Method and apparatus for determining the fracture strength of the margins of thin sheets of brittle-fracture material

A method and an apparatus for examining the fracture strength of flat samples made of brittle-fracture material are provided. The margin of the respective sample is subjected to tensile stress by bending the material in a circular arc shape.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(a) of German Patent Application No. DE 10 2014 110 855.8 filed Jul. 31, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention relates to the determination of the fracture strength of thin, flat samples of brittle-fracture material, in particular thin glass sheets, under a tensile stress σ.

2. Description of Related Art

It has been found that two mutually independent kinds of fractures can occur when such samples are placed under tensile loads: fractures that have their origin inside the surface area of the sample and those that grow starting from the margin of the sample. The present invention relates to the latter kind of fractures, that is, to those that have their origin at the margin of the sample.

Both for characterization and optimization of the properties of the edge and for ensuring a guaranteed fracture strength, it is advantageous to examine whether the margin of a sample withstands a certain tensile stress σ.

Moreover, it is advantageous to determine the stress σbof such a sample, in which the sample fractures starting from the margin (tensile stress at break).

A two-point bending method is known for the determination of the tensile stress at break of thin glass samples from, for example, S. T. Gulati: “Two Point Bending of Thin Glass Substrate,” SID Symposium, Techn. Papers Vol. 42, pp. 652-654 (2011). In this case, a thin glass sample is clamped between two support plates and bent by bringing these plates together (compareFIG. 1).

This method has a number of drawbacks. In this kind of bending, an inhomogeneous state of stress is created along the sample, with the highest stress being imposed along the margin in the middle of the sample and the stress declining with increasing distance from the middle. Therefore, this method is not adequate for characterizing extended sections of sample margins under a given stress. The generalization of the measured values of local tensile stress at break to larger sections with this method has been demonstrated to be reliable only to a limited extent. Moreover, samples with inhomogeneous thickness cause problems in this method, which can be solved only with difficulty. Furthermore, the necessity of clamping places requirements on the geometry of the samples and, in many cases, makes necessary a tedious preparation of the samples. In addition, the cost in terms of instruments and personnel required to carry out this method cannot be underestimated.

SUMMARY

The invention is therefore based on the object of reducing the drawbacks of the prior art. In particular, a possibility for simple and reliable examination of the fracture strength of the margins of samples under a mechanical tensile stress σ shall be provided.

The invention is based on the realization that, when a sample is bent, the sample material is subjected to a tensile stress σ on the outer side of the bend.

A sample that is to be characterized by the method according to the invention comprises a first lateral face and a second lateral face lying opposite to it as well as at least one margin. This margin, which is to be examined, forms a transition of the first face to the second face. It can be designed to be angular or rounded, for example, or it can comprise a border. The sample has a thickness t at this margin.

It is intended, in particular, to examine samples made of glass, preferably with a thickness of at most one millimeter, most preferably of at most 300 micrometers.

According to the invention, the margin of the sample to be examined is to be examined in terms of its fracture strength initially in the region of the first lateral face.

Accordingly, the invention comprises a method for examining the fracture strength of flat samples made of brittle-fracture material, in particular glass sheets, in which said samples have a first lateral face and a second lateral face as well as at least one margin and the first lateral face lies opposite to the second lateral face, in regard to fractures originating under a mechanical tensile stress σ from this margin of the sample to be examined, wherein the first lateral face is subjected to a tensile stress σ along the margin to be examined at the margin to be examined in a section of the sample by bending the sample in this section to be examined, so that said sample is imparted a bend along the margin to be examined, by pressing the sample in the section to be examined against the template surface of a dimensionally stable template of defined curvature, so that the curvature of the template surface is imposed on the section to be examined, wherein a template surface having a first bending radius R is used and the fracture strength of the sample is examined under the mechanical tensile stress σ corresponding to this bending radius R, and this test is repeated with successively reduced bending radius R and the thereby ensuing, increased tensile stress σ until the sample breaks, and an analysis is performed to determine the tensile stress σ or bending radius at which the sample has fractured.

Preferably, it is also determined whether the sample has fractured starting at the margin to be examined.

This method according to the invention has a number of advantages. The position of the margin region that is bent with the bending radius R along the margin is—in contrast to the known method discussed above—not predetermined by the geometry or preparation of the sample, but rather said position can be chosen at will by placing the template against the section to be examined. The method places only very small requirements even on the preparation of the sample. Essentially, it need only be possible to be able to bend the margin in the section to be examined by means of the template. In contrast to the known method, the geometry of the other margins, which are not to be examined, does not have any significant influence on the measurement. Moreover, the measurement is simpler in terms of instruments and personnel and is less sensitive to error. Furthermore, samples of inhomogeneous thickness t can also be examined. Changes in thickness remote from the margin to be examined do not generally have any influence on the method. If the thickness t changes along the margin, the local value of the tensile stress σ to which the respective section of the sample margin has been subjected changes; this can be reconstructed, if need be, by analyzing the respective local thickness of the margin.

Preferably, the samples are planar structures in a state free of external forces, which do not undergo any plastic deformation during the bending according to the invention, so that, after the measurement, they revert to their original geometry (unless they are broken).

The samples are preferably bent in a cylindrical manner, so that the section of the sample that has undergone bending in a circular arc shape assumes the form of at least a section of a hollow cylinder, wherein, in this cylindrically bent section of the sample, the first lateral face of the sample represents the outer lateral surface of this hollow cylinder and the second lateral face of the sample represents the inner lateral surface of this hollow cylinder and the margin of the sample to be examined runs along the base area of this hollow cylinder. Accordingly, in an especially preferred enhancement of the invention, the template surface is curved in the shape of the arc of a circle, so that, by pressing the sample against the template surface, a bend of circular arc shape having a bending radius R is imposed on it.

However, the bending need not be in a circular arc shape and it is still possible to subject the sample to a defined tensile stress by pressing it against the template surface. For example, the template surface can be parabolic or elliptical in shape. In the case of such a surface, the bending radius constantly changes and assumes a minimum value at the apex.

A preferred embodiment of the method according to the invention is characterized in that an extended section of the margin to be examined is subjected to the mechanical tensile stress σ by bending the sample section by section along the margin to be examined by advancing the sample along the margin to be examined either continuously or stepwise relative to the template, so that a successively enlarged section of the margin of the sample to be examined has been subjected to a defined bending predetermined by the shape of the template surface.

In particular, it is possible in this case for the template surface to have a homogeneous cross section of circular arc shape at least in one region and for an extended section of the sample along the margin to be examined to be pressed flat against this template region in a circular arc shape, so that an extended bend of circular arc shape with the constant bending radius R is imposed on this pressed margin section of the sample.

This embodiment makes it possible also to examine extended sections of sample margins without any gaps under a tensile stress σ that remains constant, or else up to a given tensile stress σ. This was not possible experimentally by means of the two-point bending method. Only the utilization of the template enables any position of the margin to be bent and, in particular, as provided for here according to an embodiment, to impose a bend over the margin.

