WO1994008229A1

WO1994008229A1 - Detection of defects in glass

Info

Publication number: WO1994008229A1
Application number: PCT/AU1993/000498
Authority: WO
Inventors: John Barry; Ross Miller
Original assignee: University of Queensland UQ
Current assignee: University of Queensland UQ
Priority date: 1992-09-29
Filing date: 1993-09-29
Publication date: 1994-04-14
Anticipated expiration: 1995-03-29
Also published as: CN1087424A

Abstract

Stress-inducing inclusions and other defects in tempered plate glass windows are detected on site by viewing the glass in cross-polarised light. A window (12) is illuminated by a light box (10) through a polariser (11), and the window (12) is viewed through an analyser (14) using either transmitted or reflected light. Despite the light patterns formed by the stresses inherent in tempered glass, small inclusions can be detected by their characteristic stress patterns. Defects in the window glass (12) can be examined in detail on site using a microscope (30) mounted on mounting means (40) releasably attached to the window by suction pads (44). An inclusion can be inspected, and photographed using a camera (33) fitted to a projector lens (32) on the microscope (30). The depth of the inclusion in the glass can be measured using a Vernier scale (34) on the microscope focussing mechanism (37).

Description

"DETECTION OF DEFECTS IN GLASS" THIS INVENTION relates to methods and apparatus for the detection of defects in glass. In particular, the invention is directed to methods and apparatus for locating nickel sulphide stones in tempered plate glass, although the invention is not limited to this particular use.

BACKGROUND ART In many modern buildings, most of the external surface is composed of glass panels. The panels are usually double-glazed units, about 1.5 x 2 metres in size. A large multi-storey office building may typically have between 5000 and 10000 such panels. In most cases, the glass is thermally tempered for reasons of safety and strength. Tempered glass is 3 to 5 times stronger than annealed glass and, if fractured, tempered glass shatters into small fragments with blunt edges.

Tempered glass has been known to undergo spontaneous fracture. The spontaneous fracture, though relative rare, is common enough to constitute a serious problem in applications (such as multi-storey office buildings) where large areas of glass are used. It is believed that about 0.8% of glass panels fail by spontaneous fracture. In a large multi-storey building this translates as a failure of between 50 and 100 panels. The failure of even one panel has the potential to cause serious injury to life and property. If defective glass panels can be identified, they can be replaced before failure. The spontaneous fracture of tempered glass can be attributed to inclusions in the glass. In most cases, the inclusions are nickel sulphide "stones". The nickel sulphide stones are metallic in appearance, polycrystalline, roughly spherical in shape, and range in diameter from 0.1 to 0.6mm.

Nickel sulphide, NiS, is known to undergo a phase transformation at 379°C. The transformation from the high temperature α-NiS to the low temperature β-NiS is accompanied by a volumetric expansion of 4.0%. It has been postulated that the volumetric expansion in NiS inclusions (of dimensions 0.1-0.6mm) located at the interior portion of tempered glass (which is in tension), could cause spontaneous breakage.

It is well known that stress in glass may be observed by placing the glass between crossed polarisers. When a single polariser sheet is placed in front of a light-beam it transmits plane-polarised light. This first sheet is called the "polariser" . If a second sheet is placed in front of the polarised light with an angle of polarisation at 90° to the first polariser sheet, no light will be transmitted through the second sheet. The second sheet is called the "analyser". If a sheet of unstressed glass is placed between "crossed" polarised sheets (that is, at 90° rotation relative to each other), no light is transmitted. However, if a piece of stressed glass is placed between crossed-polars, some light will be transmitted as the stressed-glass can partially polarise light. The technique of studying stress in glass using cross-polarised-light is well known in the glass industry.

For example, U.S. patent no. 4,026,656 describes a device for detection of stones in the sidewalls of blown glass containers. These stones cause stress patterns to be formed in the glass, and the patterns are detected when the glass container is illuminated with infrared radiation and viewed through crossed polarising filters. Stress patterns are detected automatically using a television camera having an electronic analysis circuit connected to the output thereof.

