The structure and strength of glass fibers

JOURNALOF NON-CRYSTALLINESOLIDS 1 (1968) 69--90 © North-Holland Publishing Co., Amsterdam

THE S T R U C T U R E A N D S T R E N G T H OF GLASS FIBERS

G. M. BARTENEV Department of Physics of Solids, Lenin State Teachers' Training University, Moscow, U.S.S.R. Received 12 July 1968 The strength and structure of flawless and commercial glass fibers are discussed and compared with those of bulk lasses. Particular attention is directed towards the nature of the surface layer of glass fibers which is responsible for the high strength. From a consideration of structure and defect types, it is concludedthat glass strengths can be categorized into four levels. The magnitude and nature of these four strength levels are discussed. 1. Introduction The term "constitution of glass" has a broader meaning than the term "structure of glass". The "constitution of glass" means a general atomic, molecular, and supramolecular representation of a combination and relative arrangement of different elements of structures comprising the class of glasses, and also of the nature of their rearrangement in the process of heat agitation. The term "structure of glass" reflects more detailed and distinctive features of the constitution of a given glass or even a specimen or a product manufactured by one or other process; it includes even all the possible structural defects because of an imperfect technology or during subsequent heat treatment. Thus, "structural" defects will include defects which have occurred spontaneously in the process of moulding and thermal treatment of the glass. The defects initiated by mechanical damages in technological processes or operation are not structural defects and will not be considered below. Roughly, all the defects in solids can be divided into the following three types: a) point defects - voids (vacancies), penetration of impurity atoms or molecules into the structure of a solid (impurity centers), and also substitution of the basic atoms for impurity atoms; b) group defects - bivancancies, exitons, linear defects (dislocations), phonons, etc. ; c) submicro- and microscopic defects (microcracks, inclusions, microruptures, sharp changes of the density and composition in volumes that are 69

70

G.M.BARTENEV

greater than the elements of the microheterogeneous structure of the glass, etc.). Some of these defects, e.g. dislocations, are not realized in inorganic glasses; others can hardly be called defects, e.g. "voids" or vacancies. Undoubtedly, vacancies are point defects in crystals, but vacancies in fluids are typical elements of the structure of the fluid itself, whose number, sizes and position naturally change with changing thermodynamic parameters. As a consequence of this, in glasses too, vacancies or "voids" are not defects of the structure. It must be said that up to the present there is not yet a clear and widespread definition of the term "defects in glasses". In glasses, it frequently is difficult to distinguish between defects and characteristic elements of the structure. Therefore, the approach toward defining defects of glasses proposed below must be considered debatable and requires further clarification. In glasses and glass fibres, microcracks, inclusions as well as sharp changes of the density and chemical composition in volumes much greater than the elements of the microheterogeneous structure of the glass, which are caused by fusion, crystallization or uncontrolled process factors, are typical defects responsible for their strength. The elements of the microheterogeneous structure (regions of microheterogeneities, their boundaries and junctions) are not related to defects, because they are typical repeated motifs of the structure of real glasses. The flawless microheterogeneous structure of the real glass is characterized by a natural alternation of weak and strong regions. In this case the insignificant dispersion of the strength of "flawless" glass fibers (variation factor 1-2 70) as well as flawless high-strength massive glasses (variation factor 3 - 4 ~ ) indicate that the weak regions of the structure of inorganic glasses are similar from the standpoint of their strength. The greater the size of the defects, the stronger is the effect they have on the process of fracture in glass. It is clearer to say that the greater the local overstress caused by the defect, the more dangerous it will be. Evidently, all point defects in glasses are sensitive to optical and electrical effects, but their effect on the glass strength is insignificant, because they do not cause any noticeable Stress concentrations. It is possible that some group defects have some effect on the process of fracture, but this problem needs special consideration. Of special importance among defects of this type are phonons. Under atmospheric conditions, most dangerous are defects originated on the surface of glass or glass fiber because of the effect of a surface-active medium - namely, moisture in the air. These defects can be of different types, and each type can cause a more or less different level of strength. The most outstanding property of glass fibers is their relatively high

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

71

strength as compared with many other engineering materials, and with massive glasses in particular. Until recently, the markedly displayed dependence of the strength on fiber diameter and length (Griflith, Jurkov, Anderreg, etc.) was considered as a typical property of glass fibers. Improvements of the process of manufacture of commercial glass fibers have resulted recently in a basic improvement of glass fiber properties. Thus, glass fibers now produced are not only of high strength, but also their strength hardly depends on their diameter. However, in spite of the increased strength and reduced effect of diameter modern commercial glass fibers are still characterized by a clear dependence of the strength on specimen length (curve in fig. 1) and by a wide scatter of

