International Journal of Applied Glass Science 3/2/ 107-121(2012) pplied glass sC丨ENCE Strength of High Performance Glass Reinforcement Fib lichelle L. Korwin-Edson, Douglas A. Hofmann, and Peter B. McGinnis Owens Corning Science d Technology, 2790 Columbus Rd, Granville, Ohio The practical strength of glass is highly dependent on the amount and type of damage that a glass article has experienced in its lifetime and can be 50% less than its theoretical strength. Glass reinforcement fibers in the pristine state exhibit some of the highest failure strengths of any glass form. Strength degradation is a sequential process the further from the point of formation a glass travels. Individual filament strength is important in the manufacturing process as the fiber interacts with water,HVAC, sizing applicators, contact shoes, and guide eyes and ultimately this combination impacts productivity. a dis. cussion of glass fiber strength- pristine versus usable, and the effects of temperature, humidity, and composition on glass Length follows in this manuscript. New data collected in Owens Cornings Glass Properties Laboratory on the effect of tem- erature and relative humidity on strength and modulus for Advantex glass, Owens Cornings S-glass (XStrand"S, Flite- Strand"S, and ShieldStrand"S)and H-glass (WindStrandH) are presented. Owens Cornings understanding of the effect of composition on strength and modulus, and particularly how individual oxides contribute to these properties are shared. Introduction al damage and can often be caused only by chemical damage or the presence of water near strained bonds. To It is well known in the field that the practical understand the effects of composition, temperature or strength of a given glass is dependent on that glass arti- relative humidity on failure strength, the glass fiber must cle's surface condition. The presence of Griffith faws on be collected in a careful and consistent manner and kept a glass surface govern the strength at which the glass will in a pristine state until testing. Glass strength is impor- fail. For bulk glass, such as container glass these Aaws tant not only in the end use application for the fiber but are typically created from mechanical damage during the also during the time between fiberizing and installation manufacturing process. For glass fibers these Aaws can and certainly during the manufacturing of said fibers as be made from a combination of mechanical and chemi- it can influence productivity. For a complete discussion on glass fiber applications the reader is referred to the Advanter',XStrand",ShieldStrand'WindSerand"and FliteStrand"are registered made- following manuscript entitled"Glass Fiber-Reinforced Composites: From Formulation to Application"found e 2012 The American Ceramic Sociery and Wiley Periodicals, Inc following this article
Strength of High Performance Glass Reinforcement Fiber Michelle L. Korwin-Edson,* Douglas A. Hofmann, and Peter B. McGinnis Owens Corning Science & Technology, 2790 Columbus Rd, Granville, Ohio The practical strength of glass is highly dependent on the amount and type of damage that a glass article has experienced in its lifetime and can be 50% less than its theoretical strength. Glass reinforcement fibers in the pristine state exhibit some of the highest failure strengths of any glass form. Strength degradation is a sequential process the further from the point of formation a glass travels. Individual filament strength is important in the manufacturing process as the fiber interacts with water, HVAC, sizing applicators, contact shoes, and guide eyes and ultimately this combination impacts productivity. A discussion of glass fiber strength — pristine versus usable, and the effects of temperature, humidity, and composition on glass strength follows in this manuscript. New data collected in Owens Corning’s Glass Properties Laboratory on the effect of temperature and relative humidity on strength and modulus for Advantex® glass, Owens Corning’s S-glass (XStrand®S, FliteStrand®S, and ShieldStrand®S) and H-glass (WindStrand®H) are presented. Owens Corning’s understanding of the effect of composition on strength and modulus, and particularly how individual oxides contribute to these properties are shared. Introduction It is well known in the field that the practical strength of a given glass is dependent on that glass article’s surface condition. The presence of Griffith flaws on a glass surface govern the strength at which the glass will fail. For bulk glass, such as container glass these flaws are typically created from mechanical damage during the manufacturing process. For glass fibers these flaws can be made from a combination of mechanical and chemical damage and can often be caused only by chemical damage or the presence of water near strained bonds. To understand the effects of composition, temperature or relative humidity on failure strength, the glass fiber must be collected in a careful and consistent manner and kept in a pristine state until testing. Glass strength is important not only in the end use application for the fiber but also during the time between fiberizing and installation and certainly during the manufacturing of said fibers as it can influence productivity. For a complete discussion on glass fiber applications the reader is referred to the following manuscript entitled “Glass Fiber-Reinforced Composites: From Formulation to Application” found following this article. Advantex®, XStrand®, ShieldStrand® WindStrand® and FliteStrand® are registered trademarks of Owens Corning. *michelle.korwin-edson@owenscorning.com © 2012 The American Ceramic Society and Wiley Periodicals, Inc International Journal of Applied Glass Science 3 [2] 107–121 (2012) DOI:10.1111/j.2041-1294.2012.00089.x