The crossed-polariser technique is very sensitive to stress in the glass. Tempered window-glass has a great deal of internal stress caused by its method of manufacture. Tempered glass therefore displays a considerable amount of detail when viewed in crossed- polarised light. Since the nickel sulphide stones are responsible for the glass breakage, these stones must cause extra internal pressure in the tempered glass but such internal stress is localised and may be much less than the inherent stresses found throughout the tempered plate glass. Hitherto, it had been generally believed that the polarised light stress-patterns in tempered glass would be so strong or dominant as to render any stress-patterns arising from NiS stones undetectable by eye or camera.

Although the technique used in the device of U.S. patent no. 4,026,656 is suitable for inspection of blown glass containers (which do not contain significant stresses), it is not suitable for use with tempered glass due to the stress patterns caused by the large stresses inherent in the tempered glass. Further, the apparatus of U.S. patent no. 4,026,656 is a relatively large structure which is fixed in position and requires the glass articles to move along a fixed path. Such apparatus is not readily portable and suitable for on site inspection of window glass on buildings. The use of infrared radiation, television cameras and electronic processing circuits adds to the cost and complexity of the apparatus. It is an object of the present invention to provide improved methods and apparatus for detection of defects in tempered glass.

It is a further object of the present invention to provide apparatus for the accurate inspection and measurement of the position of defects in glass.

SUMMARY OF THE INVEN ION In one broad form, the present invention provides a method of detecting small stress-inducing inclusions in tempered glass, comprising the steps of illuminating the glass with polarised light; viewing the illuminated glass through a polarising medium whose plane of polarisation is orientated orthogonally to the plane of polarisation of the illumination; and detecting localised stress patterns in the glass characteristic of the small inclusions.

It has been found that by viewing the glass in such cross-polarised light, it is possible to locate the. characteristic localised stress patterns caused by defects such as inclusions in the glass, despite the presence of patterns caused by the inherent stresses in the tempered glass. Typically, the method is used for on site detection of inclusions in window plate glass in buildings. Such inclusions are mainly caused by nickel sulphide impurities.

The cross-polarised light may be light transmitted through the glass, or reflected from the back surface of the glass. In the former method, a light source is placed on one side of the glass and polarised through a polarising sheet before transmission through the glass. The transmitted light is viewed from the other side of the glass through a second polarising sheet (or analyser) whose plane of polarisation is orientated at 90° to the first polarising sheet. In a typical application, the light source is placed inside a window in a building, and the transmitted cross-polarised light is viewed from outside the window. In the latter method, the light source and the observer are located on the same side of the window glass. Light from the source is polarised, reflected from the back surface of the glass, and viewed by the observer through an analyser. The light may be ordinary light from the visible portion of the spectrum, and the method is carried out in a darkened environment, e.g. at night or in a darkened enclosure to exclude ambient light.

It has been found by trials that visual inspection of tempered window glass in ^" cross-polarised light is able to detect NiS stones and other defects of such size as considered capable of causing spontaneous fracture. In yet another form, the present invention provides a method for detecting inclusions in tempered plate glass, comprising the step of illuminating one side of the plate glass with light, and viewing the light reflected from the glass boundary on the other side of the plate glass to detect inclusions in the glass.

Surprisingly, it has been found that inspection of the glass in reflected light can detect most defects capable of causing spontaneous fracture. This method does not require cross-polarisation of the light. Due to magnification caused by the refractive index effect of the glass, defects as small as 70 microns can be detected by the naked eye using this method.

This invention also provides apparatus for performing the abovedescribed methods.

In yet another form, the present invention provides apparatus for inspection of stress-inducing inclusions and other defects in plate glass, comprising optical magnification means; and mounting means adapted to be releasably affixed to the glass, the optical magnification means being adjustably mounted on the mounting means so as to be moveable over at least a portion of the plate glass.

Typically the optical magnification means comprises a zoom stereo microscope which is fitted with a light source.

In the preferred embodiment, the mounting means comprises a bar having suction pads at its end for releasably attachment to the glass. The microscope is mounted on the bar and slidable therealong to provide movement in a first direction. The microscope is also adjustably mounted on the bar to permit movement in a second direction, orthogonal to the first direction. The first and second directions define a plane parallel to the plane of the plate glass. In this manner, the microscope can be centred on the defect to be inspected.

The depth of the defect in the glass can be measured by the difference in the focal planes of the microscope when focussed alternatively on the glass surface and the defect, with adjustment for the refractive index. A digital Vernier scale may suitably be fitted to the focussing method to allow for accurate measurement. A cross-hair is also fitted to the viewing eye piece of the microscope to achieve better accuracy in measuremen .