This embodiment makes it possible to examine margins of nearly any length. The relative movement between the margin and the template can induce a sliding of the margin on the template. However, this sliding can also be prevented, for example, by rolling the margin on a cylindrical, co-rotating template.

In particular, a homogeneous bend of circular arc shape can be imposed on the sample over an extended region. This leads to a homogenous state of stress in this region.

Another preferred embodiment is characterized in that, at least in the margin region of the sample that has been bent in a circular arc shape, the bending radius R imposed on the section of the sample to be examined lies in an interval between a lower value Rminand an upper value Rmax, so that
Rmin≦R≦Rmax, where
Rmin=E*t/(2*σ*(1−f) and
Rmax=E*t/(2*σ*(1+f) and
where E stands for the modulus of elasticity of the sample material, σ stands for a predetermined tensile stress, and f is a number between 0 and 1, in particular 0.5, preferably 0.25, more preferably 0.1.

In this embodiment, a relation is created between the bending radius R of the sample and the tensile stress σ acting on its margin. This makes it possible to adjust the desired test tensile stress σ through the choice of the corresponding bending radius or vice versa. This is based on the following relation:

The exact bending stress σ at the margin of a sample with thickness t and modulus of elasticity E is described to good approximation by the equation
σ=E*t/(2*R).  (1)

Here, R is the bending radius of the sample in the neutral plane of the sample. The latter typically corresponds to the middle plane between lateral faces of the sample.

In accordance with the invention, thin samples, in particular, are examined, so that the difference between the bending radius R of the neutral plane and the bending radii of the lateral faces is generally negligible.

Also preferred is a method according to the invention that is characterized in that the template surface is designed as a cylinder or as a cylinder sector having a constant radius RL, so that the template surface has a cross section of circular arc shape with a constant radius RL, or is designed as a cone or as a cone sector, so that the template surface at least at one point has a cross section of circular arc shape with a constant radius RL, and wherein this template surface of circular arc shape in cross section is concave, and for the sample in the section of the sample to be examined the bend of circular arc shape with the radius R is imposed on the margin of the sample to be examined, by pressing at least one section of the first lateral face of the sample flatly and radially against this concave template surface by a pressing force, so that this pressed section of the first lateral face of the sample is in flat contact with the concave template surface, or this template surface of circular arc shape in cross section is convex, and for the sample in the section of the sample to be examined, the bend of circular arc shape with the radius R is imposed on the margin of the sample to be examined by pressing at least one region of the second lateral face of the sample flatly and radially against this convex template surface by a pressing force from outside, so that this pressed section of the second lateral face of the sample is in flat contact with the convex template surface.

This embodiment is especially advantageous, because the use of the types of templates provided for here is especially simple and tolerant of error. The flat pressing against the respective template surface enables a homogeneous bending of the sample in a circular arc shape to be achieved. In this way, it is ensured that each point on the margin of the sample is subjected to the predetermined bending and is thereby subjected to the intended tensile stress.

The intention is to impose a bend having a radius R on the neutral plane of the sample. This can be achieved in an exact manner by using a concave template surface with a radius of curvature RL, which corresponds to the desired radius R of the neutral plane of the sample in addition to the distance of this plane from the concave template surface in the pressed state of the sample. The same applies analogously for the use of a convex template, where, in this case, the radius RLof the template would be reduced with respect to R, corresponding to the distance of the neutral plane of the sample from the convex template surface. Therefore, RL=R+t/2 would generally hold for a concave template surface or RL=R−t/2 for a convex template surface. Both of these relations ensue from the fact that the neutral plane generally represents the middle plane of the sample, but a respective lateral face is pressed against the template surface. However, the difference between RLand R can be neglected in many cases and RL=R can be used.

Another embodiment is characterized in that in the section of the sample to be examined, the bend of circular arc shape with the bending radius R is imposed section by section on the margin to be examined along the margin to be examined, by moving the sample along the margin to be examined continuously or stepwise relative to the template.

This embodiment makes it possible to test margins of nearly any desired length. The relative movement between the margin and the template can induce a sliding of the margin on the template. However, this sliding can be prevented, for example, by rolling on a cylindrical, co-rotating template.

Another method according to the invention is characterized in that the template surface of the template is convex and the second lateral face of the sample is pressed against this convex template surface by the pressing force and the pressing force is transmitted onto the first lateral face of the sample or onto the second lateral face of the sample by a bendable band, wherein this bendable band preferably runs parallel to the margin of the sample to be examined and is preferably distanced from the margin to be examined and is preferably attached adhesively to the first lateral face or the second lateral face of the sample and preferably protrudes above the sample and this bendable band is preferably designed to be flexible and this bendable band is preferably designed to be self-adhering.

The bendable band enables the sample to be pressed reliably against the template. This embodiment, which is characterized as being preferred, substantially facilitates handling of the sample.

Another embodiment of the method is characterized in that the template surface is convex and comprises the following steps: adhesive attachment of a bendable band to the first lateral face of the sample or the second lateral face of the sample parallel to the margin to be examined and distanced from the margin of the sample to be examined, wherein this bendable band is preferably designed to be self-adhering and this bendable band is preferably designed to be flexible; placing the sample on the convex template surface, so that the second lateral face of the sample is in contact with the convex template surface; adjustment of the balance of forces acting on the sample particularly by means of the bendable band, so that the sample, in the section to be examined is bent over the convex template surface and the second lateral face of the sample is flatly pressed against the convex template surface, so that the second lateral face of the sample assumes at least temporarily a bend of circular arc shape with the bending radius RLof the template surface at the margin to be examined, and preferably displacement of the sample relative to the template along the margin to be examined, so that the section of the sample that has been subjected to the bending radius RLof the template surface is increased along the margin to be examined and this examined section of the margin of the sample extends over the margin to be examined.

This embodiment has the advantage that the adhesively attached bendable band can hold together the shards if the sample breaks. This is expeditious in regard to a conceivable further analysis of the broken sample. Moreover, if the method is carried out manually, the work safety is thereby increased. In this case, the bendable band can also serve as a kind of “handle” for the sample, by means of which an operator grasps the sample and, if need be, bends it over the template by exerting tensile forces. However, the balance of forces acting on the sample can also be adjusted without pulling the band in the direction of the template, because thin samples, in particular, can be wrapped around a template in many template geometries solely under the influence of their own weight.

Another embodiment of the method is characterized in that an elongated sample is used, in which the margin to be examined forms a lengthwise edge.

This is especially advantageous in the case when fractures that seldom occur per unit length of the sample margin are to be investigated, as well as for extended samples.

Also in accordance with the invention is a method in which an examination of the fracture strength of the margin of samples to be examined is performed on both sides, by examining the fracture strength of the first lateral face at the margin to be examined, as described, and then examining the fracture strength of the second lateral face at the margin to be examined, as described, wherein—preferably—the first lateral face at the margin of the sample to be examined is examined by means of the template and the sample is rotated or the template is rotated and/or displaced, so that the first lateral face and the second lateral face of the sample are exchanged relative to the template surface of the template, and then the second lateral face of the sample is examined at the margin to be examined by means of the template.