International Journal of Applied Glass Science-Korwin-Edson, Hofmann, and McGinnis Vol. 3, No. 2, 201 Review of Strength Theory as it Pertains to Glass Fiber ture. Surface faws in a reinforcement glass fiber made ng settin g can be created by contact There are already several fiber and glass strength damage with water, sizing applicators, sizing solids reviews worth noting which contain a wealth of histori- guide eyes, shoes, other fibers, cots, cutters and collets cal references. At its very core, glass strength and These Aaws may be considered physical or chemical in modulus deals with the bond strengths present in a glass nature, but can still be considered a Griffith Aaw in structure, the connectivity and the dimensionality of the either case since we are dealing with fibers on the order d work. Generally speaking, weaker bonds have greater of 10 um in diameter and a faw resulting in a signifi nces between atoms which lead to weaker glas cant stress concentration may only be a few nanometers and glasses with lower connectivity have lower stiffne deep. In 1920, Griffith stated that: Theory of Condon-Morse Eγ The Condon-Morse curve describes the attractive where or is the failure stress(strength), E is the Young's and repulsive forces at play inside the network of an modulus, y is the fracture surface energy, and a is the critical crack depth for crack growth. There is a stress rium separation distance between neighboring atoms concentration at the tip of the critical crack and the where these two forces are in harmony. The interatomic failure stress is the stress at which the strained bonds at force is F=-dUldr, where U is the thermodynamic the crack tip break and propagate causing failure and internal energy and r is the separation distance. Stiffness is s=dU/dr and the Elastic modulus is E- Sr minimizing the energy of the system. Griffith stated and is also the slope of the Condon-Morse force curve the rupture of the solid has occurred if the system can pass from the unbroken to the broken condition by a r un separation distanc process involving a continuous decrease in potential energy". It is believed that the presence of water mole Tbeory of Field Strengi cules at this crack tip reduces the stress or energy Field strength(proposed by Dietzel) is another the required to separate the glass network bonds oretical principle to describe the bond interactions fith performed two point bend experiments on glass between cations and anions(usually oxygen). The field hers, where two ends of a fiber are brought closer trength F of a cation is given by fiber, since he theorized that the critical faws must be (1) smaller due to the smaller diameter of the fiber ze is the valency of the cation and re and ra are the Orowvan's Theoretical Strength radii of the cation and the anion(oxygen).The While griffith was ng the effects of cracks force exerted by a cation on a point charge similar to an on strength, others were working on calculating the oxygen anion is Force=Re, where e is the clectronic maximum theoretical cohesive strength of solids.In arge of the cation. The equilibrium separation distance 1949, Orowan proposed the following for the theoreti useful when considering Young's modulus as it cal strength of glass depends on the relationship between an applied force and the magnitude of change in this separation distance. The field strength is useful when considering strength as high field strength cations are glass formers and low field strength cations are network modifiers where om is the Orowan stress (or Cohesive stress) and To is the interatomic equilibrium separation dis- Grifith Flaw Theory The Orowan stress is considered the stress or red to create two new surfaces. The work The practical strength of a glass fiber is determined in stressing the glass to om must at least equal by surface flaws, bulk flaws, composition, and tempera- the energy required to create the two new surfaces
