RESISTANCE WELDING METHODS AND APPARATUS

Abstract

Disclosed is a method of resistance welding between composite articles. A conductive element is provided between faying surfaces, having a plurality of lower resistivity electrode portions spaced apart along the length of the contact area between the composite articles. The electrode portions can be used to spot weld across the electrode portions, and along a longitudinal portion of the conductive element between the electrode portions by application of an electrical current. Also disclosed are apparatus for use in the resistance welding methods and composite articles and structures and elements incorporating the conductive element.

Claims

1. A composite structure comprising: a first composite article; a second composite article; a join having an area defined by respective faying surfaces of the first and second composite articles, wherein the join comprises a meltable or softenable material by which the composite articles are held together; and a conductive element in the join between the faying surfaces or embedded within one of the said faying surfaces along the join, the conductive element comprising: a longitudinal portion having a length corresponding substantially to the length of the join, and a plurality of lower resistivity electrode portions spaced apart along a length of the conductive element, each of the plurality of lower resistivity electrode portions extending across a respective width of the conductive element and comprising an electrode at the ends thereof, wherein the electrodes extend beyond a respective width of the join.

2. The composite structure according to claim 1, wherein the meltable or softenable material is present as a layer between the first and second composite articles.

3. The composite stricture according to claim 1, wherein the meltable or softenable material is a matrix material of one or both of the first and second composite articles.

4. The composite structure of claim 1, wherein the meltable or softenable material is a thermoplastic polymer.

5. The composite structure of claim 1, wherein the composite structure is a carbon fibre composite structure.

6. The composite structure of claim 1, wherein the conductive element comprises at least one of: a conductive mesh, a foil formed as a lattice, a conductive fabric, a conductive paint, and a conductive ink.

7. The composite structure of claim 1, wherein each of the first and second composite articles comprises a primary faying surface and a secondary faying surface, wherein the join further comprises a primary join and a secondary join, with a corresponding primary and secondary conductive element therein, wherein the composite structure is a closed geometry structure having an enclosed volume defined between the primary and secondary joins.

8. The composite structure of claim 7, wherein the enclosed volume defined between the primary and secondary joins is a cavity having one or more openings, wherein the enclosed volume is bounded by greater than ninety percent of external area of the enclosed volume by the composite structure.

9. An electrical connection apparatus for use in resistance welding to form a closed-geometry composite structure defining a high-aspect ratio substantially enclosed volume, the electrical connection apparatus comprising: a probe arrangement comprising: a foot portion having an engagement surface for engaging a surface of the substantially enclosed volume and comprising an electrical contact element; a shoulder portion having an engaging surface for engaging an opposite surface of the substantially enclosed volume; a reconfigurable body portion operatively coupled between the foot portion and the shoulder portion, wherein the reconfigurable body portion is remotely configurable between a contracted configuration and an expanded configuration; wherein in the contracted position the engagement surfaces are spaced apart by less than a distance between opposed surfaces of a substantially enclosed volume defined between composite articles, and in the expanded configuration the engagement surfaces are further spaced apart sufficient to engage opposed surfaces of the substantially enclosed volume and to urge the electrical contact element into electrical contact with an inner electrode within the substantially enclosed volume.

10. The electrical connection apparatus of claim 9, wherein the reconfigurable body portion is pneumatically or hydraulically reconfigurable.

11. The electrical connection apparatus of claim 10, wherein the reconfigurable body portion comprises an inflatable chamber.

12. The electrical connection apparatus of claim 9, wherein the foot portion comprises a primary electrical contact element oriented towards a first side of the probe arrangement, and a secondary electrical contact element oriented towards an opposite second side of the probe arrangement.

13. The electrical connection apparatus of claim 9, comprising two probe portions spaced apart by a longitudinal distance between longitudinally adjacent inner electrodes of the composite structure being welded.

