To fully characterize the mechanical behavior of ceramics, it is useful to study their response to both tensile and bending tests. In this investigation, a quantitative comparison is made between tensile and bending results from (circularly) cylindrical glass rods of borosilicate glass and of fused silica. These experimental results show that in each material, the Weibull exponent m is approximately the same for both types of test. The fractured samples clearly showed that all failures initiated at surface-located flaws. The stress level at which the probability of fracture in tension equals 50 pct was determined; the maximum tensile stress for 50 pct probability of fracture in bending was also determined. The ratio of these two stresses was predicted previously from simple, theoretical considerations based upon the applicability of the Weibull function and on the distribution of defects in the specimens. This theoretical result is inconsistent with the experimental observations in view of the fractography. Therefore, it is concluded that failure in the materials tested is not satisfactorily described by Weibull statistics.
Generally, the strength of optical fibers is measured in a tensile test, while their static fatigue is measured in a bending test. A useful comparison can be made if the relationship between tensile stress and maximum bending stress can be interpreted using a “weakest link in the chain” model. This is done for a Weibull strength distribution and for a modification of that function which assumes the existence of upper and lower limits for strength.
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