292 Strength of Glass

In most structural applications of glass it is necessary for the components to sustain mechanical stress. When a material is stressed, it deforms, and strains are created. At a low level of stress, most materials obey Hook's law, that is, strain is proportional to stress. While the stress level is high, most materials deform plastically. Glass is a brittle material, which cannot accommodate this plastic deformation but breaks without warning. The stress-strain curve in Figure 29.4 shows a perfect linearity from zero strain to failure. The mechanical properties of glass as an engineering material are tabulated in Table 29.1.

Generally speaking, it can be stated that the theoretical strength of a piece of glass is equal to about one tenth of its modulus of elasticity [5]. Glass in compression is extremely strong. The compressive strength can approach 10,000 MPa without breakage. However, glass in tension usually fails at stress levels less than 100 MPa. It has been pointed out that the failure of glass [6] results from a tensile component of stress. Nowadays, it is generally accepted that the failure of glass originates at surface flaws [7] at which stresses are concentrated, as shown in Figure 29.5. Since basically no plastic flow is possible in glass, these flaws lead to high stress concentrations when glass surface is in tension. Because of the random nature of the flaws, a large variability in the strength of individual pieces of glass has been observed and reported [6]. Therefore, the failure strength of glass can only be expressed by means of a statistical analysis. Based on these statistical results, we can only obtain a design value at which the risk of fracture of glass is sufficiently low, but it provides no guarantee that the glass will survive under the design load level.

FIGURE 29.4 Stress-Strain diagram.

TABLE 29.1 Mechanical Properties of Glass

E —

Young's modulus of elasticity

10.4 x 106 psi or 7.2 x 1010 N/m2

G —

Modulus of rigidity

4.3 x 106 psi or 3.0 x 1010 N/m2

m —

Poisson's ratio

0 0

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