14. A composite structure comprising: a first composite article; a second composite article; a join having an area defined by respective faying surfaces of the first and second composite articles, wherein the join comprises a meltable or softenable material by which the composite articles are held together, wherein the join has a length and a width; and a conductive element in the join between the faying surfaces or embedded within one of the said faying surfaces along the join, the conductive element having a mesh or lattice configuration with pores or apertures through which the meltable or softenable material extends through, the conductive element comprising: a longitudinal portion having a length extending at least a portion of the length of the join, and a plurality of lower resistivity electrode portions spaced apart along the length of the longitudinal portion, each of the plurality of lower resistivity electrode portions extending at least partially across a respective width of the conductive element and comprising an electrode at ends thereof, wherein the electrodes extend beyond the respective widths of the join.

15. The composite structure according to claim 14, wherein the meltable or softenable material is present as a layer between the first and second composite articles or the meltable or softenable material is a matrix material of one or both of the first and second composite articles.

16. The composite structure according to claim 14, wherein the plurality of lower resistivity electrode portions are comprised of thicker conductive material than adjacent portions of the conductive element.

17. The composite structure of claim 14, wherein the meltable or softenable material is at least one of a thermoplastic polymer, an epoxy resin material, and a vitrimer material.

18. The composite structure of claim 14, wherein the first composite article and the second composite article comprises at least one of carbon fibre, aramid fibre, glass fibre, and plant fibre.

19. The composite structure of claim 14, wherein the conductive element comprises at least one of: a conductive mesh, a foil formed as a lattice, a conductive fabric, a conductive paint, and a conductive ink.

20. The composite structure of claim 14, wherein each of the first and second composite articles comprises a primary faying surface and a secondary faying surface, wherein the join further comprises a primary join and a secondary join, with a corresponding primary and secondary conductive element therein, wherein the composite structure is a closed geometry structure having an enclosed volume defined between the primary and secondary joins.

Description

DESCRIPTION OF THE DRAWINGS

[0215] Example embodiments will now be described with reference to the following figures in which:

[0216] FIG. 1 shows a test example of composite articles prior to welding;

[0217] FIG. 2 shows a conductive element on a faying surface of a composite article;

[0218] FIGS. 3(a)-(c) illustrate steps of a resistive welding method;

[0219] FIGS. 4(a)-(f) show further examples of resistive welding methods;

[0220] FIGS. 5(a)-(f) show further examples of resistive welding methods;

[0221] FIG. 6 shows a further test example of composite articles prior to welding;

[0222] FIG. 7 shows a conductive element during assembly thereof;

[0223] FIG. 8 shows a composite structure including two composite articles resistively welded together;

[0224] FIG. 9 shows composite articles of a closed-geometry composite structure;

[0225] FIG. 10 shows a schematic plan view of region X of FIG. 9;

[0226] FIG. 11 shows a schematic cross sectional view through the region X of FIG. 9;

[0227] FIG. 12 shows an example of electrical connection apparatus;

[0228] FIG. 13 shows a schematic cross sectional view of a probe portion of the apparatus of FIG. 12;

[0229] FIG. 14 shows a schematic plan view of the electrical connection apparatus within the substantially enclosed volume of the structure shown in FIG. 9;

[0230] FIG. 15(a) shows a schematic cross sectional view of a probe portion of the apparatus of FIG. 12 within the substantially enclosed volume, in a contracted configuration;

[0231] FIG. 15(b) shows a schematic cross sectional view of a prove portion of the apparatus of FIG. 12 within the substantially enclosed volume, in an extended configuration; and

[0232] FIG. 16 shows another electrical connection apparatus.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0233] Resistance welding relies on a conductive element made of a conductive material (typically stainless steel mesh), that is placed between the faying surfaces of the components to be welded. This conductive element heats up due to joule losses when current is applied, and typically in combination with adequate pressure and cooling, can generate coherent welds for thermoplastic materials.

[0234] The traditional resistance welding process dictates that the conductive element is sized for the part weld area, i.e. contact area between the faying surfaces, and the current is applied across the entire length in one process step, welding the entire part in the process.

