GB2532489A

GB2532489A - Fixing extraction method and part therefor

Info

Publication number: GB2532489A
Application number: GB1420703.9A
Authority: GB
Inventors: Wagstaff Lee; Hoyle Mark; Powis James
Original assignee: Airbus Operations Ltd
Current assignee: Airbus Operations Ltd
Priority date: 2014-11-21
Filing date: 2014-11-21
Publication date: 2016-05-25
Also published as: GB201420703D0

Abstract

A method of extracting a fixing 14 from an aircraft structure 10 includes use of a protective cushion, for example in the form of a washer 22. At least a part of the fixing 14 is retained within a hole in the structure 10. A percussive tool 24 is used to strike the fixing to push the fixing 14 along the hole. The protective cushion (washer 22) is placed around the hole to protect against impact by the percussive tool 24. A laminar material may provide the protective cushion. Such material may be formed of multiple layers, comprising a layer (figure.8, 130) of composite material formed of woven fibre material, a metallic layer (132) and an elastomeric layer (134). There may be a fourth outer layer of adhesive material (136). There is also a method for making said protective cushion.

Description

FIXING EXTRACTION METHOD AND PART THEREFOR

BACKGROUND OF THE INVENTION

10001.1 The present invention concerns a method of extracting a fixing from a structure. More particularly, but not exclusively, this invention concerns a method of extracting a fixing from an aircraft structure using a percussive tool to assist extraction of the fixing. The invention also concerns a laminar material formed of multiple layers for use in such a method and possibly other applications. The invention also concerns a method of making such a laminar material.

100021 Fixings such as bolts are commonly used to secure an aircraft structure to some other part, such as another aircraft structure, to form an assembly (the assembly comprising the structure, the other part and the fixing). For example, a bolt may be used to secure an aircraft structure to another part by means of the shank of the bolt being received in a hole through the structure. The bolt may have a head that engages with one part of the assembly. The shank of the bolt may be secured within the hole by means of an interference fit. When seeking to remove such a bolt, it is often necessary to use a percussive tool in order to assist extraction of the fixing. Damage may be unintentionally caused by such percussive tools. Great care may therefore be needed to avoid damage. Damage caused to aircraft-grade materials can be expensive and time-consuming to fix. 10003] The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved a method of extracting a fixing from an aircraft structure using a percussive tool to assist extraction of the fixing.

SUMMARY OF THE INVENTION

100041 The present invention provides, according to a first aspect, a method of extracting a fixing from an aircraft structure, at least a part of the fixing being retained within a hole in the structure, the hole extending through the structure from a first side of the structure to a second side of the structure, wherein the method comprises the following steps: -2 -placing a protective cushion around the hole on the first side of the structure, and using a percussive tool to strike the fixing from the first side of the structure to push the fixing in a direction from the first side of the structure to the second side of the structure.

100051 According to a second aspect of the invention there is also provided a laminar material being formed of multiple layers, the layers comprising: a first layer of composite material formed of woven fibre material, a second metallic layer and a third elastomeric layer.

100061 According to a third aspect of the invention there is also provided a method of extracting a fixing in accordance with any aspect of the present invention as claimed or described herein, including any optional features relating thereto, performed using a washer made from a laminar material in accordance with any aspect of the present invention as claimed or described herein, including any optional features relating thereto. 100071 According to a fourth aspect of the invention there is also provided a washer as defined in any aspect of the present invention as claimed or described herein, including any optional features relating thereto.

100081 According to a fifth aspect of the invention there is also provided a method of making a laminar material comprising the following steps: curing an arrangement comprising a sheet of metal, at least one layer of woven fibre material and uncured resin, and thereafter bonding a polymer layer on top of the metal layer.