2

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o

o j-°

o

o 3

foo

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Jb

Fig. 1. The effect of the length on the strength of alumina borosilicate glass fibers of diameter d = 10 micron: 1 -commercial, 2 - flawless, 3 - etched in water solution of HF. test results (dispersion of the strength), the variation factor being from 15 to 30~. These factors indicate that commercial glass fibers are not free of surface defects. As far as glass fibers free of surface defects (flawless glass fibers) are concerned, they must have a strength that does not depend on length, and strength dispersion close to zero. Like strength dispersion, mean strength is a characteristic of glass fibers. Therefore, data given in figures and tables of this paper, in general, refer to the mean strength obtained by tensile testing of 25-30 specimens in air at 20 °C and times of loading of about one minute prior to rupture. 2. Flawless glass fibers and strnctural surface layer A laboratory process of manufacture of flawless glass fibers was proposed by L. K. Izmailova and the author1). Manufacture of continuous flawless

72

G,M. BARTENEV

glass fibers was ensured by at least two main factors: application of a die with a diaphragm and conditions of moulding that lead to the tightening of the bulb to a very short length. By means of a special device, samples, of glass fibers to be investigated were taken in front of the winding device, which eliminated the possibility of causing any accidental damage. Flawless alumina borosilicate glass fibers 2, 3) have a number of peculiar properties: their strength does not depend on length (curve 2, fig. 1), the strength dispersion is extremely small (1-2 ~o), and the kinetics of fracture is of the "bursting" type, where the glass fiber suddenly splits into minute particles (glassy dust). Flawless glass fibers shows only a slight dependence of strength on diameter (curve 1, fig. 2), which is due to their structural peculiarities considered below.

%

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~

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1.9" ZgO 100"

Fig. 2. The effect of the diameter on the strength of flawless glass fibers of length l = 10 mm: 1- prior to chemical etching, 2- after chemical etching, 3 - according to data by Sakka (l = 20 mm), 4- according to data by Bovkunenko (1 = 30 ram) for glass fibers of the same chemical composition after removal of the imperfect surface layer by chemical etching.

Glass fiber is a frozen-in, thermodynamically unstable system, as the sharp cooling in the process of manufacture causes a "freezing" of a high-temperature glass structure in the glass fiber. This structure is different for core and surface layer of glass fiber due to the unequal conditions of cooling its core and surface layer. In connection with this, some investigators, starting with Griffith, had proposed a hypothesis of the existence of a certain surface layer with a structure differing from that of the glass fiber core. But there had been no experimental confirmation to the hypothesis, and the latter was forgotten. It is only recently that we begin to understand why the structural surface layer could not be found previously. The thickness of a thin structural surface layer discovered at our laboratory z, 3), is one order of magnitude less than that of the cracked layer observed in commercial glass fibers. The latter is

73

THE STRUCTURE AND STRENGTH OF GLASS HBERS

absent only in flawless glass fibers which were not possible to manufacture previously. The first experiments with chemical etching have already shown that flawless glass fibers possess an outstanding property. If, after chemical etching, all glasses and glass fibers do increase their strength, this effect will be used as a method of strengthening all glasses. However, the behavior of flawless glass fibers is quite different. After chemical etching their strength decreases, and moreover it decreases greatly. As it is seen from fig. 3, the

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o

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n

Q

N

0

O

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200

0,b

--

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Fig. 3. Decreasing of the strength of alumina borosilicate glass fibers (10-micron diameter, 10 mm long) with increasing the depth of the surface layer removed by chemical etching in dilute solution of HF (0.02~ of HF). removal of a very thin layer from the surface of alumina borosilicate glass fibers to a depth of about 0.01 micron reduces the strength from 305 kgf/mm 2 down to 210 kgf/mm 2. This result was reproduced repeatedly and did not depend on the conditions of chemical etching (concentration of HF, temperature of etching bath, etc.). From these preliminary tests it is likely that there is possibly a thin strengthened layer on the surface of flawless glass fiber, which leads to a high strength uniformly distributed along its length. Detailed examination of this problem was continued4, 5) on alumina borosilicate and alumina silicate glass fibers. In investigating the light absorption by glass fibers in the ultraviolet range the maximum absorption coefficient was found to decrease sharply as thin surface layers were removed by chemical etching, but after the depth of etching achieved is 0.01-0.02 micron, it becomes stabilized and does not change as the etching proceeds further (fig. 4). The strength varies in exactly the similar way, i.e. it rapidly decreases as the thin surface layer is removed, and does not change further (fig. 4). Thin sheet glass behaves quite differently: the coefficient of light absorp-

74

G.M.BARTENEV

tion prior to and after chemical etching is the same, and its strength increases as the etching proceeds (the latter is not shown in fig. 4, because the depth of the defect layer is 30-50 microns). The structural layer taking up about one hundredth of the glass fiber volume contributes markedly to the coefficient of light absorption because a major part of the ultraviolet light passing through the rows of parallel glass fibers is repeatedly reflected from the surfaces. The reflection occurs concurrently with absorption, as the reflected light penetrates into the material to a depth of a half-wavelength (0.1 micron). Therefore, the contribution of the thin surface layer to the overall coefficient of absorption is relatively large.

