Glass Expert Performs Failure Analysis on Glass Blender Jar

The glass experts at Read Consulting in Santa Rosa California performed a glass failure analysis on a borosilicate glass blender jar that failed in a glass thermal shock test as a part of product qualification. The jar is operated with a stainless steel spoon inside. It is then subjected to thermal shock test by heating it to 140°F and then pouring a cold  mixture of vodka and ice into the hot jar. The jar must survive this test to qualify. A failure analysis was performed on a jar that failed the test. Upper left is a photograph of the cracked jar in the region of the failure origin. Upper right is a photo micro graph of the origin. The failure originated at a damage site in the interior wall of the jar (presumably caused by the spinning spoon). The failure was driven by thermal stress. Because the jar is made from low expansion glass and this is a standard qualification test, there must be an explanation as to why this jar failed the test. Examination with polarized light indicates that this jar was not completely annealed. This is shown in the center photograph taken with cross polarizers. There are significant residual stresses.

Tempered Glass Expert Discusses Glass Edge Finish

California glass expert discusses the importance of edge finish on tempered glass. Tempered soda lime glass has replaced low expansion borosilicate glass for small oven doors and glass tops for cookware. This reduces costs, but can reduce reliability. One important aspect of these tempered glass pieces is the edge quality. Poor edge finishing can introduce defects that can cause eventual failure. There are three common edge defects. These are chipping, missed seaming and "dry" seaming. Normally these pieces are scribed and broken, seamed with a wet belt sander and tempered. These three edge defects are illustrated by the figures. Read Consulting failure analysis lab has looked at numerous tempered glass failures that have originated at the edge; thus, edge finish is extremely important for any glass piece that can be preferentially heated in the center.

Coffee Pot Failure Analysis

A failure analysis was performed on a glass coffee press. In this case the glass coffee pot is a 0.085" thick borosilicate glass 3.7" diameter cylinder that is 7" high. It had received a blow on the top rim, and it broke into approximately 10 pieces. Because the pot is a cylinder, the blow from the outside on the rim created tensile stress on the inside of the pot and compressive stress on the outside of the pot. This is reflected in the fracture surface above. The crack traveled faster on the inside half of the glass fracture surface. It finished as twist hackle on the outside half of the glass fracture surface.

Failure Analysis: "Exploding" Coffee Pot

California failure analysis expert was presented with the claim that a coffee pot had "exploded" when replaced on the coffee warmer in a convenience store.First of all, there is no "stored" energy to cause an explosion. The coffee is not under pressure, and the pot is made from annealed borosilicate glass. In addition, the broken pot had been destroyed. To prove that the pot is annealed glass. The California glass expert broke an exemplar pot. It broke into large shards and the fracture surfaces were typical of those of annealed glass. Without available stored energy, an explosion is physically impossible; therefore, the injured party must have either dropped the pot or hit it against a hard object.

Glass Expert Discusses Glass Ovenware

A glass expert in California compares Pyrex glass ware from two eras. The original glass ovenware was developed by Corning Glass Works. They introduced annealed borosilicate glass that had a lower thermal expansion coefficient than the more common soda lime glass. The lower expansion coefficient reduced the level of thermal shock and allowed this glass to be used for ovenware. For a long period Pyrex was synomonous with borosilicate glass. Later the decision was made to make glass ovenware from heat strengthened soda lime glass and to continue to call it pyrex. This is the present approach to this type of cookware. Above, the upper right image is an edge stress measurement performed on an older piece of glass cookware. This measurement shows that this glass is annealed. This is an older piece of traditional pyrex cookware. On the lower right is the same measurement performed on a recently purchased piece of pyrex cookware. The presence and number of the color bands indicates that this ovenware is made from heat strengthened glass.