Question

It is stated that the higher the coefficient of thermal expansion of a glass and the lower its thermal conductivity, the higher the level of the residual stresses developed. Explain why?

Answer 1

Thermal expansion coefficient depends mainly on its component materials, composite state and environment.

Glasses have low thermal conductivity and high resistance resistivity and dilelectric strength. Their thermal coefficients are lower than those for metals and plastics and have good thermal shock resistance.

Glass can be strengthened by thermal tempering, chemical tempering and laminate strengthening.

Thermal tempering also known as physical tempering or chill tempering cools the surface of the hot glass rapidly. As a result the surfaces shrink and tensile stresses will develop on the surfaces. As the bulk of the glass begins to cool it contracts. The already solidified surfaces are forced to contract and so they develop residual compressive surface stresses while the interior develops tensile stresses. compressive surface stresses improve the strength of the glass.

The higher the coefficient of thermal expansion of the gas and the lower its thermal conductivity ,the higher will be the residual stresses developed and hence the stronger the glass becomes. Thermal tempering takes a relatively short time and can be applied to most glasses. Because of the large amount of energy stored in the residual stresses tempered glass shatters into a large number of pieces when broken.