In the preferred embodiment, a camera is fitted to a projector lens on the microscope to enable a photographic print to be obtained of the magnified image of the defect, for further analysis and measurement.

The apparatus is able to be used for on site inspection and investigation of defects in window glass by fixing the mounting means to the window such that the microscope is orientated perpendicularly to the plane of the window. Thus, once a defect has been detected using the abovedescribed cross-polarisation techniques, or reflected light technique, the defect can be further inspected, measured and/or photographed with the aid of optical magnification provided by the microscope.

In order that the invention may be more fully understood and put into practice, preferred embodiments thereof will now be described by way of example, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram illustrating apparatus for detecting defects in window glass using transmitted cross-polarised light;

Fig. 2 is a schematic diagram illustrating apparatus for detecting defects in window glass using reflected cross-polarised light;

Fig. 3 is a schematic side elevation of a microscope and camera assembly;

Fig. 4 is a schematic end elevation of a mounting arrangement for the microscope and camera assembly of Fig. 3; and

Fig. 5 is a diagram illustrating image distortion due to the refractive index effect. DESCRIPTION OF PREFERRED EMBODIMENTS The inventors have found that, unexpectedly, small inclusions such as impurity stones and air bubbles in tempered glass are visible in cross-polarised light even though the tempered glass has a great amount of internal stress resulting from the manufacture. Stones and air bubbles in the approximate range of 0.1 to 0.6mm in size can be detected by the eye. Particles less than 0.1mm in size are unlikely to cause spontaneous fracture, while particles greater than 0.6mm should be detectable without the aid of special techniques.

Any suitable light source may be used. Preferably, the cross polarised light is ordinarily light i.e. from the visible part of the spectrum. Typically, the small impurities manifest a characteristic "butterfly" pattern when viewed in cross polarised light.

The glass may be inspected using either reflected or transmitted cross-polarised light. Applications of both techniques to the detection of defects in window glass of a building will now be described with reference to Figs. 1 and 2, respectively.

As shown in Fig. 1, a light box 10 containing a suitable light source, such as one or more fluorescent tubes, is covered with a polariser sheet 11. The light distribution across the sheet 11 should be as uniform as possible. The light box is placed on the inside of a window 12 while the observer 13 views the window from the outside of the building. For highrise buildings, the observer may be transported and supported at the desired position by the window cleaner's gondola or building maintenance unit (BMU). The observer 13 views the light transmitted from light box 10 through a second polariser sheet or "analyser" 14 whose angle of polarisation is orientated 90° to that of polariser 11. For example, the observer 13 may wear goggles fitted with the polarising sheet of the analyser 14.

To avoid interference from ambient light, the inspection procedure is carried out in a darkened environment, typically at night. Alternatively, opaque enclosures may be provided on either side of the window 12 to exclude ambient light.

The characteristic butterfly patterns of NiS stones as small as 100 microns can be detected by the eye using the illustrated technique.

Small stones and other defects in window glass can also be located on site using reflected cross polarised light. As shown in Fig. 2, a light box 20 and associated polariser 21 are placed above the head of the observer 23. That is, both the light box and observer are on the same side of the window. Typically, if the observer is standing in the BMU outside the window, the light box may be mounted to the BMU frame above the observer's head. The observer 23 views light reflected from the window 22 through analyser 24.

It has been found that polarised light in reflected mode reveals strain in the glass in a manner equal to, if not better than, the transmitted mode. In the reflected mode, the incident polarised light is reflected from the back surface of the glass. The plane of polarisation does not change significantly at the reflection surface, but the light passes through the glass twice, which magnifies the strain contrast as seen by the observer.

While the transmitted cross-polarised light method is suitable for use on transparent or viewing window glass only, the reflected cross-polarised light method is suitable for use with both viewing and non- viewing windows, such as spandrel windows.

As with the transmitted light method, the reflected light method is performed in a darkened environment, e.g. at night, or within a space surrounded by opaque screens to exclude ambient light. The visibility of a defect is ^"enhanced as the image of an object inside the glass is magnified by a factor equivalent to the refractive index. Therefore, in glass having a refractive index of 1.518 an object of 70 microns in size appears to be 106 microns to the naked eye.

