WO2015071858A1

WO2015071858A1 - Hybrid fitting for termination of cables comprising high strength and high modulus unidirectional fibres

Info

Publication number: WO2015071858A1
Application number: PCT/IB2014/066024
Authority: WO
Inventors: Luca Mezzo; Michael ORANGE; Francesco ZAMPACAVALLO; Jonathan Duval
Original assignee: Future Fibres Rigging Systems S.L.
Priority date: 2013-11-14
Filing date: 2014-11-13
Publication date: 2015-05-21

Abstract

A termination fitting for cables comprising a number of yarns in tum made up of a large number of fibres of high tensile strength and high modulus of elasticity material comprises two main elements, an external socket and an internal spike. Both are elongated members, and are radially symmetric. The socket has a tapered bore defining a tapered inner surface and the spike a tapered outer surface, shaped cooperatively such that when they are assembled to one another the annular space between these surfaces is of substantially uniform cross-sectional area along their respective lengths, so that the yarns are confined along the entire length of the spike and socket.

Description

Hybrid Fitting For Termination Of Cables Comprising High Strength And High

Modulus Unidirectional Fibres

Field of the Invention

This application relates to cable terminations for securing cables made of high tensile strength and high modulus of elasticity fibres, such as carbon fibres, to structure to be supported thereby, such as the masts of a boat or ship, or a bridge.

It will be apparent to those of skill in the art that the requirements on such cable terminations are simple, but demanding. The termination must transfer the loads exerted by tension on the cable, both static and dynamic, to the structure to be supported and the support structure, e.g., the hull of a ship or boat or the towers and ground attachment points in the case of a bridge. It is also desired to provide a cable termination that is not excessively bulky, preferably having an outside diameter no more than three times the cable diameter.

According to the present invention a cable termination is provided which is better capable of transferring the loads on a cable to structure to be supported and the support structure, and which is particularly suitable for terminating cables made of high tensile strength and high modulus of elasticity fibres, e.g., carbon fibres.

Summary of the Invention

The termination fitting of the invention comprises two main elements, an external socket and an internal spike. Both are elongated members, and are radially symmetric. The socket has a tapered bore defining a tapered inner surface and the spike a tapered outer surface. These tapers differ from one another such that when they are assembled to one another the annular space between these surfaces is of substantially uniform cross-sectional area along their respective lengths.

In order to assemble the termination fitting of the invention to a cable, the yarns of a cable are separated from one another and splayed as evenly as possible between the inner surface the of socket and the outer surface of the spike. A curable material, such a heat-curing epoxy, is applied to the surfaces of the yarns where they are to be received in the fitting. Pressure is applied to urge the spike into the socket, firmly confining the yarns. The curable material is then cured, forming a solidified mass in which the individual yarns are captured between the spike and socket, securing the termination to the cable. A whipping may then be applied, covering the proximal end of the socket, where the cable enters the socket. A cap may be threaded onto the end of the socket, confining the spike therein, and to provide a means of connecting the termination to the associated structure, such as the hull or mast of a ship.

Brief Description of the Drawings

The invention will be better understood by reference to the accompanying drawings, in which:

Figure 1 shows the cable termination of the invention assembled to a cable comprising yarns made of high tensile strength and high modulus of elasticity ("HTS" and "HM" hereinafter) fibres, e.g., carbon fibres. In Fig. 1, the termination and cable are shown split along their longitudinal centerline, so as to show an elevation view in the upper half of the drawing and a cross-section in the lower half of the drawing.

Fig. 2 is a cross-section taken along line 2 - 2 of Fig. 1.

Fig. 3 shows the components of the termination in an exploded perspective view. Description of the Preferred Embodiments As can be seen, the termination fitting of the invention comprises two main elements: an external socket 10 and an internal spike 12. Both are elongated members, and are radially symmetric. The socket 10 has a tapered bore defining a tapered inner surface 10a and the spike a tapered outer surface 12a. The tapers of surfaces 10a and 12a are cooperatively designed such that the annular space therebetween is of substantially uniform cross-sectional area along their lengths, so that the individual yarns 20 of the cable 16 are securely received between these cooperating surfaces.