E,& t0

Fig. 4. Changingof the strength and absorption coefficient(in the UV region) of alumina silicate glass fibers (10 microns in diameter) with increasing the depth of the surface layer removed by chemical etching: 1-strength, 2-absorption coefficient,3-absorption coefficient of sheet glass of the same chemical composition. Of the two possible suppositions made to explain the strength decrease: 1) due to the removal of the strengthened layer, 2) due to the "spoil" of the natural glass fiber surface by chemical etching, the first, rather than the second, is valid, because when the natural glass fiber surface is "spoiled", the coefficient of absorption would have to increase, but not to decrease. Thus, these experimental results bear out the existence of a structural layer 0.01-0.02 micron thick. The strength reduction at chemical etching occurs mainly due to the removal of this high-strength layer. 3. Structure of glass fibers

It is necessary to return to the question of the existence of structural anisotropy in glass fibers in order to explain the causes of the increased strength of the surface layer. Several investigators observed the anisotropy, but others did not confirm it. Thus, Otto and Preston 6) determining the strength of glass fibers in different directions with respect to fiber axis, found that the strength did not change. Therefore, they concluded that the high

THE STRUCTURE AND STRENGTH OF GLASSFIBERS

75

strength of glass fibers cannot be explained by orientation, in particular by the orientation of the microcracks at drawing. On the contrary, J. Slayter 7) observed a chain structure in very thin glass fibers by means of electron microscopy. Such inconsistent results are possibly explained by the fact that structural anisotropy is displayed less markedly in inorganic glass fibers than in organic polymeric fibers. Meanwhile, it can be considered that all inorganic glassformers, and other glasses too, are firmly established as linear or network polymeric materials8). Therefore, during drawing, some orientation of Si-O and other bonds is possible in the fibers.

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g5

\ J

4

g g , m/ ec

Fig. 5. M6ssbauer spectra o f 7-radiation absorption (hv = 24 keV) : 1 - b u l k alkali-silicate glass containing 6,"/0 o f SnO, 2 - g l a s s fiber o f the same chemical composition.

Several investigators, e.g.K. Oscar 9), disagree with the polymeric conception of glass structure and therefore, reject the possibility of structural orientation in glass fibers. However, structural anisotropy is shown in several recent studies. The optical anisotropy of glass fibers was investigated by Merkerl0). According to Brticknern), an X-ray technique was used to observe oriented crystallization and density asymmetry. The structural anisotropy observed is not an optical one, which is caused by a type of elastic microstresses. The results of the nuclear gamma-ray resonance investigations of bulk glasses and glass fibers made of alkali-silicate glass with 6 ~ of SnO (fig. 5) at our laboratory also lead to the conclusion that glass fibers have an anisotropic structure but bulk glasses have an isotropic structure (the curve of the

76

G.M. BARTENEV

adsorption of y-quanta by tin nuclei is asymmetrical for glass fibers and symmetrical for bulk glasses). Glassy B203 is a typical linear inorganic polymer and therefore must show very markedly the structural anisotropy in their fibers. Really the diamagnetic anisotropy discovered by Bannerjee 12) in B203 fibers and by Tarasov and Semenov 13) in two-component B203.Na20 fibers, indicates that the structure of these glass fibers is anisotropic. By investigating B203 and its fibers, Shishkin 14) found them to have a clear dependence between strength and birefrigerence and hence their structural anisotropy. The fact that in order to observe structural anisotropy in silicate glass fibers a more precise technique is required, indicates that the anisotropy of ZOO" I

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100

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Heat treatment, hours

Fig. 6. Changingof the strength (at 20°C) of alkali-silicateglass fibers as a result of heat treatment at 450°C: 1-heat-treated original glass fibers, 2-the same, heat-treated glass fibers after chemical etching. the structure of such fibers is not large. This conclusion agrees with the experimental results by Motorina 15) on annealing glass fibers followed by chemical etching (fig. 6). During annealing the structure is stabilized, and its anisotropy disappears. As it is seen from curve 2 (fig. 6), the strength of the structure in the silicate glass fiber core alters very weakly during longterm anneal. These results lead us to conclude that the high-temperature glass structure "frozen" in the glass fiber core is not much stronger than the stabilized glass structure in bulk glasses, and that structural anisotropy in the glass fiber core is not great. This conclusion also agrees with the weak dependence of the strength on diameter (degree of drawing) observed in flawless glass fibers after chemical

77

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

etching (curve 2 in fig. 2). The weak dependence of the strength of flawless glass fibers on their diameter prior to chemical etching (curve 1 in fig. 2) is explained by the slight change of anisotropy of the structure with changing degree of drawing. The strength of flawless glass fibers prior to and after chemical etching is different. The role of the surface structural layer and structural anisotropy in this effect will be discussed later. From the above one can conclude that during drawing orientation of bonds takes place, but the anisotropy of the structure here is minor, and cannot be the main reason for the high strength of glass fibers.

