GB2111227A

GB2111227A - A stress observation device

Info

Publication number: GB2111227A
Application number: GB08231881A
Authority: GB
Inventors: Trevor Leslie Lowe
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-11-11
Filing date: 1982-11-09
Publication date: 1983-06-29
Also published as: GB2111227B

Abstract

A stress observation device has a platform comprising a sheet (11) of soft photo-elastic material on a rigid glass plate (12). The upper surface of the sheet, on which is placed a load whose pressure distribution pattern is to be observed, is made light absorbent, either by a thin flexible overlay (16) or a coating. Polarised light from a source (7,9) is directed obliquely up at the underside of the platform, and the refracted and reflected beam passes down through an analyser (13). The image is observable, directly or by mirrors, or recordable by a camera. The load zone may be indicated by suitable markings. <IMAGE>

Description

SPECIFICATION A stress observation device This invention concerns a stress observation device. It is primarily concerned with the determination of stress patterns generated by a load and it uses a photo-elastic technique.

Transparent photo-elastic material in conjunction with polarised light is commonly used for determining stress patterns. A basic method will be described below and it will be noted that it depends on the light passing normally through a planar model of the part to be tested made in photo-elastic material. However, not all loading tests lend themselves to this technique, and it is of course not applicable when the light cannot be passed through the object whose load distribution is to be determined.

One particular area where this difficulty arises is in orthopaedics. For example, it is extremely useful for a chiropodist to known how a foot bears on the ground. If there is a significant departure from a normal pressure pattern then measures can be taken to correct this with padded or specially formed footwear. Likewise, it is useful to know how a seat should be contoured.

It is the aim of this invention to provide means whereby stress patterns derived from opaque objects can readily be determined.

According to the present invention there is provided apparatus for indicating or measuring the contact pressure distribution over a surface, comprising a transparent rigid sheet, a photoelastic sheet, resilient compared with the rigid sheet and overlying one side thereof, the side of the photo-elastic sheet remote from the rigid sheet being light absorbing or having a coating or flexible overlay which will absorb light, a polarised light source arranged to direct light obliquely at the other side of the rigid sheet, and a polarising screen arranged to receive light reflected and refracted by the sheets and to provide on the side remote from the incident light a stress pattern image derived from a load applied to said remote side of the photo-elastic sheet.

Preferably the light source will provide substantially plane polarised light, although it could be circularly or elliptically polarised.

However, this would be a rather more expensive method, with only marginal advantages.

It is advantageous to mark the zone to be loaded so that it is visible on both sides of the surface. Thus, for example, when testing a foot, a profile of a foot will be marked on the upper side of the platform made up from the two sheets, while a corresponding profile will be visible from underneath. When there is a light absorbing overlay over the photo-elastic sheet the load zone will conveniently be indicated by a transparent sheet marked with that zone interposed between the overlay and the photo-elastic sheet.

For a better understanding of the invention one embodiment will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a diagram showing simple apparatus for observing photo-elastic patterns, and Figure 2 is a side elevation, in diagrammatic form, of apparatus for observing stress distribution generated by a load such as a foot.

In Figure 1, a light source 1 provides a substantially parallel beam of unpolarised light which is incident normally on a sheet of polaroid (RTM) 2, termed the polariser. The resultant plane polarised beam of light 3 then passes through a sheet 4 of isotropic material, such as that known as 'araldite' (RTM) or'perspex' (RTM), before reaching a further sheet of polaroid 5, termed the analyser. If the polariser 2 and the analyser 5 have their planes of polarisation crossed then an observer's eye 6 will see no light emerging from the analyser 5. However, if the sheet 4 is subject to mechanical strsss it becomes doubly refracting.

Light then passes through the analyser 5 and a pattern of light and dark areas is seen by the eye 6. By use of various combinations of plane polarised or circularly polarised light and monochromatic light or light with a spread of wavelengths (for example, white light) it is possible to obtain qualitative and/or quantitive information about the mechanical stress distribution within the sheet 4. This technique is used on models of engineering structures to detect potential weak points. However, it does depend on the model being made of translucent material.