The fracture probabilities of glass samples, in particular, can depend on the direction of bending. It can depend on which lateral face is bent outward and which lateral face is bent inward during bending. The last-mentioned embodiment enables a sample to be examined regardless of this effect.

The method according to the invention enables the tensile stress at break σbof the sample to be determined. Said tensile stress at break must lie, at the conclusion of the method, between the highest stress that the sample has withstood and the stress that corresponds to the predetermined bending radius at which the sample has fractured. The bending radii R can be achieved, for example, by using a plurality of templates having different template radii RL. If the sample has already fractured during bending with the largest radius R, then its tensile stress at break σblies below the range of measurement; if it has not fractured at the smallest bending radius R, then its tensile stress at break σblies above the range of measurement.

An apparatus according to the invention for testing the fracture strength of flat samples made of brittle-fracture material, in particular of glass sheets, having a first lateral face and a second lateral face as well as at least one margin, wherein the second lateral face lies opposite to the first lateral face and the margin forms a transition from the first lateral face to the second lateral face and the sample has a thickness t at this margin, in regard to fractures originating under a mechanical tensile stress σ at this margin to be examined, in particular by means of a method described in this application, comprises a bending device for imposing a homogeneous, convex bend of circular arc shape on the sample with a predetermined bending radius R in the region of the margin to be examined along this margin, wherein this bending device comprises: a template for predetermining the bend of circular arc shape, having a template surface for pressing of the sample, wherein this template surface is designed as a cylinder or cylinder sector or as a cone or cone sector, so that the template surface has a cross section of circular arc shape and is designed to be dimensionally stable, as well as a pressing device for flat pressing of the first lateral face or of the second lateral face of the sample against the template surface by exerting a pressing force directed radially to the template.

Moreover, another apparatus according to the invention comprises a feeding device for advancing the sample along the margin to be examined of the sample relative to the template.

This embodiment makes it possible to examine samples in an automated manner.

A multiple template according to the invention for determining the mechanical tensile stress at break σbof flat samples made of brittle-fracture material, in particular of glass sheets, having a first lateral face and a second lateral face as well as at least one margin, wherein the second lateral face lies opposite to the first lateral face and the margin forms a transition of the first lateral face to the second lateral face and the sample has a thickness t at this margin, in regard to fractures originating from this margin to be examined, in particular by means of the above-described method for determining the mechanical tensile stress at break of flat samples, in a range from a minimum tensile stress at break σmin, which is to be determined, up to a maximum tensile stress at break σmax, which is to be determined, comprises at least four, preferably at least five, templates for examining the fracture strength of samples, wherein these templates each have a template surface that is designed to be dimensionally stable and is designed as a convex cylinder or cylinder sector, and wherein these cylindrical templates each have a radius Ri, where i is a number between 0 and (N−1) and where N is the number of templates of the multiple template, and wherein these template radii Ridiffer between the templates, and the following relation applies:
Ri+1<Rifori=0 toi=(N−2), and
Ri,min<Ri<Ri,max,

and whereRi,q=R0*q^(−i)corresponds to a geometric series with a “multiplication factor” q and wherein this “multiplication factor” is
q=(σmax/σmin)^(1/N-1))=(RN-1/R0)^(1/N-1))andp is a real number between 0 and 1, where preferably p<0.99, more preferably p<0.5, most preferably p<0.01.

This multiple template is suitable especially for determining the tensile stress at break σbof samples, because, correspondingly stepped, it makes available the required templates and bending radii.

Another multiple template according to the invention for examining the fracture strength of a plurality of flat samples made of brittle-fracture material, in particular of glass sheets, in which the samples each have a first lateral face and a second lateral face as well as at least one margin and the first lateral face lies opposite the second lateral face and the margin forms a transition from the first lateral face to the second lateral face and the samples have different thicknesses ti(i=1, 2, 3 . . . ) at this margin, in regard to each of the fractures originating under a mechanical tensile stress σ from the margin to be examined of the respective sample, in particular according to one of the described methods or as a part of one the described apparatuses, comprises a plurality of templates for examining the fracture strength of flat samples with thicknesses ti, where i a whole number between 1 and N, and where N is the number of templates comprised by the multiple template, and wherein these templates each have a dimensionally stable template surface for flat pressing of samples, and wherein these template surfaces are each designed as a convex cylinder or cylinder sector having a radius Ri, wherein the following relation applies to these radii: Ri+1<Rifor 0<i<N−1, and the multiple template comprises at least three, preferably at least four, templates, wherein the radii Riof these at least three, preferably at least four, templates are different and each of them deviates by no more than 30%, preferably no more than 15%, more preferably by no more than 5%, from reference values, wherein these reference values are chosen from the set {C*20 μm, C*25 μm, C*30 μm, C*50 μm, C*70 μm, C*100 μm, C*145 μm, C*200 μm}, where C is a constant and preferably C=E/(2*σ), where E is the modulus of elasticity of the sample material (10), and σ is the tensile stress under which the samples (10) are to be examined.

This multiple template makes it possible to examine glass samples of different thickness t for their fracture strength under the same tensile stress σ.

A preferred embodiment of this multiple template is characterized in that the templates are arranged in such a way that the cylinder axes thereof are arranged parallel to one another in a plane and the individual template surfaces are arranged axially offset along the cylinder axes, and the cylindrically bent template surfaces are arranged concentrically or the template surfaces are arranged in such a way that their projections contact at one point in a plane perpendicular to the cylinder axes.

These arrangements of templates are space-saving and simplify the use of the templates, in particular the exchange between templates having various radii. These geometric arrangements of the individual templates are especially appropriate in operation.

The invention will be explained in detail below on the basis of exemplary embodiments with reference to the attached figures. In the figures, identical reference numbers refer to identical or corresponding elements.

DETAILED DESCRIPTION

FIG. 1shows a measurement arrangement for carrying out the known two-point bending method. In this method, thin glass samples10are clamped between two support plates51,52and then bent by bringing these plates51,52together.

In this case, an inhomogeneous state of stress, which is greatest in the middle10mof the sample10, is created. Accordingly, the bending radius of the sample10is also the smallest in the middle10mbetween the two support plates51,52.

The tensile stress at break can then be determined by determining the minimum bending radius Rmexisting at fracture and by determining, on the basis of this value, the corresponding tensile stress at the edge. The tensile stress σ is hereby inversely proportional to the bending radius. The bending radius Rmin, in turn, depends on the distance a between the two support plates51,52. In order to determine the tensile stress at break of the sample10, the distance a between the two support plates51,52can be recorded at the time point of fracture. Such an arrangement has the disadvantages already discussed above.