4. Nature of the high strength of glass fibers From the previous section it follows that the hypothesis of the "frozen" high-temperature structure, which is used by many investigators to explain the high strength of glass fibers as compared with that of bulk glasses, is not correct. The structure of the interior of both annealed and original glass fibers is practically of the same strength. From fig. 6 it follows that the decrease of the strength of glass fibers during annealing (curve 1) is associated mainly with the initiation of surface defects. The removal of them by chemical etching restores the strength practically to the same value which was observed after chemical etching of the original glass fibers (curve 2). This result was obtained for the first time by Sakka 16) during heat treatment of glass fibers. This conclusion is confirmed by the data given in fig. 7. Thus, the strength of the structure of the interior parts of annealed and original glass fibers is practically the same.

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2

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,

a,5

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~,0

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Fig, 7. Changing of the strength of alumina borosilicate glass fibers (10 microns in diameter, I0 mm in length) with increasing depth of the structural surface layer removed by chemical etching: I-flawless glass fibers (310 kgf/mm2), 2-slightly imperfect (mean strength- 250 kgf/mm~), 3- commercial (210 kgf/mm2), 4-glass fibers obtained at low temperature of manufacture, 5-commercial after longterm storage, 6-commercial, after heat-treatment.

78

G.M. BARTENEV

From the data (figs. 6 and 7) it follows that the strength of about 200 kgf/ mm 2 (measured after chemical etching under ordinary test conditions) corresponds to the structure of glass fibers in the core. It is not accidental that, starting with 1960, this level of strength has been observed by various investigators also for bulk glasses after chemical etching and special care taken in handling samples during their tests. Similar strength values for glass fibers and bulk glasses after chemical etching, from thoroughly conducted experiments, agree with the conclusion that the nature of the high strength of glass fibers cannot be explained by the hypothesis of the "frozen" high-temperature structure of the glass. The general level of strength observed for bulk glasses and glass fibers (200-250 kgf/mm 2 under atmospheric conditions) is directly associated with the strength of the structure of silicate glasses. This strength in vacuum is 450-500 kgf/mm 2. From this it follows that the high strength of the glass structure itself is responsible for the high strength of glass fibers which, owing to the special technological features of manufacture, have less serious surface defects than those in bulk glasses. The strength of flawless glass fibers prior to chemical etching exceeds the level of strength typical for the glass structure by about 100 kgf/mm 2 (fig. 1). This is associated with the existence of a surface layer of strength higher than that of the glass fiber core. Therefore the undamaged surface structural layer is another cause of the high strength of (flawless) glass fibers. From the total combination of experimental data it now follows that the nature of high strength of glass fibers is related to the high strength of the real glass structure itself, to the existence of the strengthened surface layer, and to the technology of manufacture which do not lead to the initiation of severe surface defects damaging the structural surface layer. Perfect technology of manufacture of glass fibers does not lead to the initiation of any surface defects ("flawless" glass fibers). 5. Causes of the origin of the surface layer and its increased strength

That the thin structural layer has an increased strength can be caused by several factors: a) freezing during moulding of a more homogeneous high-temperature structure in the surface layer as compared with the core of the glass fiber; b) initiation of tempering compressive stresses in the surface layer; c) excessive drawing of the surface layer during moulding and orientation of its structure along the glass fiber axis.

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

79

As the strengths of the high-temperature frozen structure and annealed structure in the glass fiber core still differ slightly (curve 2 in fig. 6), the increased strength of the structural surface layer can be explained partially by cause (a), taking into account that in the thin surface layer the structure is frozen at much higher temperatures than in the glass fiber core. Cause (b) is no longer valid, according to the calculations, as possible tempering stresses in glass fibers are infinitesimal. Evidently of importance is cause (c) as it is most probable and compatible with the process of moulding of the glass fiber from the "bulb" and with the polymeric structure of inorganic glasses. The surface layer is formed during glass fiber moulding owing to the fact that the temperature of the "bulb" surface is lower and the viscosity is higher, than in its core. The temperature gradient near the surface of the "bulb" is great, which is confirmed by the investigation of the temperature field of the "bulb". The gradient of viscosity changes in the surface layer can attain still higher values because of the exponential dependence of the viscosity on temperature. As a result, a considerable radial-thermal stress gradient is set uplT), and the glass layers are displaced at different velocities, the maximum velocities and stresses being concentrated in a thin surface layer with a viscosity considerable exceeding that of the interior of the "bulb". As a consequence of this the surface layer, which actually bears the main force of drawing, is subjected to greater deformation than the remaining mass of the glass. The nature of the strengthened layer can be explained by the polymeric structure of inorganic glassesa). The considerable stresses and relatively cooler surface layer of the "bulb" are subjected to a considerable degree of drawing (forced elastic deformation). At the same time, the still heated viscous mass of the glass in the interior of the "bulb" is drawn practically without any noticeable orientation of the structure, as the limit of fluidity of the glass above the glass-transition temperature 18) is rather low (about 0.1 kgf/cm2). However, the nature of the high strength of the undamaged structural surface layer is not yet fully understood. It can be attributed to the combined effects of the two causes (a and c). Undoubtedly, their relative effects can vary, depending on the chemical composition and method of production of the glass fibers. After the moulding force ceases, the stretched and oriented surface layer tends to contract along the fiber axis and compressing its core. Tensile stresses on the surface of the cooled glass fiber are also set up due to the difference of the glass structure on the surface and in the core. This is the result of uneven cooling when a less dense high-temperature glass structure

