California Glass Expert Witness Discusses Bending Failure of Tempered Glass

California Glass Expert performs a failure analysis on a tempered glass oven door that failed in bending. Normally the fracture surface of tempered glass shows two sets of Wallner lines that are separated by a band of mist hackle. However if the tempered glass window failure is due to bending, the band of mist hackle is shifted toward the tensile side of the bend. Upper left is a photo of the tempered glass failure. The failure originated at an edge and progressed inward. A representative fracture surface is shown on the right, and the failure origin is shown on the left.

California Glass Expert Examines the Edge of Tempered Glass

California glass expert uses a grazing angle surface profilometer (GASP) to study the compressive stresses at the edge of a tempered glass oven door. The edge effects during the air quench step of the thermal temper process are said to lower the surface stress to zero near the edge of the glass article. In fact, fragmentation tests of tempered glass pieces yield a "picture frame" around the edge of the part. Fir this reason, a GASP was used to examine surface stress at the edge. Upper left is an image of the GASP interference pattern in the center of a tempered glass toaster oven door. The surface compressive stress measures 17,000 psi. Upper right is a similar image measured near the edge of the part. In this region the fringes curve, and they show the surface stress going to zero. This is important because any damage to the glass in this region of reduced surface compression stress is more likely to cause failure.