California glass expert witness discusses crack travel directions during tempered glass failure. During tempered glass failure the glass sheet “self destructs” because the internal residual stresses are tensile and drive the failure. Essentially the glass failure results in the glass breaking into tiny cubes. This means that the crack is constantly changing directions. The California failure analysis expert has observed that the crack breaks through and travels in two directions. This is shown in the 20X photomicrograph above. Even though the crack is spreading in two directions, this does not indicate that this piece contains a failure origin. This is an interesting special case where the crack is spreading from a point, but it is not exiting the origin.