Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 64

As temperature rises, most materials, including glass, expand and density decreases. The coefficient of linear thermal expansion (CLTE), ε, is therefore expected to increase with increase in temperature. Figure 2.10 illustrates this obtained from a slowly heated lath of window (soda‐lime–silica) glass. Since thermal history is important for glass, it is important to mention the heating rate(s) used in the experiment. The rate of increase of specimen temperature, T, was 2 °C/minute in the range 21–100 °C, 0.5 °C/minute in the range 100–400 °C, and about 0.25 °C/minute in the range of temperature above 400 °C. As expected, ε increased with increase in T, but the slope of the ε–T curve increased rapidly after about 500 °C. However, neither linearity below 500 °C nor an abrupt drop above this temperature was noticed. This contrasts with the observations of Lebedev (1926) on the temperature dependence of refractive index, presumably for a similar glass.

Figure 2.10 Temperature dependence of the CLTE for a lapped and polished lath (152.2 mm × 25.8 mm × 5.8 mm) taken from a large commercially annealed sheet of plate glass.

Source: Sinha (1971).

In addition to the temperature dependence of the refractive index of glass, one of the most important material characteristics is the dependence of elastic modulus on temperature. Figure 2.11 shows the dependence of Young's modulus, E, of window glass on temperature. Unlike the sharp drop in refractive index, as mentioned earlier in Lebedev's studies, the value of E does not exhibit any drop between 500 and 600 °C, i.e. in the transition zone. However, E decreases monotonically with increase in temperature, but only slightly from its room temperature value. The commercially (as received) annealed glass exhibited a slightly lower value at temperatures less than 500 °C as compared to the laboratory annealed material. The difference indicates a thermal‐history‐related memory effect, similar to the thermal expansion coefficient of glass mentioned earlier. With increase in temperature and long soaking time during the mechanical testing for E, the commercially annealed glass could have approached structural equilibrium at temperatures higher than 500 °C.

Figure 2.11 Temperature dependence of Young's modulus, E, for commercially annealed and laboratory annealed automotive plate glass.

Source: Sinha (1971).