Strength and Fracture Resistance of Quartz Fibers with Polyimide Coatings

Polyimide coatings currently provide the highest performance properties of quartz fibers. The purpose of this research is to determine the strength, hardness, dynamic fatigue, performance period, and crack resistance of optical fibers with polyimide coatings. The strength limit of fibers determined by axial stretching over the distance between capstans of 500 mm is 4.8–6.0 GPa at a loading rate of 10–500 mm/min. Weibull distribution curves are plotted in coordinates that relate the probability of failure to the strength, fiber length, and parameter describing the ultimate strength. The dynamic fatigue parameter n is found, which in a physical sense corresponds to the slope tangent tan α = 1/(1 + n) in double logarithmic coordinates. The hardness and crack resistance values of quartz fibers are measured by indentation. Crack resistance K1c is calculated using the Niihara semi-empirical dependence, which connects the indentation size, radial crack length, and crack resistance. The initial crack length is calculated and the size of the characteristic defect is determined using scanning electron microscopy. Thermogravimetric analysis demonstrates that polyimide-coated fibers maintain thermal stability up to 450°C. The service life of optical fibers is determined based on the dynamic fatigue data, and it amounts to at least 25 years at a load of 0.2 GPa. The greater the difference between the lower and the upper strength levels in the tensile tests of fiber segments, the higher the distribution parameter m describing the ultimate strength of optical fibers. The values of this parameter characterize fiber quality: m = 50–100 for coated fibers and m = 1–5 for uncoated fibers.