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Mechanical reliability of silica optical fibers
Journal of Non-Crystalline Solids 316 (2003) 125–130 www.elsevier.com/locate/jnoncrysol
Mechanical reliability of silica optical fibers N. Gougeon a, R. El Abdi a, M. Poulain b
b,*
a LARMAUR, UPRES-JE 2310, IUT de Rennes, B.P. 90422, 35074 Rennes cedex 7, France Laboratoire des Materiaux Photoniques, B^ at. 10B, Universit e de Rennes1, Campus Beaulieu, F35042 Rennes, France
Abstract Dynamic and static mechanical tests were implemented using a tensile test bench and a static fatigue test under uniform curve. The incidence of aging treatments at 65 and 85 °C was investigated on two standard silica optical fibers (with polyacrylate and fluorinated coatings). Microscopic observations helped the understanding of the failure mechanism. It appears that the cyclic variations of the failure stress phenomenon, with respect to the aging time, are the result of the silicate gel which migrates towards the polymer coating. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.81.Cn
1. Introduction The huge development of telecommunication networks has been made possible by the availability of low cost and high quality silica fibers. As industrial production reaches millions of km, research focuses on networks and advanced components. Fiber reliability is not a critical issue at this time because few problems were encountered, most of them being accidental. However fiber to the home (FTTH) and future local loops will put fibers under large and permanent stress. In addition, lighter and less expensive cables could be manufactured if transient or permanent stresses have no significant influence on fiber lifetime.
*
Corresponding author. Tel.: + 33-2 23 23 62 63; fax: + 33-2 23 23 69 72. E-mail address: [email protected] (M. Poulain).
The reliability and the expected lifetime of polymer-coated silica fibers are closely related to the chemical action of water molecules on the silica structure. However, observations have shown that the polymeric coating is also a key factor contributing to the mechanical properties of the fiber. While the main role of the coating is to inhibit crack growth from the surface Griffith flaws, it also reduces the water concentration at the glass surface by restricting diffusion processes [1]. Fiber aging has been the subject of numerous studies based on dynamic and static mechanical tests [2,3] leading to theoretical models for lifetime assessment. While ground observations do not contradict these predictions, the accuracy of the models is questionable due to the complexity of the aging mechanism. In this respect, experiments implemented on a long time scale are likely to bring new information. Aging parameters usually encompass time, temperature, applied stress and water activity. While
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 9 4 4 - 0
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N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
the critical part of water in failure mechanism is well known, its real impact varies according to the physical state – liquid or vapor – and partial vapor pressure. The stress applied to the fiber may be temporary, for example during the proof test or network installation, or permanent when the fiber is bent in cable and connecting areas. The aging mechanism is assumed to enlarge or to extend the ÔGriffith flawsÕ which are spread at the fiber surface [4]. These defects may be described as micro-cracks which grow under applied stress in a wet environment [5]. Although this mechanism is believed to be irreversible, water may also induce some curing effect which could correspond to the geometrical smoothing of the crack tip. The purpose of this study was to collect quantitative information on the effect of aging on the strength of commercial optical fibers over a long time. Such observations should allow a more accurate comparison between experimental and calculated strengths and make lifetime assessments more realistic as testing periods (>2 years) become closer to the lifetime required by network users, which is 20 years. A set of measurements has been implemented on standard telecommunication fibers commercially available. They have the classical core/cladding structure of the usual single mode fibers. The coating which protects the glass surface from external erosion consists in two layers of soft and hard polymers. These polymers are usually epoxyacrylates, which is the case of the fibers Ô1Õ of this study. A second group (fibers Ô2Õ) are coated with fluorinated polymers which are hydrophobic and therefore could limit the chemical action of water. Aging was implemented in harsh conditions: fibers were rolled under a bending radius of 20 mm and immersed in deionized water at 65 and 85 °C during a period that could reach 27 months. Samples were removed from water every three months and dried in ambient air prior to mechanical testing. Then static and dynamic fatigue experiments were carried out to compare the properties with those of the as-received fibers. It was expected that aging would result in the slow
and regular decrease of the fiber strength, corresponding to shorter failure times [6]. This implied that smaller applied stresses should have been used for the most aged fibers. As will be seen, experimental results did not fit in with this expectation.