100091 It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. -3 -

DESCRIPTION OF THE DRAWINGS

100101 Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a bolt prior to extraction by means of performance of a method according to a first embodiment of the invention; Figure 2 shows the bolt of Figure 1 during a step of the method of the first embodiment, including use of a cushioning washer; Figure 3 shows the bolt of Figure 1 during a subsequent step of the method of the first embodiment; Figure 4 shows the bolt of Figure 1 being extracted during a step of the method of the first embodiment; Figure 5 shows the bolt of Figure 1 during a subsequent step of the method of the first embodiment; Figure 6 shows the extracted bolt and cushioning washer removed at the end of the method of the first embodiment a protective washer; Figure 7 shows a washer according to a second embodiment of the invention; Figure 8 shows a cross-section of the washer of Figure 7; Figure 9 shows a kit of multiple washers according to a third embodiment of the invention; Figure 10 shows a fastener prior to extraction from an aircraft structure by means of performance of a method according to a fourth embodiment of the invention; -4 -Figure 11 shows an aircraft featuring the aircraft structure shown in Figure 10; Figure 12 shows a washer with a modified shape in accordance with a fifth embodiment of the invention; Figure 13 shows a washer with a modified shape in accordance with a sixth embodiment of the invention; Figure 14 shows an initial step in a method of manufacturing a laminate material in accordance with a seventh embodiment of the invention; Figure 15 shows a subsequent step in the method of the seventh embodiment; Figure 16 shows a yet further step in the method of the seventh embodiment; and Figure 17 shows the laminate material made as a result of the performance of the method of the seventh embodiment.

DETAILED DESCRIPTION

[00111] Embodiments of the present invention relate to a method of extracting a fixing from an aircraft structure and material for use in such a method or for use with other applications. Another embodiment concerns a method of manufacturing such material. There now follows a general description outlining the general concepts embodied by such embodiments.

100121 There is provided a method of extracting a fixing, for example a bolt or other fastener, from a structure, particularly from an aircraft structure. The fixing may be secured securely within the hole so that significant force is required to remove the fixing from the hole. For example the fixing may, at least partially be secured to the structure, by means of a part of the fixing being held within the hole by means of an interference fit. By way of example, the fixing may comprise a shank terminating in a head, for example being in the form of a bolt. The fixing may be a countersunk fixing. The shank may -5 -terminate at the end opposite the head in a tail. The tail may be positioned on a first side of the structure and the head may be positioned on a second, opposite, side of the structure. (It will be appreciated that the hole may thus extend through the structure from the first side to the second side of the structure.) The force required to remove the fixing from the hole may be provided by means of using a percussive tool, for example a powered percussive tool, to strike the fixing from the first side of the structure to push the fixing in a direction from the first side of the structure to the second side of the structure. A percussive tool may thus be used in the method in order to assist extraction of the fixing. It has been recognised by the present inventors that there is a risk of significant and costly damage to surrounding aircraft structure, when using such a percussive tool, particularly in the case when using a power tool. The problem and risk is worsened when operating in tight spaces, as is often the case when working on aircraft structures. (The fixing may be located in a space in the wing for example, requiring the operator of the percussive tool to lean through a man hole in the wing skin.) It may be that there is a space restriction preventing the percussive tool from being able to strike the tail of the fixing head-on. It may be that the percussive tool is required to strike in a direction that is at a non-zero angle to the axis of the fixing. This further increases the risk of damage. The method may include a step of using an extraction tool to pull the fixing from the hole, for example after the fixing has been partially extracted from the hole. Before the percussive tool is used to strike the fixing, the head of the fixing may be in contact with the second side of the structure. The step of using the percussive tool to strike the fixing may cause the head to lift off from the second side of the structure. There may be a step of jaws of the extraction tool being used to grasp and pull the fixing from the hold, for example engaging with the underside of a head of the fixing.

100131 The method includes a step of placing a protective cushion around the hole on the first side of the structure. Such a cushion can protect the surround structure from unintentional impact from the percussive tool. The cushion need not surround the hole entirely, but it is preferred for the cushion to be in the form of plate-like material having a hole that is of a comparable size to the hole from which the fixing is to be extracted. The cushion may for example be in the form of a washer. The washer may be generally -6 -round. The washer may have one or more flat edges (when viewed in plan) -and not therefore be completely round.

[0014] The protective cushion may comprise multiple layers of different material, for example being made from laminar material being formed of such multiple layers, for example at least three layers each of different material. The various layers may each provide different properties that together enable the cushion to protect the structure around the hole from being damaged in the event of an impact on the cushion. One of the multiple layers may be a composite material layer. One of the multiple layers may be a metallic layer. One of the multiple layers may be a polymeric layer, for example being an elastomer. In certain applications, there may be a limit on how thick the laminar material can be. The total thickness of the laminar material may be less than 3.0 mm. The protective cushion may thus have a thickness that is less than 3.0 mm.