[0235] Whilst resistance welding is at the highest technology readiness level and is the most well understood process for thermoplastic welding, resistance welding is conventionally assumed to be of limited applicability; including in particular weld length limitations due to the long or large susceptors requiring massive power supplies to generate and sustain correct welding temperatures.

[0236] Resistance welding has the advantage that because it does not rely on the reinforcement material or stacking sequence or thickness (which is a limitation of alternative methods such as many induction or ultrasonic welding methods) to generate heat. Accordingly, the welding recipe used is independent of those parameters, only requiring to generate sufficient heat to reach the processing temperature of the adjacent thermoplastic material, which is often the matrix material of the composite articles being welded. Additionally, resistance welding can be used for welding non-electrically conductive materials (e.g. glass fibre).

[0237] Embodiments disclosed herein remove the high power requirement and size limitations normally associated with resistance welding.

[0238] FIG. 1 shows a test example of the inventive method. A first composite article 10, in this example a laminate sheet of thermoplastic carbon fibre composite material, in which woven plies of a carbon fibre fabric reinforcement material are impregnated with a thermoplastic matrix material, is placed on a work bench. A second composite article 12, a strip of the same composite material as the first composite article, is placed lengthwise along the first composite article. It will be understood that a range of other reinforcement materials may alternatively, or additionally, be used, such as plies of non-woven or unidirectional material, non-crimp material, etc.

[0239] For the purposes of the test example, a region of the top side of the first composite article 10 extending between electrode portions (discussed below) constitutes a first faying surface, and the region of the underside of the second composite article 12 constitutes a second faying surface. The faying surfaces define a contact area between the first and second composite articles 10, 12.

[0240] Provided therebetween is a conductive element, indicated generally as 20, formed from a stainless steel woven mesh. As disclosed herein, the conductive element may in alternative embodiments comprise one or more layers of a conductive fabric, paint, ink or the like. As shown in FIG. 2, the conductive element 20 includes a longitudinal portion 22, which is of substantially the same length and width as the contact area, and first and second electrode portions 24a, 24b.

[0241] The plurality of electrode portions 24a, 24b have a lower resistivity than the adjacent portions 20a of the conductive element 20.

[0242] The longitudinal portion 22 of the conductive element 20 is formed from a length of the stainless steel mesh, extending along a length (in the direction X) of the contact area. The electrode portions 24a, 24b are each formed from a lateral portions 26 of the steel mesh, extending across the width (direction Y) of the contact area, defining electrodes 28 at the exposed ends thereof which extend beyond the contact area. The lateral portions 26 and the longitudinal portion 22 are in electrical contact with one another, thereby resulting in the electrode portions 24a, 24b having lower resistivity. In the example shown, the lateral portions are orthogonal to the longitudinal portion, however an alternative embodiments and as may be required by the geometry of a composite article, the lateral portions may extend at an alternative angle across the width of the contact area.

[0243] The strips of steel mesh from which the conductive element is formed are in this test example simply laid on the first composite article, before laying the second composite article thereon. The composite articles are taped together as shown in FIG. 1. The composite articles are then clamped together (not shown).

[0244] FIG. 3 schematically shows the steps of the method by which the first and second composite articles 10, 12 can be continuously welded to one another to provide a continuous weld therebetween.

[0245] In a first step (FIG. 3(a)), an electrical current is applied between the electrodes 28a of the first electrode portion 24a to raise the temperature of the thermoplastic material and weld together a portion of the faying surfaces between the electrodes 28a of the first electrode portion 24a. The thermoplastic material around the region of faying surfaces that are heated is caused to melt and infiltrate the pores of the steel mesh of the conductive element such that thermoplastic material extends through the thickness of the conductive element between the composite articles, creating a welded join. In some embodiments, an additional layer or layers of thermoplastic material may be provided between the faying surfaces, or the conductive element may be pre-impregnated with thermoplastic material.

[0246] In a second step (FIG. 3(b)), an electrical current is applied between the electrodes 28b of the first electrode portion 24b to raise the temperature of the thermoplastic material and weld together a portion of the faying surfaces between the electrodes 28b of the first electrode portion 24b.

