MIT OpenCourseWarehttp://ocw.mit.edu3.22 Mechanical Properties of MaterialsSpring 2008For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.3.22 Mechanical Behavior of MaterialsWater-ageing of silica optical fibersNoémie Chocat, Sophie Poizeau, Matthew Smith, Michiel VanhoutteMIT Department of Materials Science and EngineeringCambridge, MA 02139 USAMay 13, 2008Image removed due to copyright restrictions. Please see http://www.rtcfiber.com/layout/multiflex3/images/fiber-optics.jpg• Macroscopic mechanism : stress-corrosion cracking• Subcritical crack growth in silica in the presence of water or water vapor• Stress-enhanced corrosion reaction➔ Decrease in fiber resistancefrom 4.1 GPa to 2.9 GPaover 4 weeks at 85°C and 85% humidity1• Applications :• Telecommunications• Medical imaging3.22 Mechanical Behavior of MaterialsBig Picture[1] Mauro do Nascimento, E., and Lepienski, C.M. “Mechanical Properties of optical glass fibers damaged by nanoindentation and water ageing.” Journal of Non-Crystalline Solids 352 (2006): 3556-3560.3.22 Mechanical Behavior of MaterialsMicroscopic mechanism[2] Michalske, T. A., and B. C. Bunker. “Slow fracture model based on silicate fracture models.” Journal of Applied Physics 56 (15 November 1984): 2686-2693.Slow fracture model by Michalske and Bunker• Microscopic advancement of this crack cannot occur by simultaneous bond cleavage. • Kinked front allows for localized bond rupture process. Kink can spread laterally to advance the crack front. • Atomic site becomes reactive in step 4, where the Si are forced to give up their tetrahedral molecular orbital configuration.To optimize mechanical properties we need to minimize flaw size and control exposure to water. Source : [2]Source : [2]Images removed due to copyright restrictions. Please see Fig. 6a and 7 in [2].3.22 Mechanical Behavior of Materials• Prediction :orders of magnitude ≠between H20 and N2σ = 100MPa, c = 150 nmÆ v = 5 nm/s• Optimization :– Careful handling– Hermetic coating• Diffusion of H+and OH-ions into fiber Æ cracks & attenuation• Cu tubes, water-repellant jelly, water-absorbing powder• Coating depends on environment : indoor, under water…Prediction & Optimization [3] Kurkjian, C.R., Krause, J.T., and Matthewson, M.J. “Strength and Fatigue of Silica Optical Fibers.” Journal of Lightwave Technology 7 (1989): 1360-1370.Source : [2]Images removed due to copyright restrictions. Please see Fig. 2 in