GB2240997A

GB2240997A - Strand or rope product of composite rods

Info

Publication number: GB2240997A
Application number: GB9003710A
Authority: GB
Inventors: John Mawson Walton; Chin Tong Yeung
Original assignee: Bridon PLC
Current assignee: Bridon PLC
Priority date: 1990-02-19
Filing date: 1990-02-19
Publication date: 1991-08-21
Anticipated expiration: 2010-02-19
Also published as: GB2240997B; GB9003710D0

Abstract

The product comprises a multiplicity of fibre-reinforced plastics rods extending helically, each rod being substantially free from residual bending stress. The rods are bent while plastically deformable due to heating immediately prior to or during bending, thereby preventing failure of the reinforcing fibres. The rods are uniaxially reinforced, and in a section of the rod, fibre lengths are proportional to the radii of curvature of fibres in the rod. The plastics may be a thermoplastic. Each rod may have a coating which resists heat and/or friction and/or adhesion. The rods may be tensioned during bending.

Description

STRAND OR ROPE PRODUCT OF COMPOSITE RODS This invention relates to strand and rope products comprising a multiplicity of helically extending rods of composite materia i.e. fibre-reinforced plastics material.

Whereas techniques and equipment (GB-A-2 152 089 and Gb-A-2 145 128) exist for the manufacture of long-lay strands from rods of high strength composite material, the resultant products are necessarily limited to structural applications because of the long lay length.

These techniques cannot presently be applied to shorter lay strands and ropes without exceeding the strain limit of the composite material and causing substantial damage to the rods.

The present invention provides a strand or rope product comprising a multiplicity of rods extending helically, each said rod comprising a matrix of plastics material reinforced by fibres, characterised in that each said rod is substantially free from residual bending stress in the absence of stress applied externally to the product.

Preferably the rod is uniaxially reinforced, i.e. the fibres are substantially orientated in the axial direction of the rod.

The fibres preferably occupy 50 to 80% (typically 65%) of the total volume of the rod.

Such a product can be produced, according to the invention, by a method of manufacturing a strand or rope product, comprising the steps of providing a multiplicity of rods comprising a matrix of plastics material reinforced by fibres, and forming the rods into a strand or rope product in which the rods extend helically, the forming step including subjecting each rod to bending, characterised by heating the rods in such a manner that the matrix material undergoes plastic deformation during bending.

Preferred and optional features are set forth in the sub-claims.

The invention will now be described further, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagram of an elemental section of a bent rod forming part of a strand or rope product; Figure-2 is a diagrammatic cross-section of a bent rod forming part of a strand or rope product; Figure 3 is a diagrammatic cross-section of a strand rope product.

We have devised a method of strain relief which permits composite materials to be deformed beyond their normal strain limit so that strand and rope products 1, 2 (Figs. 2 and 3) can be manufactured therefrom at much shorter lays than was previously possible. This can be achieved by elevating the temperature of the composite rods 3 immediately prior to and/or during the bending to soften the matrix material. This technique is applicable to any composite rods constructed from thermoplastic matrix materials. Simultaneously with softening of the matrix a controlled bending is introduced into the rods consistent with the helical curvature of the end product. This bending operation may thus precede the stranding or rope-closing operation or be carried out concurrently.

The bending operation may be advantageously accompanied by a controlled application of tension to the rods during the bending operation, to ensure that the fibres on the inside of the bend do not go into compression and suffer kinks. Also for rods comprised of uniaxial reinforcing fibres (e. g. of aramid, carbon, or glass), the plane of bending may be beneficially rotated about the axis of the rods to ensure a uniform distribution of the strains due to bending. Alternatively the uniform distribution of strain may be achieved by introducing an a helical lay into the reinforcing fibres of the rods prior to or during their manufacture, or by using discontinuous reinforcing fibres, such as those offered by "Verton" (Registered Trade Mark of ICI) long fibre reinforcement.