[0247] In a third step (FIG. 3(c)), an electrical current is applied between an electrode 28a of the first electrode portion 24a, and an electrode 28b of the second electrode portion 24b, to raise the temperature of the thermoplastic material weld together a portion of the faying surfaces between the electrodes 28a and 28b of the first and second electrode portions. It will be understood that in this step either of the electrodes with the respective electrode portions may be used.

[0248] In the first two steps, the lower resistivity of the electrode portions ensures minimal current leakage to adjacent portions of the conductive element, and so results in localised or spot welding across the width of the contact area. Similarly, the lower resistivity ensures that minimal re-melting or re-welding of these regions occurs when welding a portion of the length of the contact area, in the third step.

[0249] It will be appreciated that in alternative embodiments these steps can be conducted in any suitable order, for example with the steps of FIGS. 3(a) and (3b) reversed, and/or with the steps of one or other of FIGS. 3(a) and (b) being conducted after that of FIG. 3(c).

[0250] Thermocouples 14, 16 were inserted in the test example, along a part of the contact area and at the electrode portions, to monitor temperatures during the welding process. Results of this thermocouple analysis showed minimal temperature rise, and this minimal current leakage into the mesh between the electrode portions during the first and second steps; and minimal heating of the overlapping mesh of the electrode portions during the third step.

[0251] The inventive process can be iteratively repeated for larger or high aspect ratio welds, until welding of the entire composite structure has been completed.

[0252] Additionally, this process only requires access to one side of the part (e.g. welding stringers/frames from the inside of a fuselage) and/or electrode portions may be positioned where access to the electrodes is most convenient (e.g. between frame bays of an aerospace structure).

[0253] FIGS. 4(a) to (f) schematically depict methods comprising a greater number of electrode portions (four, in the examples shown). A conductive element 120 can be positioned between the faying surfaces of composite articles (not shown), generally as discussed above in relation FIG. 1.

[0254] The composite element 120 comprises a longitudinal portion 122 and a plurality of 4 electrode portions 124a-d spaced apart along a length of the longitudinal portion. The faying services adjacent to the first and second electrode portions 124a, b can be spot welded by applying a current between the electrodes 128a of the first electrode portion 124a and by applying a current between the electrodes 128b of the second electrode portion 124b (FIG. 4(a)). By applying a current between electrodes 128a and 128b of the first and second electrode portions 128a, 128b, the region 122a of the longitudinal portion 122 therebetween is then resistively welded (FIG. 4(b)) generally as discussed in relation to FIG. 3.

[0255] FIG. 4(c) depicts applying a current between the electrodes 128c of the third electrode portion 124c, to spot weld the region of the flying surface adjacent to the third electrode portion. The region 122b between the second and third electrode portions is then welded by applying a current between an electrode of the second electrode portion 124b and an electrode of the third electrode portion 124c (FIG. 4(d)). An electrical current is then applied between the electrodes 128d of the fourth electrode portion 124d, to spot weld the faying surfaces adjacent to the fourth electrode portion (FIG. 4(e)) and then the region 122c of the longitudinal portion 122 resistively welded by applying a current between an electrode of the third electrode portion 124c and an electrode of the fourth electrode portion 124d.

[0256] FIGS. 5(a)-(f) show an alternative embodiment of the resistance welding method. Spot welding across the first and second electrode portions and along the regions 122a between the first and second electrode portions is illustrated in FIGS. 5(a) and (b). Spot welding across the third and fourth electrode portions 124c and 124d is the performed (FIGS. 5(c) and (d)). These steps may be conducted in either order, before or after welding along the region 122a.

[0257] The regions 122b and 122c of the longitudinal portion between the second and third electrode portions 124b and 124c, and the third and fourth electrode portions 124c and 124d, respectively, can then be resistively welded (FIGS. 5(e) and 5(f)). Again these steps may be conducted in either order and indeed, in alternative embodiments, all of the four electrode portions may be spot welded before subsequently welding the regions 122a to 122c of the longitudinal portion 122.

