GB2194256A

GB2194256A - Strain indication in webbing straps and ropes

Info

Publication number: GB2194256A
Application number: GB08718685A
Authority: GB
Inventors: Dr Ian Michael Hutchings
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-08-13
Filing date: 1987-08-07
Publication date: 1988-03-02
Anticipated expiration: 2007-08-07
Also published as: GB8718685D0; GB8703824D0; GB2194256B; GB8619723D0

Abstract

A webbing strap or rope is provided with means for indicating when it has been subjected to excessive tensile loading, by the incorporation of strain-sensitive material either applied to its surface as a label patch or sheath, or within its structure as one or more threads or strands. The strain-sensitive material may conveniently be a plastics material which undergoes permanent stress-whitening when a predetermined tensile load in the webbing or rope is exceeded. The stress-whitening polymer may be arranged so as to cause warning lettering or markings to become visible or to be obscured.

Description

SPECIFICATION Improvements in and relating to webbing straps and ropes Background to the Invention Webbing straps are widely used to carry tensile loads, in applications such as safety harnesses, parachute harnesses, load restraint straps and industrial lifting slings. In all these applications there is the possibility that the webbing will be weakened by overloading, and that in subsequent use it will fail at a lower tensile load than similar but undamaged webbing. In other applications ropes are used, and may suffer from similar weakening by overloading. In many applications the maximum load which should be applied to the webbing strap or rope is defined by a design code or code of practice, and should not be exceeded for fear of breakage.

It - is the object of the present invention to provide an improved method of making articles incorporating webbing straps or ropes, which provides a readily apparent indication if the article or its webbing or rope parts have been subjected in use to a load greater than a certain threshold.

Summary of the Invention According to the present invention there is provided either attached to the exterior of or incorporated into the structure of the webbing strap or rope a device which will indicate by means of a readily detectable and irreversible change when some part of such webbing strap or rope has been subjected to a tensile load exceeding a certain threshold. Such device may be applied either during the manufacture of the webbing or rope or article fabricated from such webbing or rope, or may be applied as a post-manufacturing step.

Such overload indicating device may conveniently be in the form of a patch, label or sleeve of a strain-sensitive material applied to the exterior of the rqpe or webbing and affixed by stitching, adhesive bonding, crimping of ferrules or in some other way. Alternatively it may be in the form of one or more strands or threads of strain-sensitive material piaited, woven or braided with the other strands to form the rope or webbing. In a component formed by a warp and weft weave, such as woven webbing, one or more of the warp threads Fan be formed from or contain a strain-sensitive material.

The strain-sensitive material typically comprises a polymerised plastics material which on being subjected to strain beyond a certain limit exhibits the phenomenon of "stress-whitening". Such materials, for example poly(vinyl chloride), tend to change from a relatively dark colour to a lighter colour or even to a white appearance, or from transparent to opaque, when subjected to strains beyond a certain threshold. Such colour or transparency changes tend to be permanent.

The strain-sensitive material provides a readily observable indication, through permanent change in its visual appearance, when a certain threshold load has been exceeded in the webbing or rope. This change can be detected by the user or by an inspector, and can be used to indicate that the webbing or rope or component incorporating such webbing or rope has been loaded beyond a predetermined safe limit and so should be removed from service.

By careful choice of material so the stresswhitening may be employed to render visible or to obscure lettering or other marks. For example, by suitable choice of weaving patterns for the strain-sensitive threads, lettering may be created on the surface of webbing which becomes visible when the threads change in appearance by stress-whitening.

Since it is possible by choice of stress-whitening material for a patch, label or sleeve to effect a change from black to grey or even to white, it is possible to reveal printing or other marking, such as a warning, which is coloured black and is not normally visible in the surface of the material while it is black, but immediately becomes visible when the stress-sensitive material becomes grey or white throwing the black lettering or other marking into relief.

Black or other coloured lettering on an initially transparent stress-whitening material may be obscured by providing a backing layer to the material of a matching colour. When the material becomes opaque grey or white, the lettering or marking is thrown into relief.

The load in the webbing or rope at which the change in appearance of the stress-sensitive material occurs may conveniently be chosen to be lower by some suitable factor of safety than the load at which tensile failure of the webbing or rope occurs; such factor of safety or load may be specified by a design code or other specification. The visual indication may be arranged to occur at the appropriate load by suitable choice of material for the stress-sensitive component, or by suitable geometrical arrangement of the fibres or label with respect to the tensile axis of the webbing or rope. For example, a strip patch applied to the surface of a webbing strap may be attached at a suitable angle to the tensile axis to ensure that the change in appearance occurs at the appropriate tensile load in the webbing.