2. Experimental 2.1. Aging test The aging behavior of two commercial silica optical fibers (with epoxyacrylate and fluorinated coating) was studied. The fibers were rolled under a bending radius of 20 mm and aged in deionized water in large tanks at temperatures 65 and 85 °C for a maximum aging time of 27 months. The strength was measured after aging by static and dynamic fatigue tests. Samples were removed from water every three months and dried in ambient air prior to mechanical testing. 2.2. Static fatigue measurements In a static fatigue experiment, a constant stress is applied to the fiber and the time to failure is measured. The bending apparatus includes precision mandrels of different diameters. Fibers are subjected to bending stresses by winding around a mandrel. The stress level can be varied by changing the mandrel size. The technique to monitor the time to fracture is optical detection of the presence of the mandrel in a special holder. When the fiber breaks, the mandrel is pushed out of the holder and the failure time is directly recorded with an accuracy of 1 s. The applied stress on the fiber depends on the mandrel diameter according to the relation given by Mallinder and Proctor [7]: r ¼ E0 eð1 þ 0:5ea00 Þ with e¼
dglass d þ dfiber
a00 ¼ 0:75a: E0 is the zero-stress YoungÕs modulus (¼72 GPa for the silica); e the failure strain; a the correction
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
127
parameter for non-linear stress/strain behavior (typical value for a is 6); dglass ¼ 125 lm; dfiber ¼ 250 lm; d the mandrel diameter (lm). Four different stresses were used to do a complete test in a given environment. The mandrel diameter varies from 2.7 to 3.2 mm (corresponding to an applied stress value between 2820 and 3340 MPa). Sixteen samples were tested for each stress. The length of the tested area was 1 m for each sample. Here the tests were performed in the ambient atmosphere (25 °C, 50% RH). 2.3. Dynamic tensile test
Fig. 1. Failure time versus aging time for aged fibers Ô1Õ at 65 °C. Results from static test under uniform bending.
The fiber strengths were also evaluated by dynamic fatigue testing at pulling rates of 50, 500, 2500 and 5000 mm/min. Thirty samples with a length of 5 m were tested at each speed. The fibers were gripped by winding three turns of fiber around a capstan. This equipment permitted testing 10 samples simultaneously. The uncertainity of the failure load is 0.1% of the measurement. The tests were performed at standard room condition (25 °C, 50% RH).
3. Results from aged fibers 3.1. Fiber 1 An optical fiber made from doped silica with an epoxyacrylate coating (fiber 1) was studied. Figs. 1 and 2 give failure time changes with respect to the aging time for two temperatures (65 and 85 °C) when we used the static fatigue test under uniform curve. The failure time increased according to the aging time. For fibers aged at 65 °C (Fig. 3), the maximum failure time was reached for an aging period of three months. Note that the diameter / ¼ 3:2 mm led to a rather large breaking load which we could not measure. For fibers aged at 85 °C (Fig. 4), the maximum failure time was reached for a period ranging between 3 and 6 months. Beyond these maxima, the mechanical fiber strength rapidly decreased towards rather low values. This phenomenon is compared to a kind of ÔcureÕ of surface defects due to the water action on
Fig. 2. Failure time versus aging time for aged fibers Ô1Õ at 85 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 3. Failure load versus aging time for aged fibers Ô1Õ at 65 °C. Results from the dynamic tensile test.
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Failure load (N)
80
60
40 50mm/min 500mm /min
20
2500 mm /min 5000 mm /min
0 0
3
6
9
12
Aging time (months)
Fig. 4. Failure load versus aging time for aged fibers Ô1Õ at 85 °C. Results from the dynamic tensile test.
the glass surface. A more detailed explanation will be given for fibers of type Ô2Õ. Figs. 3 and 4 show the failure stress evolution with respect to the aging time for the aged fibers at 65 and 85 °C for fibers submitted to a dynamic tensile test. A decrease of the failure stress is observed for both aging temperatures.
