EFFECT OF LOADING MODE AND COATING ON DYNAMIC FATIGUE OF OPTICAL-FIBER IN 2-POINT BENDING

Two-point bending is a useful method for investigating the mechanical properties of optical fiber and has several advantages when compared t o the traditional tensile test. However, the strength of the fiber is usually determined at constant faceplate velocity rather than at const ant strain rate as in the uniaxial tensile test, and previous work cas ts doubt on the comparability of fatigue results obtained using differ ent loading modes and hence on the reliability of the bending techniqu e. This paper presents dynamic fatigue results using a two-point bend apparatus that can be programmed to operate in constant velocity, cons tant strain rate, and constant stress rate loading modes. These result s, obtained for both bare and polymer-coated fused silica optical fibe r, show no significant differences in the calculated fatigue parameter s for the three loading modes and clearly indicate the reliability of the two-point bend method at constant faceplate velocity. The results, however, show that the obtained value of the stress corrosion suscept ibility parameter, n, is dependent on the quantity used to define it, i.e., stress or strain to failure, because of the nonlinear elastic be havior of silica.