Shown schematically inFIG. 2ais the cross section of a sample margin13. The thin glass sample10has the first lateral face11and the second lateral face12. Here, the margin13of the sample10has the border13b. The sample10is extended further out of the plane of the drawing.

FIG. 2bshows, in analogy toFIG. 2a, another exemplary sample margin13. This comprises the end face13sas well as, at the transition of the first lateral face11to the end face13s, the first corner131and, at the transition of the second lateral face12to the end face13s, the second corner132.

The sample margin13illustrated inFIG. 2chas the rounded end face13sinstead of the corners131and132.

It is conventional among persons skilled in the art usually to refer to the regions of samples10named here as “margins”13also as “edges.” This applies regardless of whether they have corners131,132or, for example, are rounded.

An exemplary embodiment of the method according to the invention will be discussed below on the basis ofFIGS. 3ato 3d. The examining of the margin strength of an exemplary brittle-fracture sample10having the thickness t under the mechanical tensile stress σ will be discussed. The material of the sample has a modulus of elasticity E. The following relation exists at the margin13pof the sample10—as already described in Equation (1)—between the mentioned quantities:
σ=E*t/(2*R).

In this equation, R is the bending radius of the sample10in the “neutral plane”10n, that is, in the stress-free plane10nin the middle of the sample, in which any tensile stress and compressive stress are eliminated. Given the predetermined bending radius R, the tensile stress σ corresponding to it is determined by means of this relation and, given the predetermined tensile stress σ the bending radius R to be imposed on the sample10corresponding to it is determined.

FIG. 3ashows a perspective illustration of said exemplary thin glass sample10in the bending apparatus23prior to bending. The bending device23comprises the template20as well as the bendable band31.

The template20has a cylindrical design; its surface21has the constant bending radius RL, symbolized here by the arrow originating from the center point of this curve, which is marked as a cross. It is dimensionally stable in such a way that it is not deformed under the action of pressure. The template20shown here is made of metal. It is supported on the frame6.

The rectangular sample10is made of thin glass having the thickness t. It has the four margins13. The front margin13pis to be examined for its fracture strength. The thickness t of the sample is typically between 5 μm and 500 μm.

The adhesive band31is adhered to the first lateral face11of the sample. It runs at a certain distance from the margin13pto be examined and is essentially parallel to the latter. It has been found that this band31should be both bendable and flexible and/or stretchable. Various commercially available types of adhesive bands made of thin plastic have proven well suited for this purpose. The adhesive band31has a number of functions. On the one hand, the protruding ends31aand31bserve the operator as a convenient possibility for grasping the sample manually, laying it on the surface21of the template20, and bending it over the template20by pulling in the direction of the arrow32. On the other hand, the adhesive band31ensures that, when the sample10breaks, the shards remain adhered to this band31. The shards of the sample10ideally remain adhered to the adhesive band31as a unit. This is advantageous, on the one hand, for preventing work accidents. On the other hand, it enables the fractured sample10or its shards to be analyzed without having to collect them beforehand and reassemble them similarly to a puzzle. It has been found that, within the framework of measurement accuracy, the adhesive band31has no influence on the result of the test.

If this function of the adhesive band31can be dispensed with as an element for holding together the shards, then, instead of the adhesive band31, it is also possible to attach simple retaining pieces31aand31bto the short margins or edges of the sample without a continuous band31connecting them. If need be, the sample can also be grasped directly or can be clamped in other retaining devices.

Tests have shown that there are no special requirements placed on the geometry of the sample10. The margin13pto be examined should be straight, so that the adhesive strips31can readily be applied (a few mm behind the edge13), so as then to pull or lay the sample10over the template20. The length of the sample10is limited in practice only by handling. Ultimately, long samples10have to be drawn over the template20and thus the practical limit of the sample length is approximately 600 to 800 mm. Nor is there any fundamental limitation in the width of the sample10. If the sample10is too wide, it can “droop” in back when it is put in place/pulled, which strongly impairs the measurement in the absence of any countermeasures. It cannot be ruled out that wide samples10will also lead at the margin to be examined13(measurement edge) to additional stresses that will not be taken into consideration in a simple analysis. It is therefore advantageous when the samples10are not wider than 100 mm. Preferably, the ratio of length to width is not less than 1.

FIG. 3bshows the content ofFIG. 3ain a projection in crosswise direction. The adhesive band31is illustrated at a distance from the first lateral face11only for purpose of illustration; in reality, it is attached adhesively to it. The dotted line10nmarks the middle plane10nof the sample10.

The subject ofFIG. 3ccorresponds to that ofFIG. 3b. However, tensile forces32are now exerted on the ends31aand31bof the adhesive band31in the direction of the template20. As a result, the second lateral face12of the sample10is pressed against the cylindrical surface21of the template20. The sample10is bent, so that the second lateral face12of the sample10rests flatly against the template surface21. The second lateral face12of the sample10assumes the bending radius RLof the template surface21. Relevant for the tensile stress σ acting on the sample margin13pis the bending radius R of the “neutral plane”10nof the sample10. When regarded exactly, this is thicker than RLby half a sample thickness, that is, t/2 greater. However, this can be neglected and R=RLcan be assumed for simplicity. In the context of this approximation, the bend16of the template surface21is thus imposed on the sample10; the sample margin13pis subjected to the corresponding tensile stress σ in the region of the first lateral face11to good approximation.

In another exemplary embodiment, the adhesive band31is adhered onto the second lateral face12of the sample10. If the adhesive band31is sufficiently thin, there is no significant change in the bending radius R of the sample10. This arrangement has the advantage that any influence of the adhesive band31on the first lateral face11of the sample10is thereby excluded. Moreover, the adhesive band31can protect the second lateral face12at least partially from being scratched on the template surface21.

To be discussed on the basis ofFIGS. 4ato 4cis another exemplary embodiment that is explained on the above constructed exemplary embodiment. Once again, the margin13pof the sample10is examined for fracture strength. The template20is now designed as a rotatable solid cylinder20. The sample10has a defect41at the margin13pin the region of its first surface11.

In this example, moreover, the sample10is longer along the margin13pto be examined than in the preceding example. In this case, only a section15of the margin13pof the sample is pressed against the template at a given time. The bend of circular arc shape16of the template surface21is further imposed on this section15of the sample10. In this section15, the sample10has just been examined at the respective point in time; outside of this section15, however, the sample10is no longer in contact with the template surface21.

As shown inFIG. 4a, the sample10is placed with its second lateral face12on the cylindrical surface21of the template20. It is pressed by pressing forces30, which are directed radially to the cylindrical template20or normal to the template surface21, in the section15against the template20. This occurs in such a way that the sample10bends over the template20and the second lateral face12of the sample10or of the sample margin13pand assumes the bend of the template20having bending radius RLin the pressed section15as long as this section15is pressed. Once again, it may be assumed that the bending radius R of the neutral plane10n(not drawn in here) corresponds to that of the template20to good approximation.

