CERAMIC FIBERS NOTES
John M. Rusin
 The inherent strength of glass has been estimated to be as high as 4 x 106 psi.
 Fused-silica fibers have strengths at least as high a 2 x 106 psi.
 The practical strength of glass is much lower than the inherent strength, typically by a
factor of 100 to 1000.
 Because of the tendency of the glass surface to develop stress-concentrating flaws,
there is a strong dependence of glass strength on the surface area of the sample under
test.
 Since the stress concentration varies inversely with the square root of the depth of the
flaw, the limited flaw depth possible with fine fibers can also explain high measured
strengths in this geometry. Tensile strength increases with decrease in fiber diameter.
 The standard deviation observed in strength data from replicate samples is between 10
and 25% (or higher) of the mean.
 A standard surface abrasion reduces the standard deviation to less than 5%, but this
also lowers the mean to a strength value associated with 5% failure in the unabraded
samples.
 Gooding (1932) concluded that the results (increase in tensile strength of fibers with
decreasing diameter) could be explained on the assumption that the fracture depended
upon the occurrence of a suitable flaw, and that the probability of finding a suitable
flaw would increase with increase in surface area, that is with increase in fiber
diameter.
1960
34450
T = 14 +

T = tensile strength kg / sq.mm; d = dia, microns
d + 0.5 (d  0.5) 2
 A further consequence of this theory is that the strength should depend, not only on
the diameter, but also on the length of the fiber; this was shown to be the case by
Jurkov (1935) and Anderegg (1939).
Strength of Glass Fiber (Griffith - 1920’s)
Diameter in
in. x 10-3
4.20
2.78
2.25
2.00
1.85
1.75
1.40
1.32
1.15
Breaking stress
in 1,000 psi
42.3
50.8
64.1
79.6
88.5
82.6
85.2
99.5
88.7
Diameter in
in. x 10-3
0.75
0.70
0.60
0.56
0.50
0.38
0.26
0.165
0.130
Breaking stress
in 1,000 psi
134
164
185
154
195
232
332
498
491
Effect of Length on Strength in kg/mm2 (Jurkov - 1935)
Sample Length
5-6 cm
8-10 cm
Fiber Diameter
150 
28
19-20
100 
40-3
29-36
200 
22
15
Effect of Length on Strength (Anderegg - 1939)
Length (mm)
5
10
20
45
90
183
1560
Diameter (microns)
13
13.5
12.5
13
12.7
12.7
13
Tensile Strength
in kg/mm2
150
122
121
115
76
87
72