The typical cable comprises a large number of yarns 20, each yarn 20 in turn comprising a large number of individual fibres of an HTS and HM material, e.g. carbon. The individual fibres are secured to another to make up the yarns 20 by a matrix which is typically an elastomeric material, so as to allow the cable to be coiled, but which can also be a rigid thermosetting material. As shown in Fig. 3, in order to assemble the termination fitting of the invention to a cable 16, the HTS and HM yarns 20 of cable 16 are separated from one another and splayed as evenly as possible between inner surface 10a of socket 10 and outer surface 12a of the spike 12. A curable material, such as a heat-curing epoxy, is applied to the surfaces of the yarns where they are to be received in the fitting. Pressure is applied to urge the spike 12 into the socket 10, firmly confining the yarns between their cooperating surfaces 10a and 12a. The curable material is then cured, forming a solidified mass in which the individual yarns are captured between the spike 12 and socket 10, and thereby securing the termination to the cable. A whipping 14 may then be applied, covering the proximal end of the socket, where the cable enters the socket. A cap 18 may be threaded onto the end of the socket, confining the spike 12 therein, and to provide a means of connecting the termination to the associated structure, such as the hull or mast of a boat or ship, or to the towers, ground structure, and roadway of a bridge.

As mentioned above, and according to an important aspect of the invention, the generally tapered inner surface 10a of socket 10 and the outer surface 12a of spike 12 are shaped cooperatively so that the annular space between them is of approximately constant cross- sectional area. This typically requires that the angles made by the inner surface 10a of socket 10 and the outer surface 12a of spike 12 with respect to their centerline differ from one another, as shown in Fig. 1, and may require that one or both of the inner surface 10a of socket 10 and the outer surface 12a of spike 12 are curved in profile. In this manner, the yarns 20 fill the annular space evenly along its extent, so that the tensile load on the cable 16 in use is distributed evenly over the length of the yarns within the termination and around its circumference. This is a distinct improvement over prior art terminations involving conical sleeves receiving correspondingly-conical inner members, that is, where the angles of the conical inner surface of the sleeve and the outer surface of the inner member are the same. In such case, when the sleeve is tightened over the inner member so as to confine the yarns or wires of a cable therebetween, either strictly by friction or with the addition of a hardenable or curable material, the load is concentrated at the proximal end of the termination, where the yarns are most closely spaced. Further, and as discussed more fully below, the fact that a solid member is created with the yarns in a matrix of a solid material results in the solid member being mechanically confined between the socket 10 and spike 12.

Figure 2 is a partial cross-section taken through the socket 10, spike 12, and yarns 20 showing that the yarns 20 are closely confined in the space between the inner surface 10a of the bore in the socket 10 and the outer surface 12a of spike 12. The interstices between the yarns 20 are filled by curable resin 22.

Figure 3 is a perspective exploded view of the cable and termination of the invention, illustrating the principal steps performed to assemble a carbon fiber cable to the termination. As will be understood by those of skill in the art, the cable 16 is first passed through the bore of the socket 10, a curable resin, e.g., epoxy, is applied to the yarns, and the yarns are splayed apart as evenly as possible. The spike 12 is then inserted, pressure applied to force the spike firmly into the socket, and the resin cured. The spike may include a protruding member to be secured to the structure to be supported (for example a mast of a boat) or to the supporting structure (in the example, the hull of the boat). Alternatively the connection can be made via a cap 18 or the like secured to internal or external threads on the socket 10, as shown in Fig. 1. Wrench flats may be provided on the socket and cap for this purpose, as shown at 10b and 18a. The cap 18 can be employed to exert pressure between the spike and socket, or this can be performed in a separate fixture.

In an alternative method, the yarns can be compressed along with the spike 12 into a mold and the impregnating resin cured there with the socket assembled thereover later, or a fixture component can be used to force the yarns into the socket instead of the spike to be employed in assembling the termination fitting of the invention.

Having described the structure and method of assembly of the termination fitting of the invention, we now discuss its advantages, provide further details as to its use, and give examples.

The present invention refers to a fitting used to terminate cables made with high tensile strength (> 500 MPa as ultimate tensile strength - HTS) and high tensile modulus (> 50 GPa as tensile modulus - HM) unidirectional composite fibres, typically provided as a number of yarns comprising individual carbon fibres in a polymeric matrix, and to the method of its assembly to such a cable. Those HTS and HM fibres are intrinsically brittle in both longitudinal and transversal direction (for example, the elongation at break of carbon fibres is typically below 4%). The fitting according to the invention divides the tensile stress applied longitudinally to a cable, that is, along the fibre direction, into a mixture of shear bond, friction bond and through thickness compression (TTC) stresses. Accordingly, the fitting of the invention can be referred to as a "hybrid" fitting. The division of the stress by the hybrid fitting results in a lower off-axis stress concentration on the brittle HTS and HM fibres. This in turn has the direct consequence of improving both the ultimate tensile strength (UTS) and the durability under cyclic stresses of the whole cable, in comparison to fittings that act predominantly with a friction mechanism (see GB patent 1341013) or a shear bond mechanism (see PCT application WO2009082746). As described briefly above, the hybrid termination fitting of this invention consists of two main parts as presented in Figure 1: a) an external structure having a bore defining a female inner conical surface 10a, that is, socket 10; and

b) an internal male conical structure having a tapered external surface 12a, that is, spike 10.

