Thermal Stress and Glass ```Name: Sheryl S. Status: Educator Age: 30s Location: N/A Country: N/A Date: July 2004 ``` Question: Why do glass tumblers break on pouring hot water suddenly? Replies: When hot water is poured into a tumbler there is an uneven increase in the thermal expansion of the walls of the vessel. If the expansion is uneven enough and the strength of the glass is not strong enough, the tumbler breaks. Vince Calder This is a simple problem that is not simple to explain. There are two things to keep in mind. 1) Materials like glass are brittle and cannot bend very far without breaking. 2) Materials like glass expand when they get warmer. Simply stated, when the hot water is poured into the tumbler, some parts of the glass warm up quickly (like the sides which are thin.) These parts expand. However, they are still attached to the thicker parts of the glass (like the bottom), which have not expanded. This difference in expansion sets up strong forces in the glass which can break it. If you put a tumbler into an oven and warm it up slowly, you do not get this problem. As the glass warms up, it gets large in all directions. No stresses. Thermal stresses are a problem not only for glass, but for thick pieces of metal, masonry and so on. Almost anything that is fairly stiff and brittle can have this problem. Large metal parts sometimes require days to cool down or they will crack. You may wonder, how can a little bit of heat crack a large, strong metal or glass object? It is because thermal expansion in solids is a very strong force. If you put a solid piece of glass in boiling water, it will expand about 0.02%. Maybe thousandths of an inch. That is not a lot. But if you tried to prevent it from expanding by putting a large clamp on it, you would need to squeeze it with a force of 2000 pounds per square inch to completely eliminate the expansion. The piece of glass does not expand far, but if you try to prevent it from expanding, it will push VERY hard. Clearly, if part of the tumbler is warm and part is cold, thousands of pounds of force can occur, which will break the glass. Bob Erck When glass gets hotter, it expands. But glass is also a very poor conductor of heat. If you pour hot water into a glass vessel, the interior surface will get hot instantly, but the bulk and exterior surface will temporarily remain the same temperature. The expanded glass inside pushes outwards on the outer layers of glass, forcing them to be stretched. Glass has very little tolerance for being stretched. Any existing microscopic crack on a stretched surface will grow all the way through the thickness of the glass, abruptly. "Heat resistant" glasses such as Pyrex are tolerant partly because they are treated to have better surfaces, but mostly because the glass has a chemical composition with substantially lower thermal expansion coefficient. Jim Swenson Click here to return to the Engineering Archives

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