80

G.M.BARTENEV

with a greater coefficient of thermal linear expansion than that of the glass fiber core is fixed in the surface layer. These tensile stresses probably lead to a gradual cracking of the surface layer. Because of the existence in the surface layer of internal stresses of the two types: a) contraction (or compressive) and b) frozen high-elastic, the surface layer of the glass fiber is a mechanically unstable system. This is clearly shown under the effect of surface active media (e.g. air moisture). In glass fibers a gradual initiation of submicrocracks occurs in the weakest parts of the structure, and the strength of the glass fiber decreases during longterm storage. After a month of storage the strength of glass fibers starts to decrease gradually. If a protective silicone coating is used during moulding, the strength starts to decrease a year later. This fact indicates that the surface layer of the glass fiber is not stable and is subjected to a gradual "cracking" accelerated by the surface-active effect of moist air. Undoubtedly, the elastic constants of the surface layer and core of the glass fiber differ. If this difference is big (Young's modulus of the surface layer is 1.5 time less), at the moment of breaking the stress in the surface layer is 1.5 time less than in the glass fiber core; if this difference is slight, at the moment of breaking the stress practically is equal in the total crosssection area. By drawing a parallel with the properties of organic polymers it can be supposed that the drawing of glass in the surface practically does not lead to changes of Young's modulus19). In this instance, the difference of the strength of the flawless glass fiber prior to and after chemical etching is explained by the increased strength of the surface layer. The effect of the surface-active media, i.e. air which contains water vapor, can explain the following paradox. On the one hand, we state that the strength of "flawless" glass fibers is determined by the strength of the surface layer, while, on the other hand, it is known that the strength is determined by the weakest sections of the specimen, and fracture starts at weaker places. Such weak places would seem to be in the interior of flawless glass fibers whose strength corresponds to the strength of glass fibers after chemical etching. However, the point is that the strength of the core after chemical etching is determined under atmospheric conditions, but in the original sample the core layers are protected by the surface structural layer and must have a strength close to that which is determined under vacuum after chemical etching. As the strength in vacuum is about 2 times greater than in an air environment, the level of strength of the core layers of the original glass fibers is 2 times higher, i.e. 450-500 kgf/mm 2. Therefore, the fracture of a flawless glass fiber in air environment starts with the fracture of its structural surface layer.

81

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

6. Properties of glass fibers having surface defects The strength of glass fibers containing severe defects, which were treated in numerous investigations prior to 1960, has been the subject of numerous studies. These investigations have given rather poor information on the relationship between strength, structure and chemical composition of glass fibers. Beside the strong dependence of strength on fiber diameter, it is known that glass fiber strength is weakly dependent on its length and that the dispersion of strength is expressed by a distribution curve with one maximum. Modern commercial glass fibers are less defective; as a result of this their strength reaches 200-250, and sometimes 300 kgf/mm 2. Practically, the dependence of strength on diameter has disappeared, as was clearly determined by W. Thomas 20), but the dependence of strength on length, particularly in the range of very small lengths (curve 1 in fig. 8), have been found

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Fig. 8. The strength of alumina borosilicate glass fibers 10 microns in diameter in the range of small lengths: 1 - commercial, 2 - flawless, 3 - etched. to be more marked. It is significant that the strength of short glass fibers coincides with that of flawless glass fibers (probability of the presence of surface defects distributed along the glass fiber length with a certain mean interval between them, is less for small than for big fibers). Chemical etching of commercial short-length glass fibers, like flawless glass fibers, leads to a decrease of strength, while in the case of long lengths, it leads to its increase. The strength of commercial alumina borosilicate glass fibers after chemical etching (fig. 8) appears to be equal to a certain level of strength (210 kgf/mm 2) which does not depend on fiber length (from 1 to 25 mm). The general behavior of surface and core layers of a glass fiber is exposed by etching glass fibers to greater depths. Data given in fig. 7 lead to very

82

G.M.BARTENEV

significant conclusions. Firstly, the strength of etched fibers produced from one and the same glass mass is equal, regardless of the degree of their defectiveness. Secondly, the thickness of the layer at which the level of strength characterizing the structure of the glass is reached, depends on the degree of glass fiber defectiveness. Consequently, the strength of glass fibers at storage and in service depends mainly on processes proceeding on the surface layer. The strength of core layers is rather stable and does not depend on effects to which the fibers are exposed during moulding, storage, thermal treatment or annealing.