Review of Strength Theory as it Pertains to Glass Fiber There are already several fiber and glass strength reviews worth noting which contain a wealth of historical references.1–5 At its very core, glass strength and modulus deals with the bond strengths present in a glass structure, the connectivity and the dimensionality of the network. Generally speaking, weaker bonds have greater distances between atoms which lead to weaker glasses and glasses with lower connectivity have lower stiffness. Theory of Condon-Morse The Condon-Morse curve describes the attractive and repulsive forces at play inside the network of an ionic material such as glass.6,7 There exists an equilibrium separation distance between neighboring atoms where these two forces are in harmony. The interatomic force is F = dU/dr, where U is the thermodynamic internal energy and r is the separation distance. Stiffness is S = dU2 /dr 2 and the Elastic modulus is E = S/ro and is also the slope of the Condon-Morse force curve at ro, the equilibrium separation distance. Theory of Field Strength Field strength (proposed by Dietzel) is another theoretical principle to describe the bond interactions between cations and anions (usually oxygen).8 The field strength F of a cation is given by: F ¼ Zc ðrc raÞ 2 ð1Þ where Zc is the valency of the cation and rc and ra are the ionic radii of the cation and the anion (oxygen). The force exerted by a cation on a point charge similar to an oxygen anion is Force = F(e) 2 , where e is the electronic charge of the cation. The equilibrium separation distance is useful when considering Young’s modulus as it depends on the relationship between an applied force and the magnitude of change in this separation distance. The field strength is useful when considering strength as high field strength cations are glass formers and low field strength cations are network modifiers. Griffith Flaw Theory The practical strength of a glass fiber is determined by surface flaws, bulk flaws, composition, and temperature. Surface flaws in a reinforcement glass fiber made in a manufacturing setting can be created by contact damage with water, sizing applicators, sizing solids, guide eyes, shoes, other fibers, cots, cutters and collets. These flaws may be considered physical or chemical in nature, but can still be considered a Griffith flaw in either case since we are dealing with fibers on the order of 10 µm in diameter and a flaw resulting in a signifi- cant stress concentration may only be a few nanometers deep. In 1920, Griffith stated that: rf ¼ ffiffiffiffiffiffiffiffi 2Ec pc r ð2Þ where rf is the failure stress (strength), E is the Young’s modulus, c is the fracture surface energy, and c* is the critical crack depth for crack growth.9 There is a stress concentration at the tip of the critical crack and the failure stress is the stress at which the strained bonds at the crack tip break and propagate causing failure and minimizing the energy of the system. Griffith stated “the rupture of the solid has occurred if the system can pass from the unbroken to the broken condition by a process involving a continuous decrease in potential energy”. It is believed that the presence of water molecules at this crack tip reduces the stress or energy required to separate the glass network bonds.10,11 Grif- fith performed two point bend experiments on glass fibers, where two ends of a fiber are brought closer together creating a curve in the center section of the fiber, since he theorized that the critical flaws must be smaller due to the smaller diameter of the fiber. Orowan’s Theoretical Strength While Griffith was considering the effects of cracks on strength, others were working on calculating the maximum theoretical cohesive strength of solids. In 1949, Orowan proposed the following for the theoretical strength of glass: rm ¼ ffiffiffiffiffiffi Ec ro r ð3Þ where rm is the Orowan stress (or Cohesive stress), and ro is the interatomic equilibrium separation distance.12 The Orowan stress is considered the stress or energy required to create two new surfaces. The work done in stressing the glass to rm must at least equal the energy required to create the two new surfaces. 108 International Journal of Applied Glass Science—Korwin-Edson, Hofmann, and McGinnis Vol. 3, No. 2, 2012
amics.org/lAGS High Performance Glass Reinforcement Fiber Using this equation the theoretical strength of a silicate a higher probability of occurring on the surface com glass has been calculated to be approximately 30 GPa. pared with bulkier samples. As opposed to samples Reinforcement fibers have very small diameters (-10- tested in bending, samples tested in tension have their d their strength limiting entire volume between the test grips exposed to equal undetectable. The prefiberization protocol(dwell tem- stress assuming pure tension. Pristine fibers that have eratures), treatment of the fibers(pristine or whether been created in the laboratory under highly controlled they have undergone stress corrosion/fatigue), the conditions have a low possibility of containing bulk or method used to measure the strength of fibers(includ- surface flaws. For the tensile strength data collected on hinah emperature, humidity, speed of loading), and pristine fibers in Owens Corning Glass Properties Lab- ly whether the data are diameter dependent dete oratory, the effective volume ranges between 0.00383 mines whether the resulting data are considered extrin- and 0.00415 mmand the effective surface area ranges ic, intrinsic, inert, fatigue-free etc. as defined by between 0.782 and 0.814 mm. It is assumed that any Kurkjian et al. In this article, we discuss strength in Baws albeit rare would occur on the surface of the context and only compare data that were collected in a fibers, but as this range represents 4% of the average, similar fashion on equally treated samples unless stated size-strength scaling considerations were not taken into otherwise. For the purposes of determining the compe sitional