As noted above, the internal surface 10a of socket 10 and the outer surface 12a of spike 12 are shaped cooperatively so that between them a space is defined having substantially constant annular cross-sectional area (CAA). Definition of a space between the socket 10 and spike 12 of constant annular area will typically require that their conical surfaces make different angles with respect to their centerlines, and may require that one or both surfaces are curved in profile. The ends of the HTS and HM yarns that fit within the fitting are impregnated at assembly with a curable material that can be cured to form a rigid polymeric matrix (e.g., a thermoset epoxy), are then spaced substantially uniformly through this space of CAA, and the matrix material cured to form a composite member comprising unidirectional fibres disposed evenly throughout a matrix of cured polymer, and confined between the spike and socket.

Without limiting the invention in any way, it appears that the key reasons for the improved performance of the hybrid fitting of the invention as compared to traditional frictional or shear bond fittings are:

The HTS and HM yarns impregnated by a rigid matrix form a full composite member, which assures efficient load transfer between the cable, spike and socket and vice-versa (by a combination of friction and shear bond mechanisms) and to support the HTS and HM fibres in taking the through-thickness compression (TTC) stress. Provision of a space between the socket and spike of substantially constant annular area (CAA), and the consequent uniform distribution of the HTS and HM filament yarns radially around the spike and along the length of the spike and socket, results in a constant fibre volume fraction (F_Vf) for the composite formed in the termination. This constant F_Vf will reduce the stress risers on the HTS and HM fibres. By comparison, in the prior art fittings wherein a sleeve having an internal conical surface is attached to a central member having a conical outer surface, both surfaces having the same taper, and a cable comprising yarns or wires is confined therebetween, the volume fraction F_Vf will vary along the length of the fitting, and most of the stress will occur where the cable enters the fitting.

Preferably a long, thin spike having a low half-angle, e.g., below 3.25° at its tip, is employed to reduce the maximum stress intensity factor on the composite in the termination, together with a corresponding socket. That is, employment of a long, thin spike with a cooperating socket spreads the tensile load over a longer length of the yarns than if a shorter spike and socket combination were used. This also increases the bend radius experienced by the yarns where they enter the socket, reducing the stress riser at this point. Using a long, thin spike and socket also makes for a low profile terminal, less than three times the cable diameter.

Preferably the space between the spike and socket is designed such that F_Vf > 70% which guarantees that the tensile strength of the cable is not reduced due to the fitting (the higher the F_Vf, the higher is the tensile strength of the composite). Similarly, the higher the F_Vf, the smaller the fitting can be, reducing the weight and cost of the system. Stated differently, the radial extent of the space defined between the spike 12 and socket 10 is designed such that the yarns 20 are substantially compressed therebetween, with the curable resin 22 simply filling the interstices between the yarns 20, as illustrated by Fig. 2.

Still further, preferably high pressure (> 50 MPa) is applied between the socket and spike during curing to consolidate the composite. This pressure, which is some 80 times higher than the pressure generated by an autoclave (0.7 MPa), dramatically reduces the void content in the composite which improves fatigue (cyclical) strength.

More specifically, if the HTS and HM fibres are not impregnated (supported) by a rigid polymeric matrix, shear, friction and TTC stresses generated in the fitting when the cable is tensioned in use will damage those fibres. This damage leads to sudden and catastrophic damage to the ability of the fibres to carry load. As the tensile (axial) load increases in each fibre, the ability to withstand TTC stress by the fibres reduces and this would lead to a poor UTS of the cable.

By comparison, if the HTS and HM fibres were retained in a soft polymeric matrix (e.g. soft elastomeric rubber), the cable when "pristine" (i.e., new) would have the desired UTS properties but would not be suitable for cyclical loading. Such a system will fail only after few hundred cycles versus several hundred thousand cycles of the composite impregnated with rigid polymeric matrix.