Fig. 6. Failure time versus aging time for aged fibers Ô2Õ at 85 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 5 shows the failure time evolution with a maximum aging time of 27 months for the fiber aged at 65 °C. It can be seen that this evolution is
comparable to those observed with fiber 1. The maximum failure time was reached for an aging time of 24 months. A cyclic evolution of the failure time with an increase in amplitude is observed. For fibers aged at 85 °C (Fig. 6), the maximum failure stress was reached for an aging time of three months. The failure stress evolution of type Ô2Õ fiber submitted to aging is shown in Figs. 7 and 8. A significant decrease of the mechanical strength is observed after an aging period of nine months at both aging temperatures.
Fig. 5. Failure time versus aging time for aged fibers Ô2Õ at 65 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 7. Failure load versus aging time for aged fibers Ô2Õ at 65 °C. Results from the dynamic tensile test.
3.2. Fiber 2
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
Fig. 8. Failure load versus aging time for aged fibers Ô2Õ at 85 °C. Results from the dynamic tensile test.
3.3. Permanent deformation Fibers were aged in hot water at 65 and 85 °C as coils of 40 mm in diameter, which leads to an applied stress of 220 MPa. After drying at 25 °C at room atmosphere, these coils remained bent with a larger bending radius, exemplifying a permanent deformation. While this could be induced by the coating, we observed the same phenomenon after the removal of the coating. Note that beyond one year at 85 °C the coating was severely damaged and could be removed easily by hand.
4. Discussion The cyclic change of the failure time of aged fibers was unexpected since the chemical action of water is known to decrease the fiber strength. From these measurements, the aged fibers appear stronger under permanent stress than the asreceived ones. However dynamic fatigue measurements do not lead to the same conclusion as an overall decrease of the failure stress is observed. This behavior may have practical consequences as it suggests that the lifetime of fibers subjected to permanent stress in wet environment is likely to be much larger than previously assessed. However it raises questions about the failure mechanism in
129
relation to failure. One may assume that water molecules have a curing effect on the surface flaws. The classical view is the smoothing of the crack tips insofar as Griffith flaws may be described as micro-cracks. In a more general way, the geometry of these surface defects is likely to be modified by the chemical action of water, resulting in the decrease of the stress intensity factor associated to the defect. This could account for the observed increase of the failure stress fiber 1 aged for three months at 65 °C. However, it does not explain what happens at larger aging time. The permanent deformation of the aged fibers may result from the slow structural relaxation of the silica glass under applied stress in aqueous solution. This relaxation would counteract the external stress, inducing density changes in the volume under stress. This is surprising because 85 °C is far lower than the silica glass transition temperature, and numerical simulations would lead to the conclusion that the effect of such a relaxation is negligible. In fact, water and external stress must be taken into account, as these factors may influence relaxation. The cyclic evolution of the failure time versus aging time in static fatigue tests implies that there are also cyclic changes at the fiber surface. This suggests that when fiber is aged in water for a long time, a hydrated silica layer is formed. As this layer grows, fiber is reinforced when put under static stress – that is under stresses smaller than those used in dynamic fatigue measurements. To some extent, this hydrated layer inhibits the preexisting surface flaws. As it contains water, it also accelerates structural relaxation, decreasing the effective stress. When the thickness of the hydrated layer reaches a critical value, it starts diffusing into the polymeric coating and its thickness decreases towards a very small value. Then a new hydrated layer is formed. This hypothesis is illustrated schematically in Fig. 9. Measurements of silicon concentration in the polymer support this hypothesis as silicon concentration is significant at the coating–glass interface, corresponding to a diffusion profile of less than 10 lm (Fig. 10). This profile is not observed in unaged fibers. Additional measurements, at different time scales, are required to confirm our interpretation.
