499. Non-strengthened glass materials have relatively low tensile strength yet high compressive strength, and so most glass breakage is due to tensile stress failure. The most common glass strengthtesting standards for glass is ASTM C 158. It measures modulus of rupture and recommends that the ratio of the width to thickness of standard MOR specimens be between 2:1 and 10:1.