100151 The composite material layer may comprise fibres, preferably woven fibres, held in a polymer matrix. The polymer matrix may be in the form of a cured resin, for example an epoxy resin. The polymer matrix may be made from, or otherwise comprise, high-visibility material, for example being brightly coloured. Alternatively, or additionally, the polymer matrix may be made from, or otherwise comprise, luminescent material. For example, the polymer matrix may glow in the dark, either with or without external stimulation (for example UV light). Such visual properties of the polymer matrix may reduce the chances of the protective cushion, or a part thereof comprising a fragment of the composite material layer, from being accidentally lost or mislaid (for example if it, or a part of it, is damaged and separated from the site of the fixing) and/or may assist in finding the protective cushion or parts thereof, if so lost or mislaid. Such visual properties of the polymer matrix may be produced by means of an additive. The fibres of the composite material layer may have a yield strength of more than 2,000 MPa. The fibres of the composite material layer may comprise aramid fibres. The fibres may, for example, comprise meta-aramid fibres and/or para-aramid fibres. The fibres may be in the form of Kevlar fibre material. The fibres of the woven material may be in the form of an ultra-high-molecular-weight polyethylene. For example, the fibres may be in the form of Dyneema fibre material. The fibres may be in the form of Spectra fibre material. The -7 -fibres may have an ultimate tensile strength of more than 3,000 MPa. The composite material layer may be in the form of a single ply of woven fibres. Having just a single ply enables the end product (e.g. the washer) to be relatively thin. The thickness of the composite material layer may be less than 0.5mm. The thickness of the composite material layer will typically be greater than 0.1 mm, and may be greater than 0.2 mm, 100161 it may be that the metallic layer is sandwiched between the composite material layer and the polymeric layer. The composite material layer may act to catch or trap an impact. The metal layer may absorb the energy of the impact, deforming in the process. The metallic layer may have an ultimate tensile strength of greater than 200MPa, preferably greater than 600MPa, and possibly greater than 800MPa. The metallic layer may have a Youngs modulus of more than 80GPa, and preferably more than 100GPa. It is desirable however for the metallic layer not to be too stiff -an impact on a very stiff metallic layer may absorb too little of the energy of the impact. As such, it may be that the metallic layer has a Youngs modulus of less than 400GPa. The metallic layer may be formed from titanium and may for example be in the form of a titanium alloy. Using titanium in the metallic layer has been found to work surprisingly well. Tougher, harder, and stiffer materials might absorb too little of the impact, allowing a percussive tool or other impacting object to be deflected (or bounced off). Whilst in some applications that may be advantageous, in applications where a large object (e.g. an aircraft structure) is being worked on in a local area only such that cushioning protection is provided only in the immediate vicinity of that local area, allowing a percussive tool or other impacting object to deflect off away from the immediate vicinity of that local area may expose other areas of the aircraft structure to an unacceptable risk of damage. Tougher, harder, and stiffer materials may also be more difficult to use in manufacturing a protective cushion for a suitable application. However, using a metallic layer that is not sufficiently tough, hard, and stiff might provide insufficient protection. It is preferred that the metal layer has properties such that it is not expected to fracture or be punctured when sustaining an impact of the type to be protected against. The thickness of the metallic layer may be greater than 0.7 mm. Thicknesses above this level have been found to afford good impact absorption characteristics. The thickness of the metallic layer may be more than 150% of -8 -the thickness of the thicker of the composite material layer and the polymeric layer. There may be some applications in which the metallic layer comprises mangalloy.

[0017] The polymeric layer may deform to accommodate the deformation in the metal layer, but may be sufficiently soft as not to cause surface damage on the aircraft structure. The polymeric layer may thus act as a buffer as between the metallic layer and the aircraft structure. The polymeric layer is preferably in the form of a resilient rubber, or rubber-like compound. Thus all three layers may play an important part in the cushioning and the protection of the aircraft structure. The shore value (as measured on the "Shore A" scale) of the polymeric layer may be between 30 and 70 and is preferably between 35 and 50. The thickness of polymeric layer may be greater than 0.2 mm. The thickness of the polymeric layer may be less than 0.5mm. The thickness of the polymeric layer may be the same as the thickness of the composite material layer, within +/-25%.