effe gth, all samples were tre Bulk faws are those that exist on the nterior of a equally, considered to be pristine, having undergone no fiber and act as stress concentrators which may lead to fatigue, diameters were kept as equivalent as possible at reductions in strength. Examples of these types of faws 10 um, and tested at room temperature under tension are: devitrification, seeds, unmelted batch particles, het with a gauge length of two inches. The authors con- erogeneities, cord, and other inclusions such as refrac sider this treatment and testing of the samples to be tory or precious metals. One example is shown in pristine tensile strength. It does not fall into any of Fig. 1. Bulk flaws are common in manufacturing and the categories mentioned above as they are defined in lead to lower realized strengths, as has been confirmed Kurkjian et al's 2003 paper, but it is nearly the same by researchers. 4. I It is critical therefore to keep glass temperatures well above the liquidus during manufac- turing. For the purposes of research and the data pre- Weibull statistics sented in this paper bulk Aaws are rare and are not onsidered an is As fractures seek the weakest spot, there tends to Composition plays an important role in the theo be an inherent distribution of failure strengths in any retical strength of a glass and to a great extent in the set of samples. In 1939, Weibull created a statistical treatment of strength data which has become a com- mon way to deal with brittle fracture data, such that: Pa here Mo)is the probability of fracture under a uni form tension a and g. is the reference stress. 5 The traditional formula includes a V(sample volume before the stress ratio. He assumed that the risk of rup- ture of an elemental volume was dependent upon the stress raised to a positive power m, where m is the Weibull modulus. When m is high there is less varia- tion or distribution in the data. tensile fiber data and two point bend fiber data can have representative Wei- bull distributions if the experiments are carried out Fig. 1. An example of a bulk faw collected during produc properly. For glass fibers, the surface area to volume as a broken"bead "In this case the faw is an agglor high and therefore strength limiting Aaws have melter refractory material
Using this equation the theoretical strength of a silicate glass has been calculated to be approximately 30 GPa. Reinforcement fibers have very small diameters (~10– 20 µm) and their strength limiting flaws are practically undetectable. The prefiberization protocol (dwell temperatures), treatment of the fibers (pristine or whether they have undergone stress corrosion/fatigue), the method used to measure the strength of fibers (including temperature, humidity, speed of loading), and finally whether the data are diameter dependent determines whether the resulting data are considered extrinsic, intrinsic, inert, fatigue-free etc. as defined by Kurkjian et al.4 In this article, we discuss strength in context and only compare data that were collected in a similar fashion on equally treated samples unless stated otherwise. For the purposes of determining the compositional effect on strength, all samples were treated equally, considered to be pristine, having undergone no fatigue, diameters were kept as equivalent as possible at 10 µm, and tested at room temperature under tension with a gauge length of two inches. The authors consider this treatment and testing of the samples to be “pristine tensile strength.” It does not fall into any of the categories mentioned above as they are defined in Kurkjian et al.’s 2003 paper, but it is nearly the same as intrinsic strength. Weibull Statistics As fractures seek the weakest spot, there tends to be an inherent distribution of failure strengths in any set of samples. In 1939, Weibull created a statistical treatment of strength data which has become a common way to deal with brittle fracture data, such that: PðrÞ ¼ 1 exp r ro m ð4Þ where P(r) is the probability of fracture under a uniform tension r and ro is the reference stress.13 The traditional formula includes a V (sample volume) before the stress ratio. He assumed that the risk of rupture of an elemental volume was dependent upon the stress raised to a positive power m, where m is the Weibull modulus. When m is high there is less variation or distribution in the data. Tensile fiber data and two point bend fiber data can have representative Weibull distributions if the experiments are carried out properly. For glass fibers, the surface area to volume ratio is high and therefore strength limiting flaws have a higher probability of occurring on the surface compared with bulkier samples. As opposed to samples tested in bending, samples tested in tension have their entire volume between the test grips exposed to equal stress assuming pure tension. Pristine fibers that have been created in the laboratory under highly controlled conditions have a low possibility of containing bulk or surface flaws. For the tensile strength data collected on pristine fibers in Owens Corning Glass Properties Laboratory, the effective volume ranges between 0.00383 and 0.00415 mm3 and the effective surface area ranges between 0.782 and 0.814 mm2 . It