[0258] This “tack welding” process can be used to fix the second composite article in place before resistively welding the longitudinal portion. This can be of particular benefits for welding across large or high aspect-ratio contact areas, to prevent movement during step welding along the length of the contact area. The inventive method and apparatus provides for a continuous weld to be broken down into steps, since the difference in the resistivity between the electrode portions and the adjacent regions of the conductive element allows selected regions of the contact areas to be preferentially welded with negligible current leakage and heat generation to the surrounding regions of the conductive element.

[0259] Another advantage of the inventive process for some applications is that, by breaking the welding process into stages, providing a consistent resistance weld between composite articles with variable cross section faying surfaces is possible, such as tapered stringers as might be provided in a tapered fuselage section of an aircraft; because the welding recipe can be optimized for each step of the process.

[0260] FIGS. 6 and 7 show a further example of the application of the invention to a contoured part.

[0261] A first composite article 210, a sheet of the thermoplastic carbon fibre composite material, is placed on a work bench. A second composite article 212, a curved strip of the same composite material as the first composite article, is positioned adjacent to a correspondingly curved upper edge of the first composite article 210.

[0262] The region of the first composite article 210 adjacent to the second composite article 212 constitutes a first faying surface, and the underside of the second composite article 212 constitutes a second faying surface. The faying surfaces define a contact area between the first and second composite articles 210, 212. Electrodes 228a-228d of electrode portions spaced apart along the curved length of the contact area, extend from between the faying surfaces.

[0263] Provided between the composite articles 210, 212 is a stainless steel mesh conductive element. The conductive element is cut to size in situ, using the second composite article, curved strip 212 as a template to form the longitudinal portion 222 thereof, as shown in FIG. 7, and the lateral portions 226 and the longitudinal portion 222 laid out on the first composite article, before laying the second composite article thereon. An electrode is formed by placing a piece of foil 227 in electrical contact with the ends 226a of each lateral portion. In the figure, the electrodes have been formed along one side of the longitudinal portion. The further reduction in resistivity provided by the foil (or, in alternative embodiments, other electrode such as wire or the like) ensures that Ohmic/Joule heating occurs preferentially in the regions of the longitudinal electrode portions between the faying surfaces and not in those portions which extend therefrom.

[0264] The pieces of mesh are taped loosely in position on a backer paper 240, to assist in transferring the conductive element 220 to the desired position on the first composite article.

[0265] There first and second composite articles are then clamped together and resistively welded in the stepwise manner generally as discussed above in relation to FIGS. 4 and 5.

[0266] FIG. 8 shows a composite structure 300, comprising a first composite article 310 (made of a thermoplastic carbon fibre composite material) which has been joined by resistively welding by the stepwise methods disclosed herein, to a second composite article 312. In the embodiment shown, the composite structure is a subsection of a nose wheel well bulkhead. The first composite article 310 is an L-Chord, which functions as a stiffening element for the second composite article, which is the “web”. Visible in the figure are electrodes 328a-328d at the ends of corresponding electrode portions of a steel mesh conductive element (not visible in the figure) which are spaced apart along a (curved) length (along the curved dotted line A) of the contact area between the faying surfaces of the web 312 and the L-chord 310.

[0267] The conductive element within the composite structure 300 was cut to size in situ, using the faying surface of the L-chord 310 as a template for the longitudinal portion of the conductive element, and assembled on the faying surface along the curved upper edge of the web 312 prior to welding.

[0268] The electrodes can optionally be trimmed away, or alternatively left in place to facilitate repair at a later time.