7. Levels of strength and their physical meaning Under atmospheric conditions, defects on surfaces of glasses and glass fibers are most severe because of moisture. Surface defects can be of different types, each type can cause a more or less different level of strength. Different levels of strength are found to be very marked on surfaces of high-strength glasses and glass fibers. Thus, for example, in our work 2, 21) alumina borosilicate glass fibers were found to have four levels of strength. Some of them were discovered later in glass fibers of other chemical compositions. Table 1 gives tentative values of strength levels for ordinary conditions of testing glass sheets and glass fibers of the aluminoborosilicate group (alumina borosilicate, alumina silicate).

TABLE 1 Different levels of strength (in kgf/mm 2) of commercial glasses and glass fibers, observed during tests under atmospheric conditions at 20°C and loading time of about l rain. Levels of

Approximate values of strength levels

strength

Bulk glasses

Glass fibers

o,

5-6

Absent

Types of defects (their sizes)

Macrocracks initiated by mechanical processing

(mm) tr0

10-15

10-15

Surface microcracks initiated by moulding bulk glasses or by heat treatment of glass fibers (microns).

trl

Possible

60-80

Surface submicrocracks initiated by moulding glass fibers (0.01 micron).

o2

200-250

200-250

Bulk glasses (after etching) and microruptures of glass fiber surface layer, initiated by moulding.

cr~

Absent

300-350

"Flawless" glass fibers

83

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

I I I I

j

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I

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Fig. 9. Schematic representation of the axial cross-section of glass fibers having a structural surface layer and different types of surface defects (the boundary between the structural layer and the core of the glass fiber is obliterated).

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02

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.

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Fig. 10. The curve of strength distribution for a series of commercial alumina borosilicate glass fiber samples (10 microns in diameter, 25 mm long) according to their subsequent numbers n from the least to the highest value of the strength, ?/

Nip(~r) d~,

where N is the number of specimens in a series, p(tT) is the function strength distribution.

84

G . M . BARTENEV

Bulk glasses and glass fibers appear to have five levels of strength. The lowest level of strength % is observed in sheet glasses after mechanical processing (cutting, grinding) during which macrocracks are initiated to penetrate into the material to depths comparable to the glass thickness. The size of biggest macrocracks is about one millimeter. This level of strength is hardly affected by this size of macrocracks. Glass fibers are not observed to have this level of strength. The level of strength tro corresponds to the strength of the surface of commercial sheet glasses which have microcracks initiated during moulding by thermoelastic stresses. The size of microcracks is about several microns. Glass fibers are observed to have this level of strength owing to heat treatment at high temperatures, which causes initiation of surface microcracks that penetrate to a depth comparable to the glass fiber radius (fig. 9). The depth of microcracks in glass fibers, determined by the method of chemical etching, reaches 0.5-0.6 micron (fig. 7). This level of strength for both glass fibers and sheet glasses, is very "slurred" because of the presence of microcracks of different sizes in the sample grouped near this level. The highest levels of strength were discovered by using the so-called "method of plateau" on the integral curves of distribution of the strength of commercial alumina borosilicate glass fibers. The plateaus on the distribution curve (fig. 10) correspond to those levels of strength near which a rather great number of samples is grouped. Accordingly, the distribution curve is observed to have three maxima (fig. 11). The level of strength a~ is explained by the existence of minute surface submicrocracks initiated during moulding of commercial glass fibers. Their depth is less than the thickness of the structural surface layer (0.01 micron) (fig. 9). This level of strength also is "slurred" because of the presence of samples with submicrocracks of different sizes. Evidently, submicrocracks also exist in bulk glasses, and they are initiated at junctions of microheterogeneities. This is associated with the fact that the glass structure has fine microheterogeneities whose linear sizes are about 0.01 micron. Of the three maxima on the distribution curve, commercial glass fiber heat-treated has the lowest one corresponding to the level of strength tr1 (curve 5, fig. 11). The level of strength cr2 is found in commercial glass fibers; their surfaces are supposed to have microruptures of the structural surface layer, which appear during the moulding of glass fibers and expose a weaker structure (fig. 9). The level of strength is realized in a pure form in glass fibers as well as in glass rods and sheet glass after removing surface microcracks by chemical etching. The level of strength 0"3 is characterized by the strength of flawless glass fibers which have undamaged structural surface layer (fig. 11, curve 1).

THE STRUCTUREAND STRENGTHOF GLASSFIBERS

85

Three highest levels of strength are traced clearly on the distribution curves of the strength of glass fibers. When passing from flawless glass fibers to very imperfect fibers (fig. 11), the maxima corresponding to the strength levels ~r3 and a 2 disappears gradually, and only one maximum ~r1 remains. The same is observed on the distribution curves of commercial glass fibers (fig. 12) when passing from short to long lengths of specimens 21). The numerical values of the strength levels given in table 1 correspond to ordinary test conditions in air, when the samples are affected by moisture the surface active medium responsible for the decrease of the strength. Tests in vacuum lead to about twofold increase of strength. Hence, it must be o,oJ-

