Predicted Static Fatigue Behavior of Specially Coated Optical Glass Fibers

Long-term static fatigue behavior of optical glass fibers is controlled by their strength, fatigue resistance, and zerostress aging behavior. The effectiveness of four special coatings in preventing the long-term static fatigue deterioration of optical glass fibers was evaluated by determining the dynamic fatigue behavior and the effects of zero-stress aging on strength of the four specially coated optical glass fibers in water from 25° to 85°C. The results clearly show that the strength, fatigue resistance, and aging behavior varied significantly between these specially coated fibers. By analysis of these experimental results in terms of fracture mechanics principles, the predicted static fatigue behaviors of the four fibers were compared. Ideally the optimum fiber is one that exhibits a high strength, low strength variability, high fatigue resistance, and high aging resistance. Each of these specially coated fibers had a deficiency in at least one of these properties.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Haze-Active Protein and Polyphenols in Apple Juice Assessed by Turbidimetry

The amount of haze-active protein in apple juice was determined by adding tannic acid to induce haze followed by turbidimetry. Turbidity was essentially linear with protein concentration. PVPP treatment prior to tannic acid addition appeared to remove endogenous polyphenols and resulted in slightly weaker response. Adding gelatin to apple juices or clarified ciders induced hazes in response to content of haze-active polyphenols. At an appropriate gelatin concentration turbidity was nearly linear with polyphenol concentration. Treatment with bentonite prior to gelatin addition appeared to remove endogenous protein. Temperature control during the induction period was critical to reproducibility.     

Nucieation and Growth of Cracks in Vitreous-Bonded Aluminum Oxide at Elevated Temperatures

The nucleation and growth of cracks was studied at elevated temperatures on a grade of vitreous-bonded aluminum oxide that contained ∼8 vol% glass at the grain boundaries. Cracks were observed to nucleate within the vitreous phase, close to the tensile surface of the flexural test specimens used in these experiments. Crack nucleation occurred at a strain of ∼0.08% to 0.12% which corresponded to a crack nucleation time of ∼35% of the time to failure by creep rupture. Once nucleated, cracks propagated along grain boundaries, as long as the stress for crack propagation was maintained. The crack velocity for cracks that were nucleated by the creep process was found to be linearly proportional to the apparent stress intensity factor, whereas for cracks that were nucleated by indentation, the crack velocity was proportional to the fourth power of the apparent stress intensity factor.     

Effect of Contact Residual Stress on Proof Testing

Theoretical predictions are made of strength distributions after proof testing accounting for the possibility of a contact residual stress. Assuming a simple power-law relationship between subcritical crack velocity and the stress intensity factor, the amount of crack growth, hence strength degradation, that occurs during a proof stress cycle is determined. The results clearly show that the contact residual stress can strongly influence the after-proof strength distribution so that after-proof strengths can become weaker than the initial. The results are consistent with experimental observations with soda–lime glass; thus, the importance of accounting for the contact residual stress is emphasized.