What Is A Thermal Shock in Materials? Causes Of Thermal Shock
What Is A Thermal Shock?
Thermal shock is a sudden change in temperature that causes a mechanical load on an object. This load can come from a rapid change in temperature at a single point or from a thermal gradient, where different parts of the object expand at different rates.
This can cause stress on the object and, if the stress exceeds the material’s tensile strength, a crack can form.
To prevent failure from thermal shock, measures can be taken such as slowing the rate of temperature change, increasing the material’s thermal conductivity and strength, introducing compressive stress, decreasing the material’s coefficient of thermal expansion, and increasing its toughness through methods such as crack tip blunting or deflection.
Borosilicate glass is designed to handle thermal shock better than most other types of glass through a combination of its low expansion coefficient and high strength, although fused quartz is more superior in these areas.
Some glass-ceramic materials, particularly those made of lithium aluminosilicate, have a controlled amount of material with a negative expansion coefficient that allows for a near zero overall coefficient across a wide range of temperatures.
Alumina, zirconia, tungsten alloys, silicon nitride, silicon carbide, boron carbide, and some stainless steels are among the best thermomechanical materials.
Reinforced carbon-carbon is highly resistant to thermal shock due to the high thermal conductivity and low expansion coefficient of graphite, the strength of carbon fiber, and its ability to deflect cracks.
The impulse excitation technique is an effective tool for measuring thermal shock, and it can be used to non-destructively measure Young’s modulus, Shear modulus, Poisson’s ratio and damping coefficient. This technique can also be used to track changes in physical properties by measuring the same test-piece after different thermal shock cycles.
Causes Of Thermal Shock
Thermal shock occurs when there is a sudden change in temperature, either from hot to cold or vice versa, and it is most likely to happen in materials that are weak structurally and have poor heat conductivity.
The change in temperature causes the material to expand or contract unevenly, leading to thermal shock. Materials such as ceramics, glass, and rocks are more prone to thermal shock because they have poor heat conductivity.
Examples of thermal shock include ice shattering in a hot liquid, rocks cracking near a heat source when splashed with cold water, and head gaskets in engines failing due to sudden temperature changes.
Types Of Thermal Shock
Thermal shock can cause excessive thermal gradients on materials, leading to excessive stresses that can cause fatigue failure. These stresses can be tensile, pulling the material apart, or compressive, pushing the material together.
Thermal shock is caused by uneven heating or cooling of a uniform material or uniform heating of nonuniform materials.
When a material is heated and constrained so it can’t expand the increased temperature causes the molecules to press against the boundaries, creating thermal stresses.
Testing For Thermal Shock
Thermal shock testing involves rapidly switching a product between extreme temperatures to simulate temperature cycles or thermal shocks that may occur during normal use.
This is done using equipment with one or multiple chambers, where the product stays in one chamber with rapidly changing temperatures or is moved between hot and cold chambers using an elevator mechanism.
Glass containers are particularly susceptible to sudden temperature changes and may be tested by rapidly switching between cold and hot water baths.
Prevention Of Thermal Shock
Thermal shock can be prevented by avoiding sudden temperature changes, increasing the structural strength of the material, and enabling the material to conduct heat more efficiently, allowing for a more even expansion or contraction when encountering temperature changes.
This ensures that thermal stress is not placed on the material which could cause damage. As such, materials should be designed with the right balance between strength and heat conductivity in order to ensure that thermal shock is minimized.
