SCHOTT Technical Glasses

17
a superimposed tensile stress keeps the surface under total
compressive load. Thus, surface condition, loading rate, and
loading time do not influence the strength.
3.3 Elasticity
The ideal brittleness of glasses and glass-ceramics is matched
by an equally ideal elastic behavior up to the breaking point.
The elastic moduli for most technical glasses is within the
range of 50 – 90 GPa. The mean value of 70 GPa is about
equal to Young’s modulus of aluminum. Poisson’s ratio of
most glasses is in the range 0.21 to 0.25 and is lower than
for metals or plastics.
3.4 Coefficient of linear thermal expansion
perature (section A), the curve shows a distinct bend and
then gradually increases up to the beginning of the experimentally
detectable plastic behavior (section B = quasi-linear
region). A distinct bend in the extension curve characterizes
the transition from the predominantly elastic to the more
visco elastic behavior of the glass (section C = transformation
range). As a result of increasing structural mobility, the
temperature dependencies of almost all glass properties
are distinctly changed. This transformation range is characterized
by the transformation temperature T g according to
ISO 7884-8. Figure 13 shows the linear thermal expansion
curves of five glasses; 8330 and 4210 roughly define the normal
range of technical glasses, with expansion coefficients
α (20 °C; 300 °C) = 3.3 – 12 x 10 –6 /K. The linear thermal expansion
is an essential variable for the sealability of glasses with
other materials and for thermally induced stress formation,
and is therefore of prime importance for glass applications.
3
With few exceptions, the length and the volume of glasses
increase with increasing temperature (positive coefficient).
The typical curve begins with a zero gradient at absolute
zero (Figure 12) and increases slowly. At about room temσ
D
in MPa ––>
100
80
60
40
20
Lifetime:
10 2 s (1.7 min)
10 5 s (1.2 days)
10 8 s (3.17 years)
10 11 s (3170 years)
0
0 20 40 60 80 100 120 140
σ E
in MPa ––>
3
nominal strength
values for
99
normal
glass 95
constructions
80
chem.-
techn.
largescale
units
5
7
60
50
40
20
10
6
4
2
1
F in % ––>
10
20
S = 10000 1000 100 10 mm 2
30
50
70
100
200 300
σ E in MPa ––>
Fig. 9. Time-related strength σ D (strength under constant loading)
compared to experimental strength σ˙ E at 10 MPa/s stress increase with
lifetime t L , in normally humid atmosphere (soda-lime glass).
Fig. 10. Failure probability F for differently sized stressed areas S;
all samples abraded with 600 mesh grit, stress rate σ˙ = 10 MPa/s