Popular tea tumbler reported to shatter unexpectedly.
According to the Consumer Product Safety Commission (CPSC), there have been 302 reports of Teavana glass tea tumblers breaking or shattering unexpectedly. Six of those incidents resulted in burns and/or lacerations. The tumblers, manufactured in China, are double-walled glass and usually sold with a stainless steel infuser.
Glass is a common engineering material. It is also known for its fragility. Think of what symbolizes fragile freight. It’s a cracked glass. So why use it?
We must first establish what glass is. It is mostly silicon dioxide (SiO2), but it typically contains additional oxides of magnesium, calcium, aluminum and boron. These additions modify the mechanical, optical and thermal properties of the glass.
Glass is strong. It seems counterintuitive that strength and fragility can coexist. They can, but there are limits. Glass is very good under compression. For this reason, automotive glass and other applications requiring a more rugged character are tempered. Tempering glass creates a compressive surface force that counteracts tensile stresses.
If too much tension is applied, catastrophic failure results. If you’ve been unfortunate enough to have a side window on your car break, you know it shatters into hundreds of small pieces. That behavior is due to the internal, built-in stress of tempered glass.
When tempered, the surface is under compression and the interior is under tension. When the outside surface is compromised (say, by a crack) the interior releases that elastic energy in a violent manner. This is epitomized in a Prince Rupert’s drop (exemplified in the video below).
This brings us back to our defective tea tumbler. No cause of failure is provided by the CPSC, but we can use the common properties of glass to understand what might lead to unexpected shattering.
The tea tumbler is double-walled. That means the glass on the inside and outside are at different temperatures. Glass is susceptible to thermal shock because of low thermal conductivity and low tensile strength. Some glasses such as quartz and borosilicate (Pyrex®) are more resistant to shock.
Stress is exacerbated by flaws. Residual stress or cracks may be generated during manufacturing. A crack creates a stress concentration that can significantly weaken a material. Fracture toughness is a material’s ability to resist fracturing when a crack is present. The larger the initial crack or applied stress, the more likely failure is.
The material failure that has lead to the recall of over 400,000 tea tumblers is most likely attributable to the manufacturing of the glass. Factors such as composition, the cooling process and pre-existing flaws in the glass are possible sources of the fracturing. No matter how you split it, this time the glass is half empty.