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Fig. 9. Schematic drawing of the film thickness change as a function of the aging time: (1) hydration time; (2) diffusion of hydrated silicate film through the coating; (3) hydration time; (4) diffusion.
tures in deionized water. A permanent deformation of the aged fibers was observed. This result may be related to the slow structural relaxation of the silica glass under applied stress in water. As expected, dynamic fatigue measurements reveal an overall decrease of the failure stress. However static fatigue measurements exhibit a cyclic evolution of the failure time. For several aging time, fibers appear stronger under static stress than the as-received fibers. This behavior suggests that cyclic changes occur at the glass surface during aging. A possible explanation for these changes lies in the formation of a hydrated silica layer which implies chemical change and exchange at the polymer–glass interface with respect to the aging time.
References
Fig. 10. Diffusion profile of silicon through the polymer coating from the glass–polymer interface.
5. Conclusion Dynamic and static fatigue experiments were performed on silica optical fibers. These were submitted to different aging times and tempera-
[1] J.L. Mrotek, M.J. Matthewson, C.R. Kurkjian, J. Lightwave Technol. 19 (7) (2001) 988. [2] M.J. Matthewson, in: Fiber Optics Reliability and Testing, Proc. SPIE CR50, 1994, p. 32. [3] M.J. Matthewson, C.R. Kurkjian, S.T. Gulati, J. Am. Ceram. Soc. 69 (11) (1986) 815. [4] A.A. Griffith, Philos. Trans. R. Soc. London, Ser. A 221 (1920) 163. [5] S.M. Wiederhorn, Influence, J. Am. Ceram. Soc. 50 (8) (1967) 407. [6] W.J. Duncan, P.W. France, S.P. Craig, in: C.R. Kurkjian (Ed.), Strength of Inorganic Glass, Plenum, New York, 1985, p. 309. [7] F.P. Mallinder, B.A. Proctor, Phys. Chem. Glass. 5 (1964) 91.
Mechanical reliability of silica optical fibers N. Gougeon a, R. El Abdi a, M. Poulain b
b,*
a LARMAUR, UPRES-JE 2310, IUT de Rennes, B.P. 90422, 35074 Rennes cedex 7, France Laboratoire des Materiaux Photoniques, B^ at. 10B, Universit e de Rennes1, Campus Beaulieu, F35042 Rennes, France
Abstract Dynamic and static mechanical tests were implemented using a tensile test bench and a static fatigue test under uniform curve. The incidence of aging treatments at 65 and 85 °C was investigated on two standard silica optical fibers (with polyacrylate and fluorinated coatings). Microscopic observations helped the understanding of the failure mechanism. It appears that the cyclic variations of the failure stress phenomenon, with respect to the aging time, are the result of the silicate gel which migrates towards the polymer coating. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.81.Cn
1. Introduction The huge development of telecommunication networks has been made possible by the availability of low cost and high quality silica fibers. As industrial production reaches millions of km, research focuses on networks and advanced components. Fiber reliability is not a critical issue at this time because few problems were encountered, most of them being accidental. However fiber to the home (FTTH) and future local loops will put fibers under large and permanent stress. In addition, lighter and less expensive cables could be manufactured if transient or permanent stresses have no significant influence on fiber lifetime.
*
Corresponding author. Tel.: + 33-2 23 23 62 63; fax: + 33-2 23 23 69 72. E-mail address: [email protected] (M. Poulain).
