SCHOTT Technical Glasses

16
Time and size dependence
For testing and applying tensile strength values, the time
dependence of the load and the size of the surface area
exposed to the stress are particulary important.
As a result of subcritical crack growth due to stress corrosion,
a long lasting tensile stress may boost critical surface flaws,
thus reducing the residual strength with time. Hence the
strength of glass and glass-ceramics is time-dependent!
Stress corrosion causes the dependence of the measured
fracture probability on the stress rate as depicted in Figure 8.
Static fatique
Fracture analyses of broken specimens for the initial flaw,
its origin and growth kinetics yield further information
about the time dependence of the strength of glasses and
glass-ceramics. Some rules are established for estimating
the strength for a permanent load from fast tests with a
constant load rate, see Figure 9 for example. As a simple
rule, the permanent strength (for years of loading) will only
amount to about 1/2 to 1/3 of the strength measured in
fast experiments.
Size dependence
Increasing the stressed area increases the probability of low
strength flaws within this area. This relationship is important
for converting of experimental strengths, which are mostly
determined using small test specimens, to large glass articles
such as pipelines, where many square meters of glass may
be loaded (Figure 10).
Strengthening
The defects that reduce the strength of the surfaces become
ineffective if the glass surface ist subjected to compressive
stress. The resulting strength of a glass article may be so
high that it virtually sets no limits to technical applications.
In glass articles with simple geometries, for example flat
glass, the entire surface of the article can be put under compressive
stress.
The toughening can be caused by rapid cooling (quenching)
of the softened glass (Figure 11) or, in suitable glasses,
by an ion exchange in an approximately 50 – 200 μm thick
surface layer. In subsequent external loading (tensile or
bending), the externally induced stress adds to the internal
stress. Up to the value of the compressive surface stress,
99
95
80
60
50
40
large
small
grain
size
without deliberate
damage
nominal strength
values for
99
normal
glass 95
constructions
80
chem.-
techn.
largescale
units
60
50
40
.
σ = 0,01
0,1 1 10 100 1000 MPa/s
20
20
10
10
6
4
2
1
F in % ––>
6
4
2
1
F in % ––>
3
5
A
7
10
B
20
30
50
70
100
200 300
σ E in MPa ––>
3
5
7
10
20
30
50
70
100
200 300
σ E in MPa ––>
A: nominal strength values for chem. techn. large-scale units
B: … and for normal glass constructions
Fig. 8. Failure probability F of a predamaged surface for various rates
of stress increase σ˙ . (predamaged area: 100 mm 2 , grain size: 600)
Fig. 7. Failure probability F for samples abraded by various size grains;
predamaged surface area: 100 mm 2 , rate of stress increase σ˙ = 10 MPa/s