The sample10is transported further in the feeding direction34with continued pressing30, with the axis7of the template20remaining fixed in position and the template20rotating in the direction of rotation24shown, so that the second lateral face12of the sample10does not slide over the template surface21, but rather rolls. This makes it less likely that the test itself could lead to further surface defects, which could potentially falsify the result.

InFIG. 4b, the sample has been transported further. The section17of the sample, which has been subjected to the bend16having the bending radius R, leaves the template20in the direction34and continues to emerge. The exemplary defect41comes closer to the bending template20.

InFIG. 4c, the already bent and thereby examined section17has increased further. Moreover, the defect41has migrated into the presently bent section15. There, under the tensile stress σ, it has led to the formation of the crack42. The sample10breaks.

The cylindrical template20can be actively rotated and thus act simultaneously as a feeding device33for the sample10. However, it can also merely co-rotate, while an operator pulls the sample10manually over the template20.

In another exemplary embodiment, the template20is not rotatably mounted. Instead, the sample10is pulled in a sliding manner over the template surface21. In this case, the template20need not be a solid cylinder as well; a template surface21, which, as illustrated schematically inFIGS. 3ato 3c, forms a partial cylinder21or a cylinder sector21, is sufficient in this case.

InFIG. 4d, a template20is illustrated in an analogous manner toFIG. 4a-candFIG. 3a-c. In this case, however, the template surface21is not cylindrical, but rather parabolic in design. At the apex21k, this parabola21has a local curvature with a radius of curvature RL; the corresponding radius of curvature28of the template surface21at this point is indicated by a dashed line. In this embodiment, each point of the margin of the sample10is subjected to a bending21kcorresponding to this curvature21kin that the sample10is transported in the pressed state in the direction of the arrow35. Such a shape of the template, which does not have a cylindrically arched template surface, is also suitable for testing whether a sample10is able to withstand an increased bending stress. If, however, a sample10is stressed until it breaks, there exists the drawback that only the site of fracture would need to be localized in order to establish the stress at which the sample has fractured, since the fracture does not necessarily occur at the apex with the minimum radius of curvature RL. Cylindrically arched template surfaces, by contrast, impose a constant curvature on the sample, so that the tensile stress at break can be determined independently of the site of fracture.

It is obvious that a diversity of other template shapes20can be formed without departing from the scope of the invention. What is important is that, at least initially, a defined curvature21kis imposed on the sample10by the template surface21. This local curvature21kcan then be shifted over the sample10by a relative displacement between the sample10and the template surface21along the margin13p, so that a region13pof the sample10to be defined by this displacement has been subjected to a bend21khaving the fixed minimum radius R.

FIG. 5shows an exemplary embodiment of the method according to the invention for determining the tensile stress at break σbof the sample10. Initially, in the step52, a template radius RLor a tensile stress σ corresponding to it is predetermined and the sample10is subjected to these loads at its margin13pto be examined—in particular, as described above—to a bend16by means of a corresponding template (step53). Then, in step54, it is examined whether the sample10has fractured starting at the margin13pto be examined. Samples10that have not fractured starting from any margin or that have fractured starting from a margin13different from the margin13pof interest are usually removed (not shown).

If the sample10has not fractured, the template radius RLis reduced or the predetermined tensile stress σ is increased (step55) and the sample10is once again subjected in step53to a bend16, which now has the reduced bending radius RL.

The steps53,54, and55are repeated until, in step54, a fracture of the sample10is detected. If the sample10has fractured starting from the margin13pto be examined, then the bending stress σkcorresponding to the last bending radius RLat which the sample did not fracture is recorded. The same holds true for the bending stress σk+1that corresponds to the template bending radius RLat which the sample10has fractured. The tensile stress at break σblies in the interval σk≦σb≦σk+1. An appropriate approximation is the mean value of this interval.

If the sample10is fractured, it must be examined in any case whether this fracture originated from the margin13pof interest or else from another point of the sample (for example, from a margin13that is not of interest or from the face area). This determination, in certain cases, can be carried out on the basis of a simple criterion: If a sample10fractures starting from a point41on its margin13p, then a plurality of bundled cracks42originating from this point41often form. If such a bundle of cracks thus originates from the margin13pof interest, then it is a fracture that needs to be detected. If a fracture42has not originated from the margin13pof interest, then the corresponding stress value cannot be evaluated in the course of the observation of this margin13p.

InFIGS. 6aand 6b, a multiple template according to the invention is shown schematically in the form of a step roller300, which is set up for determining the tensile stress at break σbof samples10according to the method just discussed.

It is made up of a set300of discs301to320, which are arranged concentrically.FIG. 6bshows the step roller300in longitudinal section andFIG. 6ashows a projection of the discs301to320in the crosswise direction. The discs301to320shown here were produced individually from plywood or plastic, in particular from POM, and are joined by the axis7. However, they can also be rotated as a single workpiece, for example. It must only be possible to work the material mechanically in such a way that the discs301to320can be produced with sufficient accuracy, and the material must be strong enough that it does not deform when the glass samples10are placed on it and pulled over it. The discs301to320are held by the frame6.

The discs301to320can be mounted rotatably or not rotatably. A rotatable mounting of the axis7and/or of the individual discs301to320has the advantage that, in this way, the sample10can be moved relative to the respective disc301to320with co-rotation of this disc301to320. Thus, the second lateral face12of the sample10, which is in contact with the surface21of the respective disc301to320, does not need to slide over this surface21; the risk of scratching the lateral face12of the sample10is thereby reduced.

If the tensile stress at break σbof a sample10is to be determined, then this sample10is placed in succession on the discs301to320and pressed—for example, as discussed above on the basis ofFIGS. 3ato 3c—until it breaks. In another exemplary embodiment, the samples are pulled over the discs301to320as explained above in conjunction with the method discussed on the basis ofFIGS. 4ato 4c, for example.

In the exemplary realization of the exemplary embodiment here, the discs301to320have the following radii:

In choosing these radii RL, attention was paid to keeping the ratio of adjacent radii Riand Ri+1approximately constant. A constant ratio of the radii q:=Ri/Ri+1(with a deviation of preferably Δq<1%) according to Equation (1), in the case of equivalent samples, leads to a mutually reciprocal, likewise constant ratio of the corresponding bending stresses σi+1σi=q. This geometric gradation of the stresses or radii is especially advantageous for the characterization of ensembles of samples10, because the fracture of glass samples10under a tensile stress σ at the margin13pis a statistical effect. As a result of the gradation with constant ratio of the radii or stress ratio q, the relevant values of the distribution function can be read from the distribution of the measured values for σbwithout further transformation.

In the following table, further advantageous gradations of radii and the tensile stresses σ corresponding to them are listed in units of Megapascals (MPa) for the glasses D 263 (with the modulus of elasticity E=72.9 GPa) and AF 32 (E=74.8 GPa) for glass thicknesses of t=0.05 mm (both types of glass) as well as t=0.1 mm and t=0.2 mm (only AF 32).