In Figure 2 a light source 7 emits a substantially parallel beam of light 8 which passes through a polariser 9 to emerge as a polarised beam 10 which strikes a sheet of photoelastic materiall 1 from underneath at an oblique angle. It may be necessary to interpose a heat filter immediately after the light source and before the polariser 9, to protect the latter. The sheet 11 is of a resilient material with a modulus of elasticity of the order of 1 MNm-2 and a fringe value of the order of 0.15 kNm-l. It is translucent and a few millimetres thick, usually in the range 3-6 mm.A type of translucent urethane rubber is suitable and commercially available, with a modulus of elasticity in the range 0.7-4.1 MNm-2 and a fringe value in the range 0.14- 0.17 kNm-1. The sheet 1 1 is supported by, but not adhered to, a rigid transparent sheet of glass 12. In order to prevent the sheets 1 1 and 12 sticking together and forming residual stress patterns after a load has been removed, it is desirable to interpose a thin transparent coating of a material such as a mould release agent or dusting powder.

The refraction of the beam 10 is not illustrated, but the light does pass freely through the glass and the photo-elastic material. At the interface and at the upper surface of the sheet 11 there is reflection of a small proportion of the light and this passes downwardly at an oblique angle to an analyser 13. This may be viewed directly or there may be a mirror, camera or other optical device at 14 to carry the image elsewhere or to record it.

In use, a load such as a foot 15 bears on the platform provided by the sheets 1 and 12, and the stress patterns generated in the sheet 11 are viewed or recorded at or by means of devices 14.

There is also illustrated a thin flexible overlay sheet 16 which is of light absorbing material.

Several materials am suitable, among them black tissue paper and black p.v.c. or polythene. The sheet 16 is provided so that an excessive amount of light is not reflected at the upper surface of the sheet 11, thus overwhelming the light reflected at the lower surface and rendering interference patterns no longer clear. Instead of a sheet 16 the upper surface of the sheet 11 may simply be coated with a light absorbing film.

The platform is shown as supported on a rigid frame 17, which can also provide a mounting for the light source 7 and optical apparatus 14.

In order to define the load area, where the light beam is concentrated, the upper face of the platform may be marked, for example with the profile of a foot. This can be on the exposed surface of the sheet 16. It is also useful to have this reference profile visible at 14, and this can be done by placing a thin transparent sheet, of acetate for example, marked with the profile, between the sheets 11 and 16.

It will be understood that this technique can be used for many other applications than observation of foot pressure distribution. Its main advantage is that pressure patterns generated by opaque objects can be displayed and recorded directly, without any need for the construction of translucent models.

Claims

1. Apparatus for indicating or measuring the contact pressure distribution over a surface, comprising a transparent rigid sheet, a photoelastic sheet, resilient compared with the rigid sheet and overlying one side thereof, the side of the photo-elastic sheet remote from the rigid sheet being light absorbing or having a coating or flexible overlay which will absorb light, a polarised light source arranged to direct light obliquely at the other side of the rigid sheet, and a polarising screen arranged to receive light reflected and refracted by the sheets and to provide on the side remote from the incident light a stress pattern image derived from a load applied to said remote side of the photo-elastic sheet.

2. Apparatus as claimed in claim 1, wherein the light source provides substantially plane polarised light.

3. Apparatus as claimed in claim 1 wherein the light source provides circularly or elliptically polarised light.

4. Apparatus as claimed in claim 1, 2 or 3, wherein marking of a zone to be loaded is visible on both sides of the surface.

5. Apparatus as claimed in claim 4, wherein there is a light absorbing overlay for the photoelastic sheet, a transparent sheet with a load zone marking is interposed between the overlay and the photo-elastic sheet, and a load zone marking is provided on the remote side of the overlay.

6. Apparatus for indicating or measuring the contact pressure distribution over a surface, substantially as hereinbefore described with reference to the accompanying drawing.