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2

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Ioo

zoo

Jko

Fig. 11. Curves of strength distribution of alumina borosilicate glass fibers (d-- 10 microns) with different degrees of imperfection: 1 -flawless, strength 310 kgf/mm 2, 2-commercial, strength 193 kgf/mm ~, 3 - ditto, strength 180 kgf/mm 2, 4 - ditto, strength 150 kgf/mm 2, 5-heat-treated, strength 80 kgf/mm 2, 6-chemically etched sheet glass

86

G.M. BARTENEV

expected that the ultimate strength of flawless glass fibers of the alumina borosilicate group is about 600--700 kgf/mm 2, which is close to the theoretical strength of silicate glasses. The surface of bulk glass containing no microcracks after chemical etching, has almost the same level of strength as glass fibers after chemical etching. This level of strength and a very slight dispersion of the strength have been observed on bulk alkali-silicate glasses (rods and sheet glass) by different investigators, starting from 1960z~-~s). In all cases the authors observed that

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2.

o_t-z

,

tO0

, % ,

eoo

Jbo

460-

Fig. 12. Curves o f strength distribution o f commercial a l u m i n a borosilicate glass fibers (d = 10 microns) h a v i n g different lengths: 1 - 3 m m long, 2 - 1 0 m m long, 3 - 50 m m long, 4 - 1 5 0 m m long, 5 - 4 0 0 m m long.

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

87

the strength after chemical etching is close to 220 kgf/mm 2, which practically coincides with the strength of glass fiber after removing the structural surface layer. From this it can be concluded that the level of strength a2 characterizes the strength of the isotropic structure of glass. The strength of the cores of the glass fiber and bulk glass of the same chemical composition is practically the same. In vacuum, this strength level increases to about 450-500 kgf/mm z, and this value corresponds to the ultimate strength of the structure of commercial glasses. Thus, the finding and study of the physical nature of strength levels permits a more profound understanding of the origin of defects in glass structure and of the relationship between structure and mechanical properties. 8. Effect of chemical composition on the strength of glass fibers

The establishment of the relationship between the strength and chemical composition of glasses and glass fibers is an urgent problem. Surface conditions, production techniques, forming temperature and cooling rate have a very strong effect on the strength and greatly complicate the relationship between strength and chemical composition. It is widely known that the strength of polished glasses of different chemical compositions is practically the same and corresponds to the strength level a, (table 1). Thus, theexistence of a large number of severe defects causes a sharp decrease of the natural strength of inorganic glasses and also obliterates the difference in their chemical composition. On the other hand, slightly imperfect, especially flawless glasses and glass fibers should be expected to display a considerable dependence on strength on chemical composition. Table 2 contains numerical values of different levels of strength for alkalisilicate, alumina borosilicate, alumina silicate and quartz glass fibers and illustrates the effect of chemical composition on strength. All the strength levels were determined by the method of integral curves of strength distribution for a series of glass fiber samples (the so-called "method of plateaus"), and the strength level cr2 also by the method of chemical etching. Both methods practically yield equal results. Hillig zg) observed two plateaus in liquid nitrogen and probably two strength levels al and 0"2 (at 400 and 800 kgf/mm z) on the integral curves of strength distribution for a series of quartz glass fibers. Table 2 gives approximate (twofold reduction by passing from low temperatures) values of these strength levels at 20 °C and in atmospheric conditions (200 and 400 kgf/mm2). Holloway and Hastilow 23) obtained, by chemical etching, a level of strength of a quartz filament a z = 360 kgf/mm 2, close to 400 kgf/mm 2. The value of

88

G.M. BARTENEV

TABLE2 Levels of strength of glass fibers of different chemical composition in kgf/mm~ (when tested under atmospheric conditions at 20°C and tensile stress time of about 1 rain). Quartz glass Alumina boro- Alumina silicate fibers (accordsilicate glass glass fibers ing to Hillig, fibers (accord(according to Holloway, ing to Bartenev Bartenev and Aslanova, and lzmailova) Chernyakov) Proctor, Morley)

Levels of strength

Alkali-silicate glass fibers (according to Bartenev and Motorina)

tr0

10-15

10-15

10-15

10-15

trl

50-70

60-80

80-100

200

tr2

200

210

250

400

~2,

180

215

250

360

tr3

-

300

330

600

Remarks: The strength levels are determined by the "method of plateaus" on the integral curves of strength distribution for a series of samples. * The level is determined by the method of chemical etching.

strength levels tr 3 for quartz fibers are given according to data by Aslanova and Khasanov 30), Morley al), and Proctor 32). The effect of the total content of glass-formers (SIO2, A120 3 and B203) in glass fibers of different chemical compositions on their strength levels is shown in fig. 13. As the strength of the Si-O (106 kcal/mol), B-O (110 kcal/ mol) and A1-O (138 kcal/mol) bonds are practically the same, the effects due to SiO2, A120 3 and B20 3 should be similar whether they are present together or singly in the glass. F r o m the data it follows that chemical composition has different effects on different strength levels. The strength level tr 0 of glass fibers of different chemical composition is approximately equal. This is explained by the fact that it characterizes the strength of highly imperfect glass fibers. The existence of a large number of severe surface microcracks which exceed in size the thickness of the structural surface layer, can obliterate the effect of chemical composition. The more perfect the glass fibers are, the stronger the effect of chemical composition on strength. When passing over to higher strength levels the effect of chemical composition on the observed strength is stronger. In flawless glass fibers the effect of chemical composition appears to be strongest. Thus, from the data given it follows that the chemical composition of the glass from which glass fibers are made, has a different effect on low-strength