Even more unexpectedly, the inventors have found that small stones and defects in window glass can be detected using reflected light which is not polarised._. The window glass is viewed in a manner similar to that illustrated in Figure 2, but the analyser polarising sheet 24 is not used. With uniform illumination from the light box, the light reflected from the window displays a uniform white background. Against this background, the small stones or defects cast a dark shadow. Stones as small as 70 microns are visible to the naked eye.

Once a NiS stone or other defect has been detected, it can be analysed in more detail using optical .enhancement techniques. This invention also provides apparatus for optical analysis of a particular defect in the glass, and an embodiment of this apparatus is illustrated in Figs. 3 and 4. The apparatus can be used to: (i) photograph the defect,

(ii) measure its diameter, and

(iii) measure the distance of the defect from the front surface of the glass. The apparatus illustrated in Figs. 3 and 4 can be constructed simply and economically using an off-the- shelf stereo optical microscope and camera. As shown in Fig. 3, a zoom stereo microscope 30, having a magnification range of typically 9 to 60 times, is provided with a fluorescent ring illuminator 31 as its light source. A filter 35 is preferably added to correct for the colour of the fluorescent light source 31, and a cross-hair is fitted to the microscope eye piece 36 to ensure accuracy in focussing.

A 3.3x projector lens 32 is fitted to the microscope 30 to project the image onto^' the back of a camera 33. In addition, a digital Vernier scale 34 is fitted to enable measurement of variation of the focal plane. The digital Vernier scale 35 is suitably connected to a rack-and-pinion type microscope focussing assembly 37. Magnification is controlled by microscope zoom control 38.

The microscope 30 is normally orientated vertically for conventional use. However, when used in this application to vertical window glass, the microscope 30 is mounted to a mounting device 40 (Fig. 4) to operate in a generally horizontal orientation. The microscope assembly 30 is fixed in a cradle portion 41 of a cradle and slide assembly. The cradle portion 41 is attached to a slide portion 47 which slides on a horizontal bar 43.

Vacuum pads 44 are mounted to both ends of the horizontal bar 43. These vacuum pads 44 are used to releasably attach the mounting device 40 to a window, and the vacuum pads 44 may be of the type used as suction lifting-units for plate glass. To attach the mounting device 40 to a window, the vacuum pads 44 are placed against a window, and a vacuum is created by turning handles 45 on the pads. The cradle and slide assembly allows the microscope 30 to move in the X and Y directions. The slide portion 47 is a sleeve-like assembly which surrounds the horizontal bar 43. Fixed Teflon-capped screws in the front and back sides of the sleeve-like portion of the slide 47 maintain torsional rigidity of the cradle and slide assembly relative to the horizontal bar 43.

The slide 47 is supported on the bar 43 by two screws 46 which are tipped by Teflon (Trade Mark) plugs. The two screws 46 are rotated simultaneously to move the cradle and slide assembly vertically (in the Y direction). Because of the low friction contact between the Teflon plugs and the top of the horizontal bar 43, the cradle and slide assembly is able to be translated smoothly in the horizontal (or X) direction.

The microscope is focussed (and thereby moved in the Z direction) using the rack and pinion focussing device 37 on the microscope 30 itself. The cradle and slide assembly, and the horizontal bar 43, are suitably made of aluminium, or other strong lightweight material.

In use, once a defect has been detected usually in the window glass using cross-polarised light, the microscope and camera assembly are mounted onto the window using mounting device 40, and the microscope is centred onto the defect. The defect is able to be photographed through the microscope. A ruled scale is also photographed in order to calibrate the magnification onto the camera. For the illustrated embodiment, at maximum magnification of 90 times, a 10 x 15cm photographic print corresponds to a field view of 1.2 x 1.8mm on the window glass. The size of the defect in the glass can be measured from the photographic print.

The view through the stereo microscope 30 is off-centre, i.e. the optic axis is not perpendicular to the window glass surface. The use of off-centre optics provides an advantage in that, except for an object on the back surface of glass, it is possible to photograph the front of the object directly, and photograph the back of the object by viewing its reflection from the back surface of the glass.