The purpose of the hot-forming process is to impart a permanent helical shape to the rods and thereby substantially relieve the residual stresses that would otherwise result from the forming operation (i.e. at ambient temperature). This objective may also be expressed in terms of the disposition of the reinforcing fibres 4 within the helical rods 3, by considering an elemental length of rod between two planes normal to its axis, as shown diagrammatically in Figure 1, which shows a section through the rod in the plane of bending. The neutral axis AB of the rod, which has a radius of curvature R, remains unchanged in length when the rod is bent but fibres remote from the neutral axis will assume a different length, depending on their radial distance from the neutral axis.The extremes of this effect are represented by the inner and outer most fibres where if r is the radius of the rod; Length of neutral axis (AB) = RtE Outer-most fibre length = (R+r)8 Inner-most fibre length = (R-r)AO Therefore in a properly formed rod the length of fibre in an elemental section should be directly proportional to its radius of curvature. If this condition is not achieved there will be in-built strains in the fibres giving rise to residual stresses.

The rods 3 may also be sheathed or coated with a heat-resistant material (that is being able to withstand the temperature of the heating operation without melting), to prevent cohesion of adjacent rods during or immediately after the strand or rope forming process.

If this is also a low-friction material such a PTFE then it will serve the second purpose of reducing abrasive wear between the rods when the strand or rope product is worked.

The heating of the rods may be effected for example by exposure to radiant heating or by forced convection through a heated fluid. To facilitate the forming process the rods should be heated to a temperature above the softening point and below the crystalline melting point of the matrix material. For example, with polypropylene the Vicar softening point typically lies between 145-150-C, and the crystalline melting point is between 165-170*C, depending upon the exact grade of material employed.

With some materials it may even be desirable to exceed the melting point of the matrix locally in order to expedite the process. This will be particularly appropriate where large rod sizes are involved, having regard to the heat transfer rates through the material, and/or where the matrix material is enclosed within a high-temperature resistant coating.

If necessary, cooling (e.g. by forced convection or liquid spraying) is applied to the strand or rope product after the forming process.

Claims

CLAIMS: -

1. A strand or rope product comprising a multiplicity of rods extending helically, each said rod comprising a matrix of plastics material reinforced by fibres, characterised in that each said rod is substantially free from residual bending stress in the absence of stress applied externally to the product.

2. A product as claimed in claim 1, in which each said rod is uniaxially reinforced and in which, in an elemental section of the rod between planes normal to its axis, the section lengths of the fibres are proportional to their radii of curvature in the rod.

3. A product as claimed in claim 1 or 2, in which the matrix is a thermoplastic material.

4. A product as claimed in any preceding claim, in which the curvature of at least some of the helical rods is such that a similar straight rod bent at room temperature to the same curvature would suffer failure of at least some of its reinforcing fibres under the resulting bending strain.

5. A product as claimed in any preceding claim, in which each said rod has a sheath or coating which resists heat and/or friction and/or adhesion to a greater extent than the matrix material.

6. A method of manufacturing a strand or rope product, comprising the steps of. providing a multiplicity of rods comprising a matrix of plastics material reinforced by fibres, and forming the rods into a strand or rope product in which the rods extend helically, the forming step including subjecting each rod to bending, characterised by heating the rods in such a manner that the matrix material undergoes plastic deformation during bending

7. A method as claimed in claim 6, including subjecting the rods to tension during bending, so as to prevent compressive strain of the reinforcing fibres.

8. A method as claimed in claim 6 or 7, in which the rods are heated during bending.

9. A method as claimed in any of claims 6 to 8, in which the rods are heated immediately before being bent.

10. A method as claimed in any of claims 6 to 9, in which the heating is such tht the temperature of the rod during bending is between the softening point and the melting point of the matrix material.

11. A method as claimed in any of claims 6 to 9, in which the heating is such that part of the cross-section of the heated rod is at a temperature above the melting point of the matrix material.