Thermal Shock of Car Stickers [video clip] January 23 2014

Another freezing day in Ottawa! A perfect time for more winter testing, so let's THERMAL SHOCK some car stickers!

Thermal shock occurs when the temperature of an object is changed quickly. Most materials shrink in the cold and expand in the heat. For example, it's possible for a bathroom window (assume just normal glass) to experience thermal shock. When you take a really hot shower, and it's -40°C outside, the glass might experience a temperature gradient. The inside surface will expand slightly due to heat from the hot shower. The outside surface is not expanding at the same rate because it's freezing cold outside, this causes stresses to build up in the entire glass, and the whole piece might crack. 

What might happen when thermal shocking a tailribbon:

The hot water will make the top surface of a tailribbon sticker instantly warmed up, mostly likely causing the vinyl material to expand by a small rate. The bottom side that is attached to the car is really cold, and doesn't really want to expand at any rate. This difference between expansion rates creates unwanted strains (ε). Strain is related to stress (σ) by a formula, σ = Eε, where E is a constant depending on the material used. Stress (σ) is defined as force over an area, and this force is what might cause thermal cracks!

Let's see what happens:

What was observed in our test:

In the end we haven't observed any cracks on any of the 2 tailribbons. It's possible that the strains and stresses created by the temperature gradient were not high enough to generate a force necessary to cause a crack. More likely though, is that the heat from the hot water propagated through the tailribbon fast enough to not cause any difference between expansion rates of the top and bottom surfaces.

Another example of common thermal shock: why does ice crack in a glass of water?

To make iced coffee, you need some ice cubes from the freezer. Let's say your freezer is at -20°C. When you put them into your hot coffee, the outer surface of the ice cube warms up really quickly compared to the inside. Lets say, for a split second, that it's -2°C on the outside and -20°C on the inside of the cube. This creates an extreme temperature gradient. Since the the outside is warmer, it contracts (water expands when freezing, contracts on thawing). This puts pressure all around the cube, causing it to crack!

Posted by: Eli