THE STRUCTURE AND STRENGTH OF GLASS FIBERS

89

600 -

400"

200

/ / /

/

~o

/

Content of gloss-formers

Fig. 13. The effect of the total content of glassformers in glass fibers of different chemical composition on their strength levels.

a n d high-strength glass fibers a n d o n different strength levels for these glass fibers. W h e n passing from low to high strength levels a n d to "flawless" glass fibers one can observe a more definite effect of the chemical composition. Therefore, the more perfect the technology o f glass m a n u f a c t u r e is, the more i m p o r t a n t becomes the chemical c o m p o s i t i o n of the original glass.

References 1) L. K. Izmailova and G. M. Bartenev, Glass and Ceramics, USSR, N3 (1964) 12. 2) G. M. Bartenev and L. K. Izmailova, Proc. Acad. Sci., USSR 146 (1962) 1136; Physics of Solids, USSR 6 (1964) 1192. 3) G. M. Bartenev, The Chemical Engineer, N 182, (1964) CE 249; Silikattechnik 18 (1967) 315. 4) G. M. Bartenev and R. G. Chernyakov, Proc. Acad. Sci., USSR 174 (1967) 800. 5) G. M. Bartenev, Glass and Ceramics, USSR, N8 (1967) 4. 6) W. Otto and F. Preston, J. Soc. Glass Techn. 34 (1950) 65T. 7) J. Slayter, Amer. Ceram. Soc. Bull. 31 (1952) 8, 276. 8) G. M. Bartenev, The Structure and Mechanical Properties o f Inorganic Glasses (Construction Publishing House, Moscow, 1966). 9) K. Oscar, Silikattechnik 16 (1965) 281. 10) L. Merker, Symposium sur la R6sistance M6chanique du Verre, Florence, 1961 (Compte Rendu, Charleroi, 1962) pp. 567-587. 11) R. Bri.ickner, Zur Structur der Glasfasern, VI1 Intern. Congress on Glass, Brussels, 1965.

12) B. K. Bannerjee, Glastechn. Ber. 33 (1960) 8. 13) V. V. Tarasov and L. V. Semenov, Glassy State; The MechanicalProperties and Structure o f Inorganic Glasses Vol. 3, issue 2 (Leningrad, 1962) pp. 52-54.

90 14) 15) 16) 17) 18) 19)

20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32)

G.M. BARTENEV N. I. Shishkin, ibid, pp. 54-55. G. M. Bartenev and Mrs. L. I. Motorina, Proc. Acad. Sci., USSR 155 (1964) 1302. S. Sakka, Bull. Inst. Chem. Res., Kyoto 34 (1957) 316. S. Bateson, J. Appl. Phys. 29 (1958) 13. G. M. Bartenev and A. S. Eremeeva, Highmolecular Compounds, USSR 2 (1960) 508 ; 3 (1961) 740. G. M. Bartenev and Yu. S. Zuyev, The Strength and Fracture of High Elastic Materials (Publishing House "Chemistry", Moscow, 1964) ; Strength and Failure of Visco-Elastic Materials (Pergamon Press, Oxford, 1968). W. F. Thomas, Phys. Chem. Glasses I (1960) 4. G. M. Bartenev and A. B. Sidorov, Silikattechnik 16 (1965) 347. B. Proctor, Nature 187 (1960) 492; Phys. Chem. Glasses 3 (1962) 7. D. Holloway and P. Hastilow, Nature 189 (1961) 385. C. Symmers, J. Ward and B. Sugarmen, Phys. Chem. Glasses 3 (1962) 76. J. Cornelissen and A. L. Zijlstra, Symposium sur la R~sistance M~ehanique du Verre, Florence, 1961 (Compte rendu, Charleroi, 1962) pp. 337-358. J. Ritter and A. Cooper, Phys. Chem. Glasses 4 (1963) 76. F. F. Vitman, G. S. Pugachev and V. P. Pukh, Physics of Solids, USSR 7 (1965) 2717; Inorganic Materials, USSR 2 (1966) 194. L. G. Baikova, F. F. Vitman and V. P. Pukh, Physics of Solids, USSR 9 (1967) 2185. W. B. Hillig, J. Appl. Phys. 32 (1961) 741. M. S. Aslanova and V. E. Khazanov, Glass and Ceramics, USSR, N1 (1967) 22. J. G. Morley, P. A. Andrews and I. Whitney (ibid N25, pp. 417-428). B. A. Proctor, I. Whitney and J. W. Johnson, Proc. Roy. Soc. A 297 (1967) 534.