In some embodiments of the invention it may be convenient to protect the stress-whitening material from damage or distortion by bending, indentation, impact or abrasion. In such embodiments it may be convenient to provide a protective sleeve or sheath wholly or partly surrounding the length of webbing or rope carrying the strain-sensitive material. The protective sleeve may be rigid or flexible, and may be made from a transparent material, or be made with a window or aperture, so that the change in appearance of the strain-sensi tive material is visible.

Although the invention has been described so far almost exclusively with reference to stress-whitening polymer, it is to be under stood that the invention is in no way limited to such materials.

The invention thus lies in any component or article when manufactured in accordance with the present invention and in any rope or webbing strap or article incorporating such having as part of its structure or having ap plied or bonded to at least part of its surface a strain-sensitive material which changes its visual appearance if subjected to a strain of greater than a predetermined amount.

The invention will be described further by way of example only, with reference to the accompanying drawings in which: Figure 1 is a plan view of a length of webb ing, which may for example form part of a vehicle seat belt or lifting sling or restraint strap; Figure 2 is an edge view thereof; Figure 3 is a graphical representation of force versus extension for the webbing strap of Figs. 1 and 2; Figure 4 is a view of ropes to which stress whitening material has been applied, and Figure 5 is a sectional view of an arrange ment to render printing visible by stress-whi tening.

In Fig. 1 a webbing strap 10 has stitched to its surface a strip or patch of stress-whitening polymer 12, a double line of stitching being denoted at 14 and 16. By choice of material and thickness, so the strip can be made to change its appearance due to the stress-whi tening phenomenon if the overall tensile load on the webbing strap 10 causes greater than a predetermined lengthwise extension of the webbing.

The graph of Fig. 3 demonstrates the force which is needed to cause actual fibre break age in the webbing (ie 3.3 kN for the material in question) and the level of 1.9 kN beyond which it is felt that the webbing should not be loaded in normal use. Having defined this force, it is a simple matter to select an appro priate strip of stress-whitening polymer which if subjected to the unit extension exhibited by the webbing from no load to 1.9 kN will itself exhibit a stress-whitening change at or beyond 1.9 kN loading, and to stitch the appropriate strip onto the surface of the webbing 10 to form a patch 12. The latter -may for example be a label containing the manufacturer's name or other information or it may simply be a warning strip to indicate if the safe loading limit of the webbing has been exceeded.The values shown in Fig. 3 are derived from tests on webbing 30 mm wide. The actual values will obviously vary with the type of webbing and its dimensions and tests are needed to ascertain the appropriate values for different types of webbing, as used for example in lifting slings or vehicles seat belts.

It is then a simple matter for an inspector to spot that a webbing component, such as a lifting sling or vehicle seat belt, has been subjected to an undesirable level of loading and to withdraw the component from service.

Similar considerations apply to the application of the invention to ropes and hawsers as illustrated in Fig. 4 where the strand or sheath of stress-whitening material will indicate if the rope or hawser has been subjected to overloading. A strand of stress-whitening material 40 may be woven into the structure of a rope, or may be incorporated into the weave of a braided rope or braided rope covering, or more than one strand may be used. Alternatively, a sleeve partly or wholly of stress-whitening material 42 may be attached to the rope, by adhesive bonding or by stitching or by crimping of ferrules or by some other method.

Fig. 5 illustrates how stress-whitening can be used to render visible previously obscured printing. A piece of suitable transparent poly mer 50, which may be in the form of a sheet, strip or sleeve or other shape, is coated on the one side with a black coating 52 and is marked with printing or other markings of black or any other colour on the other side 54. The material is viewed from the latter side, and the markings are invisble against the black background. When the polymer stresswhitens, it becomes opaque and light in colour, rendering the markings 54 visible.

Claims

1. A webbing strap or rope to which is affixed a sleeve, patch or label made in whole or in part from a strain-sensitive material which changes in visual appearance on being strained beyond a certain predetermined limit.

2. A webbing strap or rope incorporating in its structure at least one thread or strand made in whole or in part from a strain-sensitive material which changes in visual appearance on being strained beyond a certain predetermined limit.

3. A webbing strap or rope according to either of the preceding claims wherein the strain-sensitive material is arranged so as to render visible or invisible lettering or other markings when the load on the webbing or rope exceeds a certain level.

4. A webbing strap or rope according to one of the preceding claims wherein the strain-sensitive material is protected against damage by a sleeve, patch or sheath or some other protective device.

5. A webbing strap or rope according to one of the preceding claims wherein the strain-sensitive material is a plastics material which exhibits the phenomenon of stress-whi tening.

6. A webbing strap or rope according to claim 5 in which the plastics material is composed wholly or in part of poly(vinyl chloride).

7. A webbing strap substantialy as herein described and illustrated in Figs. 1 and 2.

8. A rope or hawser substantially as herein described and illustrated in Fig. 4.

9. A lifting sling, safety harness, vehicle seat belt or other assembly or article containing or composed of at least one webbing strap or rope according to one of the preceding claims.