The HTS and HM fibres to be used in connection with the fitting of the invention are those now known to the art or later developed. As examples, while not limiting the invention, these fibres can be carbon, of either "pan" or "pitch" based varieties , glass (E, R, S...), basalt, quartz, ultrahigh molecular weight polyethylene, para-aramide, poly(p-phenylene-2,6-benzobisoxazole), metallic alloys, ceramics, and combinations of these.

The material of which the spike and socket of the hybrid fitting of the invention are made can be any type of metal alloy (e.g. steel, titanium, aluminium, magnesium) or composite reinforced polymer (both short and/or long fibres) or thermoplastic or thermoset polymers. The socket and the spike can be made of different materials. The hybrid fitting will have dimensions according to the stress level to which the cable will be subjected and the type of material will be selected according to the weight, dimensions and cost limitations. The annular space defined between the socket and spike will depend on the packing ratio of the yarns, the fibre filament diameter and the amount of fibres (i.e., on the total cable diameter) and is designed to achieve a final F_VF of at least 70% and preferably 80% or more. As above, a high FVF value results in a high performance, low weight system.

The contact area between the HTS and H M yarns and the internal wall of the socket and the outer surface of the spike is engineered so that the radial pressure does not exceed the ultimate off-axis strength of the fibre or the compressive strength of the composite.

The curable material used to form the matrix between the yarns should have an elongation at break of between 5 and 80%, allowing for some slight motion without failure as the cable is tensioned.

The procedure to assemble the HTS and H M fibres into the final hybrid fitting is very simple and straight forward and will typically include the following steps:

1. Impregnate an appropriate length (to suit the termination system) of the HTS and HM yarns with the material to be cured to form the rigid matrix (typically thermoset epoxy resin).

2. Place the socket over the end of the cable, and splay the fibres evenly around the

socket.

3. Assemble the spike to the socket, ensuring that the spike is placed concentrically inside the evenly radially distributed HTS and HM fibres.

4. Push the spike and socket together until they are firmly wedged together, ensuring that the spike and the socket are aligned co-axially. 5. Apply pressure between the spike and the socket to reach an internal surface pressure > 50 GPa.

6. Cure the matrix by following the cure cycle recommended by the resin supplier.

Examples.

Example 1.

66 yarns of HexTow IM8 12K 446 tex (nominal tensile modulus of 310 GPa and 1.8% of nominal elongation at break as quoted in the supplier's technical data sheet) carbon fibres previously impregnated with an elastomeric matrix were put together and cut to form a cable with a length of about 1.5 m. The spike contact length and the socket contact area were respectively 50.82 mm and 2,105 mm² and the overall length and diameter of the hybrid fitting were 53.09 mm and 19.05 mm, respectively. The yarns were homogeneously impregnated by hand for a length equivalent to the spike length plus another 5 mm with a bi-component epoxy resin. The spike tip was inserted in the centre of the impregnated carbon bundle and the whole system (spike + socket) assembled with care taken to maintain the spike and the socket co- axially together. The excess resin coming out of the hybrid fitting was removed. The assembled and finished hybrid fitting was then cured, following the resin supplier's recommendations.

The pristine cable had a nominal breaking load (NBL) of 4,473 Kg. The cable was then tested under quasi-static axial loading (approximately 400 mm/min) and broke, near the socket, at 5,570 Kg, substantially exceeding the N BL. F_VF of this element was approximately 70% and socket half-angle was 3.1⁹.

A second similar cable was then subjected to 100,000 cyclic axial stresses. Each cycle goes from 0 load until maximum stress load (MSL), where the MSL was about 30% of the NBL. At the end of those cyclic axial stresses the cable was tested under quasi-static axial loading (approximately 400 mm/min) and was found to have retained 89% of its NBL. Noting that the leading industrial standard Germanischer Lloyd for nautical applications of cable terminations requires that they retain at least 62.5% of the NBL after 100,000 cyclic stresses, it will be apparent that this was a very successful test of the invention.

After both tests the hybrid fittings were disassembled, and the quality of the composite fibres between the socket and the spike were examined, showing no fibre degradation due to abrasion. This is clear proof that the hybrid fitting of the invention stresses the fibres equally and homogeneously, keeping the fibre damage to a minimum.

Example 2.

122 yarns of the same HexTow IM8 12K 446 tex carbon fibres previously impregnated with an elastomeric matrix were put together and cut to form a cable with a length of about 1.5 m. The spike contact length and the socket contact area were respectively 76.67 mm and 3,397 mm² and the overall length and diameter of the hybrid fitting were 80 mm and 25.40 mm, respectively. The carbon fibre impregnation and hybrid socket assembly procedure was the same as in example 1.