The depth of the defect within the glass is measured by placing a mark on the glass surface, e.g. by using a marker pen. The microscope is focussed on the glass surface, and the digital Vernier scale is set at 0. The microscope is then focussed on the defect itself, and the depth of the defect can be read off the digital scale 34. The digital scale is accurate to 0.01mm. However, the accuracy of focus with the eye piece is typically only 0.1mm. The cross-hair in the microscope eye piece is therefore very advantageous in obtaining accuracy in the focus position. The thickness of the glass is^" measured in a similar manner, i.e. by focussing on the mark on the front surface, resetting the digital scale to 0, then focussing on the reflection in the back surface of the front-surface mark. The thickness of glass is half the distance measured by the digital scale 34.

The ability to measure the position of the defect within the glass is highly advantageous. In tempered glass, approximately 20% of the glass thickness at both surfaces is under high pressure (the compression zone), whereas the middle 60% of the glass thickness is under high tensile force (the tension zone) . A defect in the compression zone will generally be harmless. However, a defect in the tension zone may cause the glass to shatter spontaneously. By measuring the distance from the front surface, and the glass thickness, it is possible to ascertain whether the defect is in the tension zone or not. The defects which are dangerous in tempered glass are found to range in size from typically 0.1mm to 0.6mm. The abovedescribed focussing method enables these defects to be depth-located in the glass with an accuracy of 0.1mm, which is quite adequate.

The refractive index of glass has an appreciable effect on the perceived size and position of a defect within the glass. As shown in Fig. 5, light entering the glass is bent at the surface due to the change in refractive index at the boundary, and the apparent position of the stone is closer to the observer than the actual position. Further, the apparent image is magnified relative to the actual object. It is well known, that because of this effect, the defect will appear to be closer to the front surface by a factor of the refractive index, and is magnified by a factor of the refractive index. That is, for normally tempered window glass, the true distance equals the apparent distance multiplied by 1.52 (the refractive index of the glass), and the true size equals the apparent size divided by 1.52. In measuring the defect size and position, the effect of the refractive index is taken into account.

The foregoing describes only some embodiments of the invention and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention. For example, the glass can also be inspected at manufacturing stage and not only after installation. Further, the abovedescribed inspection methods can be automated using a computer-controlled scanning mechanism, a video camera, and a computer and/or other data storage means.

Claims

CLAIMS :

1. A method of detecting stress-inducing inclusions in tempered glass, comprising the steps of illuminating the glass with polarised light; viewing the illuminated glass through a polarising medium whose plane of polarisation is orientated orthogonally to the plane of polarisation of the illumination; and detecting localised stress patterns in the glass indicative of the inclusions.

2. A method as claimed in claim 1, wherein the glass is window plate glass and the method is performed on site in a darkened environment.

3. A method as claimed in claim 2, wherein the illuminated glass is viewed from the opposite side of the plate glass to the source of illumination using light transmitted through the glass.

4. A method as claimed in claim 2, wherein the illuminated glass is viewed from the same side of the plate glass as the source of illumination using light reflected from the distal surface of the plate glass.

5. A method of detecting inclusions in tempered plate glass, comprising the steps of illuminating one side of the plate glass with light, and viewing the light reflected from the glass boundary on the other side of the plate glass to detect inclusions in the glass.

6. Apparatus for inspection of stress-inducing inclusions and other defects in plate glass, comprising optical magnification means; and mounting means adapted to be releasably affixed to the glass, the optical magnification means being adjustably mounted on the mounting means so as to be moveable over at least a portion of the plate glass.

7. Apparatus as claimed in claim 6, wherein the optical magnification means comprises ^"a zoom stereo microscope.

8. Apparatus as claimed in claim 7, wherein the mounting means comprises an elongate member having suction pads mounted thereto for releasable attachment to the glass; a microscope mount slidable along the elongate member, and moveable in a direction transverse to the axis of the elongate member, the mount being adapted to have the microscope mounted thereon in use.

9. Apparatus as claimed in claim 8, wherein the microscope has a Vernier scale thereon for measuring variation in the position of the focal plane.

10. Apparatus as claimed in claim 8, wherein the microscope has a projector lens fitting, further comprising a camera mounted to the projector lens fitting.

11. Apparatus as claimed in claim 10, wherein the optic axis of the microscope when mounted on the microscope mount, is obliquely angled to the surface of the plate glass.