Thermal shock is the process by which a temperature gradient causes separate areas of a material to expand differently, creating stress on the material. Once the stress exceeds the strength of the material, a crack forms and will begin to propagate through the material until the material ultimately fails.
Thermal shock is the process by which a temperature gradient causes separate areas of a material to expand differently. When the material expands at different rates in different areas, a large amount of stress is created within the material. Once the stress exceeds the strength of the material, a crack forms and will begin to propagate through the material until the material ultimately fails.
Avoiding Thermal Shock
In order to avoid failure by thermal shock, many steps can be taken. For example, the temperature gradient can be reduced by either increasing the thermal conductivity of the material or by changing the temperature more slowly. Additionally, the material can be altered by reducing its coefficient of thermal expansion, increasing its strength, introducing residual compressive stress, decreasing its Young’s modulus, or by increasing its toughness through crack tip blunting or crack deflection.
Materials with Resistance
Some materials are naturally more resistant to thermal shock than others. For example, borosilicate glass has a reduced expansion coefficient and greater strength than most other forms of glass allowing it to withstand thermal shock reasonably well. Reinforced carbon-carbon is also very resistant to thermal shock, because graphite has an extremely high thermal conductivity and low expansion coefficient. In addition, carbon fiber possesses a very high strength and deflects structural cracks reasonably well. A different approach to reducing thermal shock is taken in glass-ceramic materials like lithium aluminosilicate (LAS) materials. These materials have a proportion with a negative expansion coefficient. In this way, the overall coefficient of expansion can be reduced to nearly zero over a wide temperature range.
Thermal Shock Testing
Thermal shock testing is generally performed by exposing materials to low and high temperatures in rapid sequence – generally more than 15 degrees Celsius per minute. This accelerates failures caused by temperature cycling during normal operation.
Examples of Thermal Shock
A common example of thermal shock failure is the cracking of ice cubes that have been suddenly dropped into a glass of warm water. This happens because the surface of the ice cube heats up much faster than the interior. Ice is less dense than liquid water, so as the outer surface warms up, it shrinks and melts. However, the interior of the ice cube is largely unaffected by this process. The different layers change in volume quite rapidly and cause a huge amount of stress to build within the ice cube. Once the stress grows beyond the strength of the material, the ice cube cracks. Additionally, it has been hypothesized that the famous crack in the Liberty Bell was caused by thermal shock. After casting, the bell was cooled much too quickly, and therefore the material was much weaker than it should have been. Therefore, the first time the bell was rung, a large crack appeared and propagated all the way up the bell.
In applications where large temperature gradients or cycles will be present, it is always important to take into account the coefficient of thermal expansion of the material that will be used. If it is large, then thermal shock must always be accounted for in the design of the system.
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