A novel four-point bend test for strength measurement of optical fibers and thin beams. I. Bending analysis

A novel four-point bend apparatus is described for strength measurement of thin compliant beams that avoids the loading and gripping problems associated with other techniques. The apparatus has proved particularly useful for strength measurement of relatively weak optical fibers. In this four-point bend system, loading pin displacement rather than applied load is the measured quantity from which failure stress is calculated, avoiding the load based instability at high deflection. A single-ended support design for the loading pins permits the specimens to be conveniently immersed in the test environment and enables several specimens to be tested simultaneously. Nonlinearities in the deflection/stress relationship are analyzed and a correction factor to the linear bending theory is presented. Friction between the specimen and support pins is found to increase local stresses at the pins. In the second part of this work, a statistical analysis is presented that determines the effective tested length in bending and the tension to bending strength ratio. The predictions of the analysis are confirmed by strength measurements on a weak silica fiber