Tutorial: Tempered Glass
Strength of Tempered Glass: Tempered glass is glass with an outer compressive
layer; therefore, the center of the glass is in tension. The compressive stress
is additive. Therefore, if a piece of glass normally breaks at 4000 PSI tensile
stress, and it’s tempered to have an outer layer of 10,000 PSI compressive
stress, the resulting breaking stress is raised to 14000 PSI (or higher).
Breaking Characteristics: Once a defect enters the tensile portion of the
glass, tempered glass fails almost instantly into blocky pieces. Once the
failure initiates, it is driven by the residual tensile stress in the center of
the glass, and the crack travels at an approximate velocity of 3,384 m/sec. This
is almost instantaneous to an observer. There may be a sound associated with
crack propagation; however, the glass is not exploding. In the absence of
external forces, the failure is driven from internal stresses. Therefore, the
failed glass will stay interlocked until external forces are applied. Figure #1
is a schematic of the stress state in a tempered piece of glass. In this
illustration, the depth of the compressive layer is 20% of the glass thickness.
This varies with the level of the temper stress, but 20% is a reasonable number
to work with. It is important to realize that the compressive stress and its
depth is less at corners in the glass. Figure #4 shows the effect of the surface
compressive stress from temper on the breaking strength of glass. It is
important to remember that this strength is additive, and it must be considered
in evaluations applying fracture mechanic principles.
Figure #1: Schematic cross-section of a piece of tempered glass. This schematic
illustrates that the outer layer of the glass is in compression. Because glass
only breaks in tension, the compressive stresses must be overcome before the
glass surface can be put in tension. Therefore, the compressive stress is
additive to the stress necessary to break the glass.18
Introduction to Specific Problem
Glass fractography is the most
effective method for determining why a glass object, such as a bottle, failed.
This technique consists of examining the fracture surfaces of the failure for
artifacts such as Wallner lines and using them to trace the crack back to its
origin. Once the origin has been identified, it can be examined in detail with a
microscope to determine the cause of the failure. In the case of tempered glass,
the object breaks into thousands of small blocks of glass, and a failure
analysis is impossible. In this particular failure the part of the backboard
that contained the fracture origin was preserved between two steel plates, and a
failure analysis is possible.
The subject backboard had been in
service for 25 years; so, it is not likely that it was defective. However, it is
worthwhile to examine it for educational purposes. The structure for the support
of the hoop was held by two steel plates (Figure #1) that were clamped through
holes to the 1/2” thick tempered glass backboard. The bolts through the glass
were hollow, and this allowed the hoop to be bolted to the structure. The glass
was properly protected from the steel hardware by polymer plates and washers.
Figure #2 is an overview of the glass remaining between the steel plates.
Figure #1: Remainder of the backboard. This was all that was
submitted for failure analysis.
Figure #2: Overview of the remainder of the tempered glass preserved between the
steel plates. The glass was isolated from the steel hardware by compliant
polymer sheets and washers. The fracture pattern indicates that the failure
started at the lower left hole and spread from that point.
Figure #3: Close up of the region where the failure initiated. In this picture
one can see samples of the compliant buffers that separated the glass from the
steel. This piece of glass was properly isolated from the steel hardware.
This backboard had been in use for 25 years, and the failure initiated at one of
the holes machined in the glass. It is hypothesized that during use one of the
cracks generated by machining a hole in the glass must have grown into the
tensile region in the center of the tempered glass, and the glass “self
destructed”. It is believed that this example proves that small edge cracks in
tempered glass can grow over time and cause failure.