is assumed that any flaws albeit rare would occur on the surface of the fibers, but as this range represents 4% of the average, size-strength scaling considerations were not taken into account. Bulk flaws are those that exist on the interior of a fiber and act as stress concentrators which may lead to reductions in strength. Examples of these types of flaws are: devitrification, seeds, unmelted batch particles, heterogeneities, cord, and other inclusions such as refractory or precious metals. One example is shown in Fig. 1. Bulk flaws are common in manufacturing and lead to lower realized strengths, as has been confirmed by researchers.14,15 It is critical therefore to keep glass temperatures well above the liquidus during manufacturing. For the purposes of research and the data presented in this paper bulk flaws are rare and are not considered an issue. Composition plays an important role in the theoretical strength of a glass and to a great extent in the Fig. 1. An example of a bulk flaw collected during production as a broken “bead.” In this case the flaw is an agglomerate of melter refractory material. www.ceramics.org/IJAGS High Performance Glass Reinforcement Fiber 109
International Journal of Applied Glass Science-Korwin-Edson, Hofmann, and McGinnis Vol. 3, No. 2, 201 Table I. Comparison of Four Glass Types and their Pristine Tensile Strength Along with the mol% of Glass Former Pristine tensile strength SiO2+ Al2o Glass name Glass type mol (kpsi)(MPa) ASTM D578 E for general applications -75 500 ≈3450 Advantex(Owens Corning) Boron Free EC-R glass 77 ≈3800 XStrand S(Owens Corning) Alumino silicate glas 740 ≈5100 Ref. 16 820 ≈5700 Pristine tensile strength is measured as described in Experimental Method. practical strength as well. Higher intrinsically strong The graph in Fig. 3 shows a set of Owens Corning glasses have higher hardness and tend to be more dam data and the effect of fiber diameter on measured pris- age resistant to strength limiting flaws in the first place tine tensile strength for an E-glass. As Otto showed in In comparing just four glass types in Table L, a trend is 1955, there is still no appreciable effect of fiber diame seen in the pristine tensile strength with compositional ter on strength in this size range. These diameters changes, specifically the overall amount of Al2O3 and shown (6-24 um) are typical of those sold in a major- SiO2 combined. The Sio2 data came from Kurkji ity of reinforcement fber products. The strengths are and paek somewhat lower for the smaller diameter fibers. this The chart in Fig. 2 shows the effect of sample could be a result of the increased tension found in form, test conditions, composition, and surface condi- forming finer fibers if the temperature and composition tion on measured strength. Clearly it is seen that there of the glass remains the same a large discrepancy between the theoretical strength and the actual data collected. the trends observed are Efects of Temperature 1. The smaller the sample form the larger the One simple example of the effect of measured strength as shown in going from bulk emperature on strength is seen in Fig. 2 as the Pristine glass to rods to fibers. This is due to the fact that there is a higher probability for larger grif- fith flaws as the sample size increases and the effects of Weibull strength-size scaling and 2. Samples tested under conditions such as liquid 8 nitrogen show drastically improved strength(30 -50% increase). This occurs because the main reactive species water is frozen at 77 K and kept from being mobile on the glass surface 00sa7152536a5 and participating in the bond breaking reaction 3. Going from alkali silicate to a traditional E-glass (containing Boron) to Owens Corning S-glass ncreasing streng removal of alkali ions and borate and additions of silica, alumina, and magnesia. The effects of Fig. 2. Tensile strength as it relates to sample form, composition rength will be discussed in of glass, and surface conditions(S-Glass source was Owens Corn- more detail later on in this article ing's XStrand" S and LN2 means submerged in liquid nitrogen)
practical strength as well. Higher intrinsically strong glasses have higher hardness and tend to be more damage resistant to strength limiting flaws in the first place. In comparing just four glass types in Table I, a trend is seen in the pristine tensile strength with compositional changes, specifically the overall amount of Al2O3 and SiO2 combined. The SiO2 data came from Kurkjian and Paek.16 The chart in Fig. 2 shows the effect of sample form, test conditions, composition, and surface condition on measured strength. Clearly it is seen that there is a large discrepancy between the theoretical strength and the actual data collected. The trends observed are such that: 1. The smaller the sample form the larger the measured strength as shown in going from bulk glass to rods to fibers. This is due to the fact that there is a higher probability for larger Grif- fith flaws as the sample size increases and the effects of Weibull strength-size scaling and effective area or volume are predominant. 