The pristine cable had a NBL of 8,945 Kg. The cable was been then tested under quasi-static axial loading (approximately 400 mm/min) and broke at 9,840 Kg, thus substantially exceeding the NBL.

A second cable was subjected to 100,000 cyclic axial stresses with the same procedure described in example 1. At the end of those cyclic axial stresses the cable was tested under quasi-static axial loading (approximately 400 mm/min) and broke at 80.5% of the NBL, thus again well exceeding the industry standard. Also in this case the carbon fibres showed no visible degradation after the fatigue test. Example 3.

396 yarns of the same HexTow I M8 12K 446 tex carbon fibres previously impregnated with an elastomeric matrix were put together and cut to form a cable with a length of about 1.5 m. The spike contact length and the socket contact area were respectively 150 mm and 9,903 mm² and the overall length and diameter of the hybrid fitting were 171 mm and 44.45 mm, respectively. The carbon fibre impregnation and hybrid socket assembly procedure was the same as in example 1.

The cable was subjected to 100,000 cyclic axial stresses with the same procedure described in example 1. At the end of those cyclic axial stresses the cable was tested under quasi-static axial loading (approximately 400 mm/min) and broke at 32,150 Kg, retaining 120% of the NBL. Again in this case the carbon fibres showed no visible degradation after fatigue testing.

Example 4.

396 yarns of the same HexTow I M8 12K 446 tex carbon fibres previously impregnated with an elastomeric matrix were put together and cut to form a cable with a length of about 1.5 m. The socket contact length and the socket half angle were respectively 250 mm and 1.5⁹ and the spike overall length and overall diameter of the hybrid fitting were 240 mm and 37 mm, respectively. F_VF in this example was approximately 60% .

The carbon fibre impregnation and hybrid socket assembly procedure were different than example 1 in that the socket whipping was done prior to assembly. Then the socket was passed over the yarns. The yarns were then cured in a mould with the spike installed, and then the socket was pressed into and bonded onto the composite cone comprised by the spike and the composite yarns/curable material. The cable was subjected to 200,000 cyclic axial stresses with the same procedure described in example 1. Again in this case the carbon fibres showed no visible degradation after fatigue testing.

While a preferred embodiment of the invention had been described, it will be appreciated that the invention is not to be limited thereby, but only by the following claims.

Claims

What is claimed is:

1. A termination fitting for a cable comprising a plurality of yarns each comprising a large number of fibres of high tensile strength, high modulus material, said fitting comprising:

a socket comprising a generally cylindrical elongated member having a generally tapered bore therein defining an inner surface;

a spike comprising an generally cylindrical elongated member defining a generally tapered outer surface;

wherein said inner surface of said bore in said socket and said outer surface of said spike are shaped correspondingly, such that when said spike is inserted into said bore to a predetermined depth suitable for confining said yarns firmly between said inner surface of said bore and said outer surface of said spike, a space of substantially constant annular area is defined therebetween.

2. The fitting of claim 1, wherein at least one of said generally tapered inner surface of said bore in said socket and said generally tapered outer surface of said spike are curved in profile so as to define said space of constant annular area therebetween.

3. The fitting of claim 1, where the half-angle of the taper of said spike is no more than about 3.25° at its tip.

4. The fitting of claim 1, further comprising a cap threadedly attached to the socket for securing the fitting to associated structure.

5. A method for securing a termination fitting to a cable comprising a plurality of yarns each comprising a large number of fibres of high tensile strength, high modulus material, said method comprising the steps of:

providing a socket comprising a generally cylindrical elongated member having a generally tapered bore therein defining an inner surface; providing a spike comprising an generally cylindrical elongated member defining a generally tapered outer surface;

wherein said inner surface of said bore in said socket and said outer surface of said spike are shaped correspondingly, such that when said spike is inserted into said bore to a predetermined depth suitable for confining said yarns firmly between said inner surface of said bore, and said outer surface of said spike, a space of substantially constant annular area is defined therebetween;

passing the end of the cable to be secured to the fitting through the bore in the socket;

applying a material curable to form a solid to the yarns;

splaying the yarns of the cable substantially uniformly apart;

inserting the spike into the bore in the socket, so as to confine the yarns between the spike and socket; and

curing the curable material.

6. The method of claim 5, comprising the further step of applying pressure between the spike and socket prior to curing the curable material to force the spike into the bore in the socket and firmly capture the yarns of the cable therebetween.