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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