Preferred, in particular, are sets of at least five discs having different radii, which are chosen from the above table.

The classification of the individual radii into a geometric sequence is illustrated once again below. A range of measurement of bending stresses with the upper limit σmaxand the lower limit σminis predetermined. This range is to be subdivided geometrically into N individual stresses σn. The maximum radius R1corresponds via Equation (1) to σminand the minimum radius RNcorresponds to σmax. Regarded strictly mathematically (omitting the sample thickness), the “multiplication factor” for this series of radii would be:
q=σi+1/σi=(σmax/in)^(1/(N-1))=(R1/RN)^(1/(N-1)).

This results in the following equation for the individual radii:
Ri+1=q*Ri=R1*q^(i-1).

It has been found that, in a range from σmax/σmin≈1.5 for four gradations, a multiplication factor of q≈(1.5)^(1/5)to (1.5)^(1/4), that is, q≈1.09, affords experimentally reasonable results.

The calculated radii Riand gradations can be obtained roughly as follows. Appropriate is a real value
Ri,realε[Ri,min,Ri,max]
with
Ri,min=Ri*(q^(−1/2);Ri,max=Ri*(q^(+1/2)),

and further preferably
Ri,min=Ri*(q*p)^(−1/2);Ri,max=Ri*(q*p)^(1/2)
with the positive distance factor p<1, which is preferably 0.99, more preferably 0.5, and most preferably 0.01.

In the following table, exemplary radii Ri, including the lower and upper limits for p=1 and p=0.99, are given:

It is self-evident that the values and relations given as “radii” or “disc radii” in the present application are not limited to disc-shaped templates in the scope of the concrete exemplary embodiments, but rather are especially advantageous as bending radii also for use in the context of individual templates. The same holds true for (single or multiple) templates of any shape (for example, parabolic) with respect to the local radius of curvature at least at one point on the respective template surface21at the site21kat which the sample10is to be pressed or is pressed against this surface21.

Whereas, in the example shown inFIGS. 6aand 6b, the discs301to320are arranged concentrically with respect to one another,FIG. 7shows an alternative arrangement to this. It shows a projection of the discs in the corresponding crosswise plane, so that the cylinder axes project out of the plane of the drawing. The discs301to305are arranged in such a way that the projections of their surfaces come into contact at a common point300p. In the longitudinal direction (out of the plane of the drawing, not shown), the surfaces form a straight line, which connects the disc surfaces, at this point300p. This embodiment has the advantage that, when a measurement is performed, it is easy to switch between various discs301to305.

Another exemplary embodiment contains, instead of one or a plurality of discs20or301to320, one or a plurality of templates20, which has (have) a template surface21of circular arc shape21only in a narrow (angle) range and, moreover, can be shaped in any way. In other words, what are involved here are small sections taken from cylinder outer lateral surfaces and mounted on support stands. The respective sample10is pulled over these templates20. This is of advantage, in particular, for very small bending radii R, because, in this case, the long samples10have to be pulled over the template20in any case and the fabrication of the templates20is thus simplified. However, in this case, it needs to be ensured that each point of the sample10has been subjected at least briefly to the corresponding bend16. It has been found that it generally already suffices when, for this purpose, the sample10passes over a cylindrical sector21having a sector angle of 10° and the sample10is pressed flatly against the template20in this range21. The reliability can be increased still further, if need be, by increasing the pressed angular region21up to at most 90°.

Furthermore, the multiple template can also be designed to be “stepless” in the form of a cone or truncated cone. In this case, the tensile stress at break σbof samples10can be measured in a stepless manner in that the respective sample10is placed on a region of the template with a larger bending radius RLand moved under pressure on this multiple template to smaller bending radii RLuntil the sample10breaks. Viewed in this way, a cone or truncated cone can thus be regarded as a set of infinitely many, infinitesimally narrow templates20.

The sample10can be pressed by means of the adhesive band31or another bendable band31. However, it can also be pressed against the template20by means of a counterpiece (“die”) that matches the template20.

Moreover, instead of an outer cylinder21, against which the second lateral face12of the sample10is pressed from the outside, it is also possible to use an inner cylinder (or a section of an inner cylinder). The first lateral face11of the sample10is pressed against it radially from the inside.

On the basis of fracture tests, it is also possible to carry out a method for the production of a glass article having at least one margin with a guaranteed fracture strength with respect to a specified mechanical tensile stress σgat this margin, wherein a glass article is produced and the fracture strength of the margin of this glass article is tested under the specified tensile stress σg, preferably under 1.1 times the specified tensile stress σg, more preferably under 1.2 times the specified tensile stress σg, at least in a section of this margin, preferably at least along 0.01 times the length of this margin, more preferably at least along 0.1 times the length of this margin, in particular along the entire margin, by imposing a bend of the glass article on a template surface having a defined curvature, and the article is discarded provided that it breaks.

This method makes it possible to provide glass articles whose edge fracture strength has been examined.

A method for the manufacture of a thin, flat glass article can be further developed by examining the edge fracture strength with samples of such a glass article by means of one of the described methods and, on the basis of the edge fracture strength, determining a minimum bending radius RMand bending a glass article that is equivalent to the glass article from which the samples were obtained, with the bending radius RBnot being less than the determined minimum bending radius RM. In this case, the glass article from which the samples were obtained can also comprise the glass article to be examined. For example, the samples can be obtained by cutting off a section of the glass to be examined.

This method makes it possible on the basis of the samples to characterize and ensure the quality of glass articles that cannot be examined themselves.

Another method is characterized in that, as the glass article, a ribbon of glass is produced and wherein, after its production, the ribbon of glass is rolled up into a roll, with the bending radius RRof the ribbon of glass on the inner side of the roll not being less than the minimum bending radius RM.

This method enables a rolled sheet of glass to be created without having to accept the existence of too many glass fractures.

Shown inFIG. 8is a glass product103according to the invention. What is involved here is a rolled ribbon of glass100. For this ribbon of glass, the minimum allowable bending radius was initially determined by means of the method according to the invention and the ribbon of glass was then rolled up. In the process, attention was paid to making the bending radius of the glass on the inner side104of the roll103larger than the allowable minimum bending radius. A sheeting material107(for example, paper or plastic) was rolled up into the roll with it in order to prevent the surfaces of the ribbon of glass from being scratched.

In order to be able to ensure an especially low rate of fracture even for very large-area glass articles, such as, in particular, thin ribbons of glass, it is also possible according to an embodiment of the invention to combine the method for examining the fracture strength of flat samples10with a bending test of a glass product103. This embodiment of the invention is based on the idea that, by means of the method for examining the fracture strength of flat samples10, initially one or a plurality of parameters relating to fracture strength, in particular one or a plurality of statistical parameters, are determined and, on the basis of these parameters, a bending radius for a bending test of the entire glass article is determined and the glass article is then subjected to the bending test, with the glass article being discarded if it breaks under the bending load at the fixed bending radius. It is appropriate to choose the bending radius of the glass article or the radius of curvature of the corresponding template surface of the template used to be greater than the mean value of the bending radius at which the samples10have broken.