The reliability and the expected lifetime of polymer-coated silica fibers are closely related to the chemical action of water molecules on the silica structure. However, observations have shown that the polymeric coating is also a key factor contributing to the mechanical properties of the fiber. While the main role of the coating is to inhibit crack growth from the surface Griffith flaws, it also reduces the water concentration at the glass surface by restricting diffusion processes [1]. Fiber aging has been the subject of numerous studies based on dynamic and static mechanical tests [2,3] leading to theoretical models for lifetime assessment. While ground observations do not contradict these predictions, the accuracy of the models is questionable due to the complexity of the aging mechanism. In this respect, experiments implemented on a long time scale are likely to bring new information. Aging parameters usually encompass time, temperature, applied stress and water activity. While
0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 9 4 4 - 0
126
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
the critical part of water in failure mechanism is well known, its real impact varies according to the physical state – liquid or vapor – and partial vapor pressure. The stress applied to the fiber may be temporary, for example during the proof test or network installation, or permanent when the fiber is bent in cable and connecting areas. The aging mechanism is assumed to enlarge or to extend the ÔGriffith flawsÕ which are spread at the fiber surface [4]. These defects may be described as micro-cracks which grow under applied stress in a wet environment [5]. Although this mechanism is believed to be irreversible, water may also induce some curing effect which could correspond to the geometrical smoothing of the crack tip. The purpose of this study was to collect quantitative information on the effect of aging on the strength of commercial optical fibers over a long time. Such observations should allow a more accurate comparison between experimental and calculated strengths and make lifetime assessments more realistic as testing periods (>2 years) become closer to the lifetime required by network users, which is 20 years. A set of measurements has been implemented on standard telecommunication fibers commercially available. They have the classical core/cladding structure of the usual single mode fibers. The coating which protects the glass surface from external erosion consists in two layers of soft and hard polymers. These polymers are usually epoxyacrylates, which is the case of the fibers Ô1Õ of this study. A second group (fibers Ô2Õ) are coated with fluorinated polymers which are hydrophobic and therefore could limit the chemical action of water. Aging was implemented in harsh conditions: fibers were rolled under a bending radius of 20 mm and immersed in deionized water at 65 and 85 °C during a period that could reach 27 months. Samples were removed from water every three months and dried in ambient air prior to mechanical testing. Then static and dynamic fatigue experiments were carried out to compare the properties with those of the as-received fibers. It was expected that aging would result in the slow
and regular decrease of the fiber strength, corresponding to shorter failure times [6]. This implied that smaller applied stresses should have been used for the most aged fibers. As will be seen, experimental results did not fit in with this expectation.
2. Experimental 2.1. Aging test The aging behavior of two commercial silica optical fibers (with epoxyacrylate and fluorinated coating) was studied. The fibers were rolled under a bending radius of 20 mm and aged in deionized water in large tanks at temperatures 65 and 85 °C for a maximum aging time of 27 months. The strength was measured after aging by static and dynamic fatigue tests. Samples were removed from water every three months and dried in ambient air prior to mechanical testing. 2.2. Static fatigue measurements In a static fatigue experiment, a constant stress is applied to the fiber and the time to failure is measured. The bending apparatus includes precision mandrels of different diameters. Fibers are subjected to bending stresses by winding around a mandrel. The stress level can be varied by changing the mandrel size. The technique to monitor the time to fracture is optical detection of the presence of the mandrel in a special holder. When the fiber breaks, the mandrel is pushed out of the holder and the failure time is directly recorded with an accuracy of 1 s. The applied stress on the fiber depends on the mandrel diameter according to the relation given by Mallinder and Proctor [7]: r ¼ E0 eð1 þ 0:5ea00 Þ with e¼
dglass d þ dfiber
a00 ¼ 0:75a: E0 is the zero-stress YoungÕs modulus (¼72 GPa for the silica); e the failure strain; a the correction
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
127
parameter for non-linear stress/strain behavior (typical value for a is 6); dglass ¼ 125 lm; dfiber ¼ 250 lm; d the mandrel diameter (lm). Four different stresses were used to do a complete test in a given environment. The mandrel diameter varies from 2.7 to 3.2 mm (corresponding to an applied stress value between 2820 and 3340 MPa). Sixteen samples were tested for each stress. The length of the tested area was 1 m for each sample. Here the tests were performed in the ambient atmosphere (25 °C, 50% RH). 2.3. Dynamic tensile test
Fig. 1. Failure time versus aging time for aged fibers Ô1Õ at 65 °C. Results from static test under uniform bending.
