Strength, fatigue resistance, and zero-stress aging behavior control the long-term mechanical reliability of optical fibers. Zero-stress aging refers to the loss of strength of high-strength glass fibers after exposure to some corrosive environments in the absence of stress. Understanding the effect of the chemical environment under zero stress on the subsequent fracture strength of optical fibers is important because optical fibers in service will probably encounter water and other chemical species while exposed to zero-or low-stress conditions. In this work, the strength of fibers aged under zero-stress conditions at 80°C in deionized water has been measured. Scanning tunneling microscopy was also used to measure the roughening of the fibers from corrosion at intervals during the aging. The product of the median inert strength of fibers aged for various times and the square root of the roughness depth of fibers was constant within experimental error. The results show that surface roughening contributes to zero-stress aging in silica fibers.

No abstract

No abstract

No abstract

An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows crack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield strength, silicon-modified, AISI 4340 steel tested under zero-to-tension cyclic loading. Good agreement is shown between acoustically determined crack depth and that measured by post-fracture optical microscopy. The monitoring of changing crack depth-to-surface length ratios during tests is also demonstrated. Acoustically determined crack opening stresses were found to be about 10% higher than values determined by measurements of crack tip opening displacements by scanning electron microscopy. Effects on crack growth of two different specimen surface preparations, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress. NOMENCLATURE u/c = ratio of crack depth to half surface crack length C = crack front E = modulus of elasticity AK, A& = range of K , and of effective K , CTOD = crack tip opening displacement K , = mode I stress intensity factor N = number of cycles P = power input to acoustic transducer rb = reversed crack tip plastic zone size R = stress ratio = urn in/^,,, wb = beam width at transmitting transducer = crack reflection coefficient K = wave number (2n/I) J. = acoustic wavelength v = Poisson's ratio p = distance between origin of surface crack and tangent line to crack front u* = acoustic stress uOp = opening stress usat = saturation stress urnin, u,,, = minimum and maximum applied stress uCTOD = opening stress based on CTOD measurements Aueff = range of effective stress u; = cyclic yield strength @ = elliptic integral of second kind o : angular frequency of surface acoustic wave