In this way, the edges of glass articles, such as, in particular of glass sheets having defined radii, are bent and a bending stress is generated, which is sufficiently great so that the edges break at critical weak points, but, on the other hand, is sufficiently small that uncritical edge defects are not enhanced by the bending. In particular, it is to be ensured that the delivered glasses have warranted strength properties with adequate statistical reliability.

In order to design the bending radii for the bending test of the glass article, statistical strength tests are carried out on glass samples10in accordance with the method according to the invention, by means of which the “base strength” of the glass article is determined. The bending test should correspond to the range in which the strength properties of the glass article deviate in a negative way from those of random samples, that is, the fracture test according to the invention on glass samples with successively smaller bending radii.

According to an embodiment, by means of the method according to the invention for examining the fracture strength of flat samples10, a random sample of N values is taken for the fracture bending radii R1. . . RN, and, for the values of these random samples, the mean value

〈R〉=1N⁢∑i=1N⁢Ri
and the variance

Then the entire glass article, in particular a ribbon of glass, is bent, preferably by feeding it over rollers, so that, depending on the relative variance s/<R>, the bending radius RPTfor the two bending directions lies in the range defined by the curves inFIG. 9, that is, between the two curves RPT,maxand RPT,min. In the diagram shown inFIG. 9, the ratio of the bending radius of the glass article to the mean value <R> of the bending radius when the samples10break is plotted on the ordinate as a function of the relative variance s/<R>.

It is preferred that the number of random samples N is at least 20, that 10% of the largest values and 10% of the smallest values are discarded from the random samples, and that, from the rest (of the so-called “supported random samples”), only the mean value and the variance are derived according to the above two equations. The glass article is then subjected to a bending test by pulling it over rollers, for example, so that, depending on the relative variance s/<R>, the bending radius RPTthereof lies in the region defined by the curvesFIG. 10for both bending directions.

The values RPT,maxand RPT,minare functions of the relative variance s/<R>. If the relative variance is high, this means that there is a large scatter of the bending stresses or, correspondingly, the bending radii in the fracture tests. Correspondingly, with higher scattering, larger bending radii are also chosen for the fracture test of the entire glass article in order to be able to ensure a specific fracture strength corresponding to the chosen bending radius.

By choosing a bending radius between the curves RPT,max/<R>(s/<R>) and RPT,min/<R>(s/<R>) according toFIG. 9orFIG. 10, which are oriented to the relative variance of the fracture strength of samples of an equivalent glass article and define a narrow region with a bending radius that is as small as possible, it is possible to ensure very high fracture strengths and still limit the rejects in the fracture test to glass articles that actually have significant weak points.

The regions between the curves RPT,max/<R> and RPT,min/<R> ofFIG. 9andFIG. 10are slightly displaced against each other. For both of the test conditions mentioned above, namely, on the one hand, that of a random sample for which the values in the range of RPT,max/<R> to RPT,min/<R> according toFIG. 9are favorable and, on the other hand, that of an adjusted random sample for which corresponding bending radii in the range of RPT,max/<R> to RPT,min/<R> result according toFIG. 10, it is possible to specify an enveloping region. For this purpose,FIG. 11shows the curves RPT,max/<R> and RPT,min/<R> ofFIGS. 9 and 10as well as preferred limit values Rmin/<R>(s/<R>), Rmax<R>(s/<R>), between which the curves RPT,max<R> and RPT,min/<R> lie. According to an embodiment of the invention, the bending radius for the fracture test of the entire thin, flat glass article is therefore chosen in such a way that it lies in the range of Rmin(s/<R>), Rmax(s/<R>). In this case, the following relations can be employed for the bending radii Rmin(s/<R>), Rmax(s/<R>):

Accordingly, the invention also relates to a method for providing a plate-shaped glass article having high fracture strength, wherein by means of a method according to the inventions for examining the fracture strength of flat samples10made of brittle-fracture material, in particular of glass sheets, a plurality of samples10(total number N) are used to evaluate the bending radius or tensile stress at which each of the samples10breaks,

from these values, the mean value <R> of the bending radii Riat which each of the samples (10) has broken is calculated and, using the mean value <R>,

the variance s is calculated according to

s=1N-1⁢∑i=1N⁢(Ri-〈R〉)2
and wherein then a glass article made of the same glass material as the samples10, preferably a ribbon of glass (100), is bent in order to test whether the glass article withstands a bending radius RPTor a corresponding tensile stress, wherein the bending radius RPTis chosen in such a way that it lies in the range of the radii Rminto Rmax, which are dependent on the relative variance s/<R> in this range, wherein the radii Rminand Rmaxare given by the above-given Equations i) and ii).

For this purpose,FIG. 12shows schematically a glass article in the form of a ribbon of glass100, which is advanced along the arrow and fed over rolls or rollers8,9, so that a bend having a bending radius predetermined by the rollers8,9is imposed locally on the ribbon of glass. The glass article is preferably tested with bends in opposing bending directions. For this purpose, in the example ofFIG. 12, the rollers8,9are arranged above and below the ribbon of glass100, so that the ribbon of glass is bent with each of the two lateral faces11,12around one of the rollers8,9.

According to an enhancement, in order to be able to ensure that the entire glass article, which is preferably very large in surface area and elongated, in the form of a ribbon of glass, withstands the bending stress with a bending radius in the range of Rminto Rmax, its longitudinal edges are bent along at least % of its total length, preferably along its entire length, at least with the bending radius RPT.

As already mentioned, an adjusted random sample can be used for determining the values of <R> (mean value of the bending radius at fracture) and s (variance of the bending radii at fracture), by determining the bending radii or tensile stresses at fracture are determined for at least twenty samples and by discarding the largest and smallest values, preferably 10% of the largest and smallest values, of the bending radius or tensile stress, and by calculating the mean value and the variance by using the remaining values.

The embodiment of the invention described above can then be used to produce a plate-shaped glass article having a guaranteed or predetermined fracture strength under a bending load at a predetermined bending radius, in which the edge thereof or at least one edge of the glass article withstands a bending load with a bending radius RPTalong its entire edge length, with the bending radius, corresponding to Equations i), ii) given above, lying in the range of

Rmin=〈R〉·{0.7+exp⁡(s〈R〉·0.053-2.3)}⁢⁢toiii)Rmax=〈R〉·{3.4+exp⁡(s〈R〉·0.05-2.1)}⁢iv)
where <R> is the mean value and

s=1N-1⁢∑i=1N⁢(Ri-〈R〉)2
is the variance of the bending radii at fracture of a plurality of N samples made of the same glass material as the glass material of the glass article, with the bending radii Riat which each of the samples10breaks being determined preferably by means of the method according to the invention for examining the fracture strength of flat samples10made of brittle-fracture material.