The fiber strengths were also evaluated by dynamic fatigue testing at pulling rates of 50, 500, 2500 and 5000 mm/min. Thirty samples with a length of 5 m were tested at each speed. The fibers were gripped by winding three turns of fiber around a capstan. This equipment permitted testing 10 samples simultaneously. The uncertainity of the failure load is 0.1% of the measurement. The tests were performed at standard room condition (25 °C, 50% RH).
3. Results from aged fibers 3.1. Fiber 1 An optical fiber made from doped silica with an epoxyacrylate coating (fiber 1) was studied. Figs. 1 and 2 give failure time changes with respect to the aging time for two temperatures (65 and 85 °C) when we used the static fatigue test under uniform curve. The failure time increased according to the aging time. For fibers aged at 65 °C (Fig. 3), the maximum failure time was reached for an aging period of three months. Note that the diameter / ¼ 3:2 mm led to a rather large breaking load which we could not measure. For fibers aged at 85 °C (Fig. 4), the maximum failure time was reached for a period ranging between 3 and 6 months. Beyond these maxima, the mechanical fiber strength rapidly decreased towards rather low values. This phenomenon is compared to a kind of ÔcureÕ of surface defects due to the water action on
Fig. 2. Failure time versus aging time for aged fibers Ô1Õ at 85 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 3. Failure load versus aging time for aged fibers Ô1Õ at 65 °C. Results from the dynamic tensile test.
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N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
Failure load (N)
80
60
40 50mm/min 500mm /min
20
2500 mm /min 5000 mm /min
0 0
3
6
9
12
Aging time (months)
Fig. 4. Failure load versus aging time for aged fibers Ô1Õ at 85 °C. Results from the dynamic tensile test.
the glass surface. A more detailed explanation will be given for fibers of type Ô2Õ. Figs. 3 and 4 show the failure stress evolution with respect to the aging time for the aged fibers at 65 and 85 °C for fibers submitted to a dynamic tensile test. A decrease of the failure stress is observed for both aging temperatures.
Fig. 6. Failure time versus aging time for aged fibers Ô2Õ at 85 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 5 shows the failure time evolution with a maximum aging time of 27 months for the fiber aged at 65 °C. It can be seen that this evolution is
comparable to those observed with fiber 1. The maximum failure time was reached for an aging time of 24 months. A cyclic evolution of the failure time with an increase in amplitude is observed. For fibers aged at 85 °C (Fig. 6), the maximum failure stress was reached for an aging time of three months. The failure stress evolution of type Ô2Õ fiber submitted to aging is shown in Figs. 7 and 8. A significant decrease of the mechanical strength is observed after an aging period of nine months at both aging temperatures.
Fig. 5. Failure time versus aging time for aged fibers Ô2Õ at 65 °C. Results from static test under uniform bending. The scale of the right-hand side is related to the diameter / ¼ 3:2 mm.
Fig. 7. Failure load versus aging time for aged fibers Ô2Õ at 65 °C. Results from the dynamic tensile test.
3.2. Fiber 2
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
Fig. 8. Failure load versus aging time for aged fibers Ô2Õ at 85 °C. Results from the dynamic tensile test.
3.3. Permanent deformation Fibers were aged in hot water at 65 and 85 °C as coils of 40 mm in diameter, which leads to an applied stress of 220 MPa. After drying at 25 °C at room atmosphere, these coils remained bent with a larger bending radius, exemplifying a permanent deformation. While this could be induced by the coating, we observed the same phenomenon after the removal of the coating. Note that beyond one year at 85 °C the coating was severely damaged and could be removed easily by hand.