100181 There may be a further outer layer of adhesive sheet material, for example a sheet with an adhesive coating on both sides, one side for sticking the sheet material to the polymeric layer, the other side thus facilitating temporary or permanent fixing of the laminar material to a structure to be protected. The adhesive on that other side may be covered, when the laminar material is not in use, by a backing sheet. The layer of adhesive sheet material may have a thickness of more than 0.1 mm. It is preferred that the layer of adhesive sheet material has a thickness of less than 0.5mm.

[0019] There may be other applications, other than a cushioning washer, in which a laminar material being formed of multiple layers, comprising a composite material layer formed of woven fibre material, a metallic layer and a polymeric layer is of use. For example, there may be other objects requiring impact protection. The protective cushioning properties of the laminar material of the washer could for example be used in armour for protecting an object against impact. As such the thickness of the layers mentioned above may be varied to suit a particular application [0020] There is also provided a method of manufacture of a laminate material. Such a method may comprise a step of curing an arrangement comprising a sheet of metal, at least one layer of woven fibre material and uncured resin, for example by means of heat and/or pressure (for example curing in an autoclave). Thereafter a polymer layer may be -9 -added on top of the metal layer. The curing step may be performed with the metal, woven fibre material and resin being supported on a suitable (removable) substrate. The resin may be added or may be provided as part of a fibre mat being pre-impregnated with such resin.

100211 There may be a step of roughening one or both sides of the metal sheet to aid adhesion of resin/polymer/adhesive to the surface. The roughening process may involve blasting the surface of the metal with particulate material (such as a sand-blasting or grit-blasting process). The polymer layer may be added on top of the metal layer by means of bonding the polymer layer to the metal layer. The bonding of the polymer layer to the metal layer may be effected by means of curing the polymer in situ. An adhesive may be used to bond the already cured polymer to the metal sheet. There may be an additional step of adding a sheet the outer facing side of which is covered with an adhesive, for example a reusable adhesive. There may be a backing sheet provided to cover the adhesive that would otherwise be exposed. The laminate material once so formed may be machined to an appropriate shape and size for a particular application. The laminate material may be cut to size using a water jet cutting process.

100221 There now follows a description of specific embodiments, including the first and subsequent illustrated embodiments.

100231 The first embodiment concerns a method of removing a bolt from an aircraft structure, the method being illustrated with reference to Figures 1 to 6. Figure 1 shows parts 10 of aircraft structure help together by a nut 12 and bolt 14. The bolt 14 has a head 16, a shank 18 and a tail 20. The bolt 14 is secured to the aircraft structure 10 partly by mean of an interference fit. The bolt 14 is part accommodated within a hole that extends through the structure 10 from a first (lower) side of the structure to a second (upper) side of the structure. Figure 1 shows in plan view a cushioning washer 22 to be used in the method. The washer 22 is sized to fit around the tail 20 of the bolt and has a structure that is capable to absorbing and cushioning impact. The cushioning washer 22 is able to locally deform to accommodate an impact and is also able to resist penetration by such an impact. The nut 12 is removed from the bolt with a spanner and the washer 22 is placed over the tail 20. A percussive tool 24 is then used to knock the bolt outwards (upwards in -10 -the orientation shown in Figures 1 to 6) by applying percussive knocks to the tail 20 of the bolt. (it will be appreciated that a safety guard -not shown -may be provided to cover the head 16 of the bolt to prevent the bolt from unintentionally being ejected at speed from the hole). When using the percussive tool 24 to knock the tail of the bolt outwards, it may be necessary -for example as a result of restricted access -to strike the tail 20 of the bolt in a direction that is at a non-zero angle a to the axis 14a of the bolt. As such, there is the possibility of the percussive tool 24 inadvertently making contact with the structure immediately surrounding the hole in which the bolt 14 is received. With the use of an appropriately sized washer 22, the structure immediately surrounding the hole may be protected against such inadvertent contact. (It will be appreciated that the various parts shown in Figures 1 to 6 are not shown to scale and also that the diameter of the washer may be chosen to provide more or less protection in terms of the area covered as required.) The bolt 14 is progressively knocked outwards until the tail end of the bolt is substantially flush with the washer 22, as shown in Figure 3. This then provides sufficient clearance, c, to enable the bolt to be grasped by its head 16 by a suitable extraction tool, the jaws 26 only of which being shown in the Figures. Figure 4 shows the jaws 26 of the extraction tool being used to pull the bolt from the hole, the bolt 14 eventually being completely extracted from the hole 28 (as shown in Figure 5). As a final step, as illustrated by Figure 6, the washer 22 may then be removed from the structure 10. In some set-ups, the thickness of the washer may have an effect on the amount of clearance that can be provided. For example, if the fastener can only be pushed outwards until the tail end is flush with the surface of the washer, and the amount of the protrusion of the tail end from the structure (before being pushed outwards) is limited, there may be a limit to how much clearance can be provided.