It is especially preferred that the glass article is a thin ribbon of glass having a length of at least 20 meters, preferably at least 50 meters. Preferably, the ribbon of glass100is tested along the entire edge length of at least 20 meters for fracture strength and it is possible to ensure a corresponding bending radius that the ribbon of glass withstands.

The glass article is preferably composed of a lithium aluminum silicate glass, a soda lime silicate glass, a borosilicate glass, an alkali aluminosilicate glass, or an alkali-free or low-alkali aluminosilicate glass. Such glasses are produced, for example, by means of drawing methods, such as a downdraw method, overflow fusion, or by means of float technology.

Advantageously, a low-iron or iron-free glass, in particular one with a Fe2O3content less than 0.05 wt %, preferably less than 0.03 wt %, is used, because said glass has reduced absorption and thus makes possible, in particular, an increased transparency. However, for other applications, gray glasses or colored glasses are preferred.

According to an embodiment, a glass or a glass ceramic that is prestressed for its use is used. This glass or glass ceramic can be prestressed chemically by ion exchange or thermally or by a combination of thermal and chemical methods.

An optical glass can also serve as a glass material, such as, for example, a dense flint glass, a lanthanum dense flint glass, a flint glass, a light flint glass, a crown glass, a borosilicate crown glass, a barium crown glass, a dense crown glass, or a fluorine crown glass

Advantageously, a low-iron or iron-free glass, in particular one with a Fe2O3content of less than 0.05 wt %, preferably of less than 0.03 wt %, can be used, because said glass has reduced absorption and thus, in particular, an increased transparency.

For other applications, however, gray glasses or colored glasses are also preferred.

The invention is suited, in particular, for optimizing the mechanical properties of glasses that are already of high strength. High-strength glasses are typically used for applications in which the glasses are also subjected to a high mechanical load. Such glasses are consequently designed to withstand bending stresses acting on the glass surface area. In this case precisely, the edges of the glasses represent significant weak points. Ultimately, a glass pane made of high-strength glass indeed breaks very quickly if the edge of the pane has flaws and is also subjected to a bending load. It is then possible to examine by means of the invention whether the edges remain constant in their quality when, for instance, individual glass panes are finished by dividing a larger pane. Thus, for example, it is conceivable that, owing to wear, a cutting wheel leaves behind damage at the glass edges. If this is the case, the strength of the entire glass pane is markedly diminished. The method can then be used to determine very precisely such changes in the manufactured product and to test the efficiency of improvements in the formation of edges. Given below are high-strength glasses for which an increase in strength can be achieved by monitoring the edge strength by means of the invention.

Suitable according to an embodiment are glasses having the following constituents, with the molar composition being given in mole percent:

In addition, it applies as an auxiliary condition that the quotient of the molar content of fluorine to the molar content of B2O3, that is, F/B2O3, lies in a range from 0.0003 to 15, preferably from 0.0003 to 11, especially preferably 0.0003 to 10. These glasses can be prestressed chemically and can be used in mobile displays as cover glasses.

The composition in this case preferably contains the following components:

Especially preferably, the composition contains the following components:

Alkali-free borosilicate glasses constitute another suitable group of glasses. In this case, the following compositions in weight percent are preferred:

These glasses are also described in US 2002/0032117 A1, the content of which is also made the subject of the present application in full scope with respect to glass compositions and glass properties. A glass of this class is marketed by the applicant under the trade name AF32.

The following table lists the contents of components of further alkali-free borosilicate glasses as well as, in the right column, a composition range of a class of glasses having similar properties based on this glass:

Yet another class of preferred glass types comprises borosilicate glasses having the following components in weight percent:

A glass of this class of glasses is the Schott glass D 263. Glasses with more precise compositions are also described in US 2013/207058 A1, the content of which is also made the subject of the present invention in full scope with respect to the compositions of the glasses and the properties thereof.

Soda lime glasses are also suitable. Listed in the following table are two exemplary embodiments and the proportion in weight percent of the components in accordance with a preferred composition range:

The glass 2 is especially well suited for the manufacture of sheet glass by the float method.

According to yet another embodiment of the invention, lithium aluminum silicate glasses of the following compositions are used for the glass material, consisting of (in wt %):SiO255-69Al2O319-25Li2O 3-5Total Na2O+K2O 0-3Total MgO+CaO+SrO+BaO: 0-5ZnO 0-4TiO20-5ZrO20-3Total TiO2+ZrO2+SnO22-6P2O50-8F 0-1B2O30-2,as well as, if need be, additives of coloring oxides, such as, for example, Nd2O3, Fe2O3, CoO, NiO, V2O5, Nd2O3, MnO2, TiO2, CuO, CeO2, Cr2O3, rare earth oxides in contents of 0-1 wt %, as well as refining agents such as As2O3, Sb2O3, SnO2, SO3, Cl, F, CeO2of 0-2 wt %.

In another embodiment, the brittle-fracture material is a glass ceramic, in particular in the form of a glass ceramic pane, wherein the glass ceramic consists of a ceramized aluminosilicate glass or a lithium aluminosilicate glass, in particular of a chemically and/or thermally cured, ceramized aluminosilicate glass or lithium aluminosilicate glass. In another embodiment, the brittle-fracture material comprises a ceramizable starting glass, which, in the event of a fire, is ceramized under the action of heat and thereby brings about increased fire protection safety.

Preferably, a glass ceramic or a ceramizable glass having the following composition of the starting glass is used (in wt %):

In another embodiment, a glass ceramic or a ceramizable glass having the following composition is preferably used as the starting glass (in wt %):

In another embodiment, a glass ceramic or a ceramizable glass having the following composition is preferably used as the starting glass (in wt %):

The glass ceramic preferably contains high-quartz mixed crystals or keatite mixed crystals as predominant crystal phase. The crystallite sizes are preferably smaller than 70 nm, especially preferably smaller than exactly 50 nm, quite especially preferably smaller than exactly 10 nm.

It is obvious to the person skilled in the art that the embodiments described above are to be understood as being given by way of example and that the invention is not limited to them, but rather can be varied in diverse ways without departing from the protective scope of the claims. It is further obvious that the features, independent of whether they are disclosed in the description, the claims, the figures, or otherwise, also define individually essential components of the invention, even if they are described together with other features.

LIST OF REFERENCE NUMBERS

6frame7axis8,9roller10sample10nneutral plane10mmiddle of the sample in a two-point bending11first lateral face12second lateral face13margin13bborder13pmargin to be examined13send face131first corner132second corner15currently bent section of the sample16bend a in circular arc shape17examined section of the sample20template21template surface23bending apparatus24direction of rotation of the template28circle of curvature of the template surface30pressing force31bendable band/adhesive band31aprotruding end of the band31bprotruding end of the band32tensile force33feeding device34direction of feeding41near-margin defect of the sample42crack51,52support plates100ribbon of glass103roll of glass104inner side104of103107sheeting material300step roller300ppoint of contact of the surface projections301-320discs of the step roller