2. Samples tested under conditions such as liquid nitrogen show drastically improved strength (30 –50% increase). This occurs because the main reactive species water is frozen at 77 K and kept from being mobile on the glass surface and participating in the bond breaking reaction. 3. Going from alkali silicate to a traditional E-glass (containing Boron) to Owens Corning S-glass “handled” fibers show increasing strength with removal of alkali ions and borate and additions of silica, alumina, and magnesia. The effects of composition on strength will be discussed in more detail later on in this article. The graph in Fig. 3 shows a set of Owens Corning data and the effect of fiber diameter on measured pristine tensile strength for an E-glass. As Otto showed in 1955, there is still no appreciable effect of fiber diameter on strength in this size range.17 These diameters shown (6–24 lm) are typical of those sold in a majority of reinforcement fiber products. The strengths are somewhat lower for the smaller diameter fibers, this could be a result of the increased tension found in forming finer fibers if the temperature and composition of the glass remains the same. Effects of Temperature One simple example of the effect of measurement temperature on strength is seen in Fig. 2 as the Pristine Table I. Comparison of Four Glass Types and their Pristine Tensile Strength Along with the mol% of Glass Former Glass name Glass type SiO2 + Al2O3 Measured pristine tensile strength* mol % (kpsi) (MPa) E ASTM D578 E for general applications ~75 500 3450 Advantex® (Owens Corning) Boron Free EC-R glass ~77 550 3800 XStrand® S (Owens Corning) Alumino silicate glass ~92 740 5100 SiO2 Ref. 16 100 820 5700 *Pristine tensile strength is measured as described in Experimental Method. Fig. 2. Tensile strength as it relates to sample form, composition of glass, and surface conditions (S-Glass source was Owens Corning’s XStrand®S and LN2 means submerged in liquid nitrogen). 110 International Journal of Applied Glass Science—Korwin-Edson, Hofmann, and McGinnis Vol. 3, No. 2, 2012
amics.org/lAGS High pe nce Glass Reinforcement Fiber tion brin. rgy present to conduct the well known reac- kinetic ene een adsorbed water and stressed siloxane O≡SiO-Si≡→2Si-OH Equation (5)is the standard reaction which oc curs during stress corrosion or environmental fatigue. It is thought that water molecules are present at the tip of Avg actual Avg measured tengu图r a crack on a glass surface where the water reacts with the siloxane bond at the crack tip leading to slow growth or propagation of the crack. However, the glass has to be under some form of tensile stress for this to occur. In a pristine fiber, however, there are no classi- Fig. 3. E-glass strength as a fiunction of fiber diameter for cal Aaws on the surface and if this fiber is not put orcement fiber p I2. under an applied stress there should be no reduction in strength, but there is. Gupta et al stated that the mechanism of fatigue in the absence of cracks is not Owens Corning S-Glass Fiber tested in liquid nitrogen. clear Some work has been done in this area from liquid There may not be cracks present on a pristine fiber Helium temperatures of 4 K to temperatures of surface, but there must be irregularities at the very least -900K, primarily for silica at the lower tempera- in the structure of the glass itself to cause stress concen- tures Proctor showed increasing strength of silica trations thus targeting bonds for destruction. Afiber fibers measured under decreasing temperature with 12- that is not under an applied load is still under internal 13 GPa at 77 K and 14 GPa at 4 K 8 Cameron stress because of the nature in which the fiber is showed that the strength of E-glass went from 820 kpsi formed. The fiber is elongated at high speeds and high at -190oC to approximately 580 kpsi at 29C with temperature which freeze in a slightly oriented and many measurements in between at various humidities. 2 strained structure. This is known because of experi- Duncan measured both silica and sodium borosilicate ments done on annealing fibers which show changes in glass in two point bend and showed that the strain of properties such as density and refractive index. So silica was 18% at -196C and was 5% at 100.C, for can be said that even pristine fibers under no load can sodium borosilicate the strain was 15% at C and still undergo stress corrosion in the presence of was2.5%at100°C, with sever adsorbed water on the surface. Gy estimated the size of between showing both the effect of temperature and a surface crack for a fiber which exhibited a room temper- humidity. Otto collected data at much higher tem- ature tensile strength of 3000 MPa to be approximately eratures for E-glass and showed the strength to be 200 A and at 77 K the strength being twice as high 580 kpsi at room temperature and then 200 kspi at would suggest a surface crack of only 50 A.Cameron 620 C both at 0% RH (relative humidity). These calculated the Aaw to be 6 A at 77 K20 Gy goes on to data are shown graphically later on in the article along- say that". the fact that the fracture originates from side more the surface, has not been directly evidenced: high speed It is well established that measurements of strength images recording of the fracture process shows that the at 77 K under liquid nitrogen are free of environmental