100241 Figure 7 shows a washer 122 according to a second embodiment of the invention. The washer 122 of the second embodiment may be used as the washer used in the method according to the first embodiment of the invention. There may be other applications for such a washer. It will also be appreciated that other types of washer may be used in the first embodiment. The washer is generally ring-like in shape when view in plan, as shown in Figure 7. Figure 8 shows a cross-section of the washer taken along the plane A-A of the view shown in Figure 7. The washer 122 thus has four layers including a first layer of composite material 130 formed of woven fibre material, a second layer of titanium metal 132, a third elastomeric layer 134 of polyurethane and a fourth sticky-tape layer 136, having a removable backing sheet 138. The woven fibre material 130 is in the form of a single ply of Kevlar® fibre set in polymer resin (an epoxy resin) and has a thickness of about 0.3mm (equivalent to a 140gm-2 weave). The layer 132 of titanium is about I mm thick. The titanium used is an alloy additionally comprising 6% aluminium and 4% vanadium, typically referred to as 6A1 4V Titanium alloy. It has a Youngs modulus of about 110 GPa, and an ultimate tensile strength of about 1,000 MPa. The polyurethane layer 134 is about 0.3mm thick. The sticky-tape layer is about 0.2mm, and the backing sheet has a thickness of less than 0.1mm. The total thickness of the washer is therefore just under 2mm. In use, the composite material layer 130 acts to reduce the chance of an impact from an object puncturing through the washer. In the case of a glancing blow from an object, or a non-perpendicular impact, the composite material layer 130 may act to capture the object to reduce the chance of the object sliding off, or rebounding off, the surface of the washer 122. The strength and toughness of the titanium layer 132 is able to absorb the energy of an impact, typically deforming in the process. The deformation of the titanium layer 132 is accommodated by the soft elastomeric layer 134, thus providing a buffer between the titanium layer 132 and the structure (not shown in Figure 8) being protected by the washer 122. The sticky-tape layer 136 enables the washer 122 to be temporarily fixed to a structure so that the washer 122 remains in place, which may be particularly useful if gravity alone is insufficient. The backing sheet 138 may be reused.

100251 In accordance with a third embodiment of the invention, Figure 9 shows a kit of multiple washers 122, of the sort used in the first and/or second embodiments, for use in different applications that require different inner or outer diameters of washer. As illustrated by the broken lines shown in Figure 9, two washers 122a, 122b of the kit have the same outer diameter, but differing inner diameters, and two washers 122b, 122c of the kit have the same inner diameter, but differing outer diameter.

-12 - 100261 In accordance with a fourth embodiment of the invention, Figure 10 shows a cross-sectional view of a structure 310 in an aircraft (for example of the sort shown in Figure 11) from which a countersunk titanium fastener 314 needs to be removed. The method of removal, accordance to this fourth embodiment, seeks to avoid damage to the surrounding structure 310 by using a protective washer (not shown in Figure 10), of the sort used in the first and/or second embodiments, in combination with a percussion power-tool fitted with a mushroom snap device. The method is suitable for removing fasteners of up to I 5mm diameter. Washers are contained in a kit, similar to that of the third embodiment, there being washers suited for varying sizes of fastener diameter. The washers are labelled to make clear the diameter of fastener with which they can be used. After removal of the nut (not shown in Figure 10) with a suitable nut-runner or spanner, the appropriate washer is then prepared for use. The area surrounding the fastener 314 tail 320 is cleaned with an approved solvent wipe to remove residual contamination from the surface. A double-sided backing protection is peeled from the washer and retain in a suitable place to allow re-use when the fastener removal operation is complete. The washer is placed over the tail of the fastener 314 and pressed firmly into position so that the washer sticks to the structure surface. When the washer is correctly positioned, a snap-type percussion power-tool fitted with a mushroom snap (or, alternatively an aluminium 'drift' or bucking bar) is positioned over the fastener tail and then operated to cause the mushroom snap to drive the fastener-tail from the hole up to the point of contact with the Kevlar washers. The mushroom snap device (not shown in Figure 10) is made of hardened steel. The pressure applied via the power tool effectively shocks the fastener 314 from the surrounding bore. If, due to the shocking motion, the mushroom snap device loses contact with the tail end 320 of the fastener 314, it is likely to impact only on the washer, which absorbs the impact thus protecting the relatively soft surrounding aircraft-grade structure, which might otherwise be damaged by a relatively hard snap tool impacting on it.