4. Discussion The cyclic change of the failure time of aged fibers was unexpected since the chemical action of water is known to decrease the fiber strength. From these measurements, the aged fibers appear stronger under permanent stress than the asreceived ones. However dynamic fatigue measurements do not lead to the same conclusion as an overall decrease of the failure stress is observed. This behavior may have practical consequences as it suggests that the lifetime of fibers subjected to permanent stress in wet environment is likely to be much larger than previously assessed. However it raises questions about the failure mechanism in
129
relation to failure. One may assume that water molecules have a curing effect on the surface flaws. The classical view is the smoothing of the crack tips insofar as Griffith flaws may be described as micro-cracks. In a more general way, the geometry of these surface defects is likely to be modified by the chemical action of water, resulting in the decrease of the stress intensity factor associated to the defect. This could account for the observed increase of the failure stress fiber 1 aged for three months at 65 °C. However, it does not explain what happens at larger aging time. The permanent deformation of the aged fibers may result from the slow structural relaxation of the silica glass under applied stress in aqueous solution. This relaxation would counteract the external stress, inducing density changes in the volume under stress. This is surprising because 85 °C is far lower than the silica glass transition temperature, and numerical simulations would lead to the conclusion that the effect of such a relaxation is negligible. In fact, water and external stress must be taken into account, as these factors may influence relaxation. The cyclic evolution of the failure time versus aging time in static fatigue tests implies that there are also cyclic changes at the fiber surface. This suggests that when fiber is aged in water for a long time, a hydrated silica layer is formed. As this layer grows, fiber is reinforced when put under static stress – that is under stresses smaller than those used in dynamic fatigue measurements. To some extent, this hydrated layer inhibits the preexisting surface flaws. As it contains water, it also accelerates structural relaxation, decreasing the effective stress. When the thickness of the hydrated layer reaches a critical value, it starts diffusing into the polymeric coating and its thickness decreases towards a very small value. Then a new hydrated layer is formed. This hypothesis is illustrated schematically in Fig. 9. Measurements of silicon concentration in the polymer support this hypothesis as silicon concentration is significant at the coating–glass interface, corresponding to a diffusion profile of less than 10 lm (Fig. 10). This profile is not observed in unaged fibers. Additional measurements, at different time scales, are required to confirm our interpretation.
130
N. Gougeon et al. / Journal of Non-Crystalline Solids 316 (2003) 125–130
Fig. 9. Schematic drawing of the film thickness change as a function of the aging time: (1) hydration time; (2) diffusion of hydrated silicate film through the coating; (3) hydration time; (4) diffusion.
tures in deionized water. A permanent deformation of the aged fibers was observed. This result may be related to the slow structural relaxation of the silica glass under applied stress in water. As expected, dynamic fatigue measurements reveal an overall decrease of the failure stress. However static fatigue measurements exhibit a cyclic evolution of the failure time. For several aging time, fibers appear stronger under static stress than the as-received fibers. This behavior suggests that cyclic changes occur at the glass surface during aging. A possible explanation for these changes lies in the formation of a hydrated silica layer which implies chemical change and exchange at the polymer–glass interface with respect to the aging time.
References
Fig. 10. Diffusion profile of silicon through the polymer coating from the glass–polymer interface.
5. Conclusion Dynamic and static fatigue experiments were performed on silica optical fibers. These were submitted to different aging times and tempera-
[1] J.L. Mrotek, M.J. Matthewson, C.R. Kurkjian, J. Lightwave Technol. 19 (7) (2001) 988. [2] M.J. Matthewson, in: Fiber Optics Reliability and Testing, Proc. SPIE CR50, 1994, p. 32. [3] M.J. Matthewson, C.R. Kurkjian, S.T. Gulati, J. Am. Ceram. Soc. 69 (11) (1986) 815. [4] A.A. Griffith, Philos. Trans. R. Soc. London, Ser. A 221 (1920) 163. [5] S.M. Wiederhorn, Influence, J. Am. Ceram. Soc. 50 (8) (1967) 407. [6] W.J. Duncan, P.W. France, S.P. Craig, in: C.R. Kurkjian (Ed.), Strength of Inorganic Glass, Plenum, New York, 1985, p. 309. [7] F.P. Mallinder, B.A. Proctor, Phys. Chem. Glass. 5 (1964) 91.