fiber is fully pulverized by the release of the stored elas- corrosion or fatigue as the reaction kinetics of water tic energy, making hopeless any attempt to obtain an and the silica structure are minute at this temperature. insight into this question from an examination of the As such, the strength values obtained at this tempera- fracture surfaces. "0 Tomozawa has even said that water ture are much higher owing to the fact that the energy will diffuse into the structure of a silica glass under a required to create two new surfaces is greater when the tensile load removing even the requirement for surface surfaces do not have readily available species with which water to cause strength reductions. Gy and guillemet to react. On the other hand, at elevated temperatures tested fibers under high temperature and were able to there is a reduction in strength due to the increased preserve the fracture surfaces. Analysis of these
Owens Corning S-Glass Fiber tested in liquid nitrogen. Some work has been done in this area from liquid Helium temperatures of 4 K to temperatures of ~900 K, primarily for silica at the lower temperatures.18–22 Proctor showed increasing strength of silica fibers measured under decreasing temperature with 12– 13 GPa at 77 K and 14 GPa at 4 K.18 Cameron showed that the strength of E-glass went from 820 kpsi at 190°C to approximately 580 kpsi at 29°C with many measurements in between at various humidities.20 Duncan measured both silica and sodium borosilicate glass in two point bend and showed that the strain of silica was 18% at 196°C and was 5% at 100°C, for sodium borosilicate the strain was 15% at 196°C and was 2.5% at 100°C, again with several points in between showing both the effect of temperature and humidity.19 Otto collected data at much higher temperatures for E-glass and showed the strength to be 580 kpsi at room temperature and then 200 kspi at 620°C both at 0% RH (relative humidity).22 These data are shown graphically later on in the article alongside more recent data. It is well established that measurements of strength at 77 K under liquid nitrogen are free of environmental corrosion or fatigue as the reaction kinetics of water and the silica structure are minute at this temperature. As such, the strength values obtained at this temperature are much higher owing to the fact that the energy required to create two new surfaces is greater when the surfaces do not have readily available species with which to react. On the other hand, at elevated temperatures there is a reduction in strength due to the increased kinetic energy present to conduct the well known reaction between adsorbed water and stressed siloxane bonds:23,24 H2O Si-O-Si ! 2Si-OH ð5Þ Equation (5) is the standard reaction which occurs during stress corrosion or environmental fatigue. It is thought that water molecules are present at the tip of a crack on a glass surface where the water reacts with the siloxane bond at the crack tip leading to slow growth or propagation of the crack. However, the glass has to be under some form of tensile stress for this to occur. In a pristine fiber, however, there are no classical flaws on the surface and if this fiber is not put under an applied stress there should be no reduction in strength, but there is. Gupta et al.25 stated that the mechanism of fatigue in the absence of cracks is not clear. There may not be cracks present on a pristine fiber surface, but there must be irregularities at the very least in the structure of the glass itself to cause stress concentrations thus targeting bonds for destruction. A fiber that is not under an applied load is still under internal stress because of the nature in which the fiber is formed. The fiber is elongated at high speeds and high temperature which freeze in a slightly oriented and strained structure. This is known because of experiments done on annealing fibers which show changes in properties such as density and refractive index. So it can be said that even pristine fibers under no load can still undergo stress corrosion in the presence of adsorbed water on the surface. Gy estimated the size of a surface crack for a fiber which exhibited a room temperature tensile strength of 3000 MPa to be approximately 200 A˚ and at 77 K the strength being twice as high would suggest a surface crack of only 50 A˚. 10 Cameron calculated the flaw to be 6 A˚ at 77 K.20 Gy goes on to say that “… the fact that the fracture originates from the surface, has not been directly evidenced: high speed images recording of the fracture process shows that the fiber is fully pulverized by the release of the stored elastic energy, making hopeless any attempt to obtain an insight into this question from an examination of the fracture surfaces.”10 Tomozawa has even said that water will diffuse into the structure of a silica glass under a tensile load removing even the requirement for surface water to cause strength reductions.26 Gy and Guillemet tested fibers under high temperature and were able to preserve the fracture surfaces.27 Analysis of these surFig. 3. E-glass strength as a function of fiber diameter for typical reinforcement fiber product sizes. www.ceramics.org/IJAGS High Performance Glass Reinforcement Fiber 111