[0027] Once complete, a fastener extraction kit 340 may then be used to extract the fastener 314. In this case, fastener extraction kit 340 includes a cap head 342 that is screwed into a thread that has been formed within the shank of the fastener 314, the cap -13 -head 342 being grasped by the jaws 344 of a fastener puller 346, supported on a bridge 348. The fastener puller 346 is driven upwards (in the orientation shown in Figure 10) by means of rotating a nut 350 (that engages with a thread on the fastener puller 346) the nut 350 reacting, via thrust bearing 352, against the bridge 348, thus drawing the fastener 314 full from the hole. The washer (not shown) is inspected after use, and provided that it remains undamaged, has its backing protection replaced and is then placed back into the kit box. Damaged washers are replaced.

[0028] In particularly tight spaces there may be insufficient clearance around the hole from which a fixing needs to be extracted to fit a washer of constant outer diameter. In such cases a modified shape of washer may be needed. One such washer 222a with a modified shape is shown in Figure 12, in accordance with a fifth embodiment of the invention. The washer 222a includes a flat edge 222f. Another such washer 222b with a modified shape is shown in Figure 13, in accordance with a sixth embodiment of the invention. The washer 222b includes two flat edges 222f. Other shapes could of course be provided in accordance with other (not illustrated) embodiments of the invention.

[0029] The washer of the second embodiment is made by water-jet cutting the washer from a larger sheet of material. The method of manufacturing such a material will now be briefly described, with reference to the seventh embodiment. As shown in Figure 14, a substrate in the form of a glass sheet 440 is provided, on which a layer of woven fibre material 430 is placed, the woven fibre material having been pre-impregnated with epoxy resin (the layer 430 being in the form of a section of "pre-preg" composite material). On top of this is placed a sheet of titanium 432, which has been sand-blasted on both sides to provide roughness, thus allowing resin, polymer, adhesive or the like to "key-in". The layers each measure about 300mm by 200mm in plan. Then a vacuum bag 442 is taped down onto the substrate 440 and air removed to form a vacuum seal. The resulting assembly 444 is then cured in an autoclave, resulting in the titanium layer 432 and the composite material layer 430 being bonded together. The glass substrate is then removed from the bonded layers 430, 432. A mix of an isocyanate-containing liquid and a polyolcontaining liquid is then applied (see Figure 15) to the exposed surface of the titanium layer 432 and allowed to set (cure) to form a layer of polyurethane 434 (with a hardness -14 -on the Shore A scale of about 40), so as to form a three-layer material with the titanium layer 432 sandwiched between a composite material layer 430 and a elastomer layer 434. Figure 16 shows this three-layer material (having been inverted from its position in Figure 15). The three-layer material can at this stage be cut to size. In the case where the end application is a washer of the type described in relation to the second embodiment, the shape of the washer may be cut (for example, water-jet cut) from the sheet. A double-sided sticky sheet 436 (i.e. a PVC or kraft-paper-based sheet on which both sides of the sheet are coated with a suitable adhesive, for example an acrylic-based adhesive, that is protected by a thin plastic -e.g. polypropylene -outer layer on both sides) may then be attached, by exposing the adhesive layer on an uppermost side (e.g. by removing a first protective sheet 437). Figure 17 shows the end product 422, which may be attached to a structure or object to provide it with impact protection by means of removing the backing sheet 438 to reveal the adhesive coated layer 436 underneath. It will be appreciated that one application of the multilayer laminate material product 422 so produced is a protective washer, such as that of the second embodiment, and that there may be other applications.

100301 In a further embodiment not separately illustrated, but closely based on the washer shown in Figure 7 and 8, the epoxy resin that forms the polymer matrix of the composite layer includes an additive that fluoresces under UV light. In the event that a fragment of, or the whole of, the washer is mislaid during use -for example if the washer breaks off during use -a UV lamp may be used to find the washer or broken part thereof. This may be particularly important when extracting a fixing from an aircraft structure forming part or, or being adjacent to, a fuel tank. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0031] The adjacent layers of the washer shown in Figures 7 and 8 need not be the same shape or size in plan view. One layer may overhang an adjacent layer for example. Different ways of manufacturing cushioning protective devices are envisaged. The -15 -cutting of a larger laminate sheet to size (for example to create a washer) could be performed before adding the elastomeric layer for example. Additional layers could be added.

[0032] Instead of having an additive in the composite material layer that fluoresces under UN' light, a brightly coloured dye may be added, for example bright red, so that the washer or part thereof can easily be spotted in normal light conditions.

[0033] Another type of fibre weave found to be particularly well-suited to the application concerning use of a cushioning washer when extracting a fixing from an aircraft component, is a para-aramid plain weave fabric material that is sold under the name Twaron (a brand of Teijin Aramid) with a mass per surface area of 170g/m2 -a fabric which appears to offer particularly good impact resistance.

100341 The elastomeric later could be made by using the two-part polyurethane system provided by Delta Resins Limited under product name "DX 830", which is a two-part polyurethane mix which cures at room temperature to form an elastomeric layer with a hardness on the Shore A scale in the region of 55 to 65.

[0035] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

-16 -Claims 1. A method of extracting a fixing from an aircraft structure, at least a part of the fixing being retained within a hole in the structure, the hole extending through the structure from a first side of the structure to a second side of the structure, wherein the method comprises the following steps: placing a protective cushion around the hole on the first side of the structure, and using a percussive tool to strike the fixing from the first side of the structure to push the fixing in a direction from the first side of the structure to the second side of the structure.
2. A method according to claim 1, wherein the fixing is secured in the hole by means of an interference fit.
3. A method according to claim 1 or claim 2, wherein the percussive tool is a power tool.
4. A method according to any preceding claim, wherein the method includes a step of using an extraction tool to pull the fixing from the hole, from the second side of the structure.
5. A method according to any preceding claim, wherein the fixing comprises a shank terminating in a head, the head being positioned on the second side of the structure.
6. A method according to claim 5 when dependent on claim 4, wherein before the percussive tool is used to strike the fixing, the head contacts the second side of the structure, and after the step of using the percussive tool to strike the fixing, the head is lifted off the second side of the structure, -17 -the step of the extraction tool being used to pull the fixing from the hole including using jaws of the extraction tool to engage with the underside of the head.
7. A method according to any preceding claim, wherein the protective cushion is in the form of a washer.
8. A method according to any preceding claim, wherein the protective cushion comprises multiple layers of different material.
9. A method according to claim 8, wherein one of the multiple layers is a composite material layer.
10. A method according to claim 8 or claim 9, wherein one of the multiple layers is a metallic layer.
11. A method according to any of claims 8 to 10, wherein one of the multiple layers is an elastomeric layer.
12. A laminar material being formed of multiple layers, the layers comprising: a first layer of composite material formed of woven fibre material, a second metallic layer and a third elastomeric layer.
13. A laminar material according to claim 12, wherein the metallic layer has an ultimate tensile strength of greater than 600MPa.
14. A laminar material according to claim 12 or claim 13, wherein the metallic layer has a Youngs modulus of between 100GPa and 400GPa.

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15. A laminar material according to any of claims 12 to 14, wherein the composite material layer is in the form of a single ply of woven fibres.
16. A laminar material according to any of claims 12 to 15, wherein the fibres of the composite material layer have a yield strength of more than 2,000 MPa.
17. A laminar material according to any of claims 12 to 16, wherein the total thickness of the material is less than 3.0 rum, the thickness of the first composite material layer is greater than 0.2 mm, the thickness of the second metallic layer is greater than 0.7 mm, and the thickness of the third elastomeric layer is greater than 0.2 mm.
18. A laminar material according to any of claims 12 to 17, wherein the multiple layers comprise a fourth outer layer of adhesive sheet material.
19. A method according to any of claims 1 to 11 performed using a washer made from a laminar material according to any of claims 12 to 18.
20. A washer as defined in claim 19.
21. A method of making a laminar material comprising the following steps: curing an arrangement comprising a sheet of metal, at least one layer of woven fibre material and uncured resin, and thereafter bonding a polymer layer on top of the metal layer.
22. A method according to claim 21 performed so as to produce a laminar material according to any of claims 12 to 18 or a washer according to claim 20.