[0269] As discussed above, two composite articles may define a closed geometry composite structure, having a substantially enclosed volume. FIG. 9 shows an example of an aircraft skin panel 310 (a first composite article) and stringers 312 on the panel. Each of the stringers includes two faying surfaces (such that corresponding faying surfaces are defined as regions of the skin panel 310), and define an elongate, trapezoidal in cross section, tubular substantially enclosed volume 318 between each stringer 312 and the skin 310. FIG. 10 schematically illustrates in plan view selected features of the highlighted region X of FIG. 9. FIG. 11 is a schematic cross sectional view through the region X, through the stringer 312.

[0270] The stringer 312 has a primary faying surface 341 and a secondary faying surface 342. The skin 310 has corresponding primary and secondary faying surfaces 351, 352. Between the respective primary and secondary contact areas 340, 350 between these faying surfaces are primary and secondary conductive elements 3201 and 3202, each generally as disclosed above in relation to other embodiments, having longitudinal portions 3221 and 3222 and electrode portions 3241 and 3242 in electrical contact therewith. Only two electrode portions 3241a and 3241b, and 3242a and 3242b of each conductive element are shown, but it will be understood that for high aspect ratio parts such as panels reinforced with stringers, a larger number of electrode portions will typically be used.

[0271] Each of the contact surfaces has an inner edge portion 340i, 350i, within the enclosed volume 318. Each of the electrode portions has, extending out from the inner edge portions 340i, 350i, of contact surfaces 340, 350, are inner electrodes 3281ai, 3281bi, 3282ai and 3282bi. Extending out from the outer edge portions 340o, 350o of the contact surfaces 340, 350 opposite to the inner edge portions, are outer electrodes 3281ao, 3281bo, 3282ao and 3282bo; at the oppose ends of the respective electrode portions 3241a, 3241b, 3242a, 3242b.

[0272] Access to the inner electrodes 3281ai, 3281bi, 3282ai and 3282bi is hindered by the composite materials and access becomes progressively impractical the further from the open ends of the stringers that an electrode is positioned.

[0273] Were resistive welding to be attempted using electrode portions extending across both contact areas, the regions within the enclosed volume 318 would not be adjacent any meltable or softenable matrix material (in this instance thermoplastic). Consequently, applying current would overheat these regions, causing damage and reliable bonds between the faying surfaces could not be ensured. The use of separate conductive elements makes resistive welding possible, using electrical connection apparatus capable of remotely establishing electrical connection with inner electrodes.

[0274] The inner electrodes 3281ai, 3281bi, 3282ai and 3282bi are oriented diagonally away from the skin 310 during assembly, ready for remote electrical connection.

[0275] FIG. 12 shows an electrical connection apparatus 1000, adapted to remotely connect to the inner electrodes and, whilst connected, perform thermal welding operations as disclosed herein.

[0276] The apparatus 1000 includes a first probe arrangement 1100 and a second probe arrangement 1200, attached to the distal end of an elongate umbilical 1300. The umbilical is a tubular member through which electrical cables and pneumatic supply lines extend, as discussed in further detail below. The probe arrangements 1100, 1200 are connected via a further tubular member 1310. The probe arrangements are positioned a distance apart equal to the distance between the pairs of electrode portions 3241a and b, and 3242a and b. The conductive elements 3201, 3202 can be assembled so that the electrode portions are suitably spaced.

[0277] The apparatus 1000 also includes a drive arrangement 1400, through which the umbilical 1300 extends. The drive arrangement includes a frame 1410, to which is mounted an electronic motor 1420. The umbilical 130 extends through the motor 1420 and its longitudinal position controlled using drive wheels (not visible in the figure). The drive arrangement 1400 also includes guide wheels 1430. The umbilical is longitudinally incompressible, under the forces encountered in normal use, but is provided with a degree of flexibility to allow excess length of umbilical to be coiled until required for use.

[0278] FIG. 13 shows a schematic cross sectional view of a probe arrangement 1100. The probe arrangement has a foot portion 1110 having a lower surface 1112 that engages the skin panel 310. The probe arrangement 1100 has a body portion, indicated generally as 1140, having a cross sectional shape sized to fit within the enclosed volume 318. The body portion 1140 has an inflatable member 1142, and a shoulder portion 1160, formed as a reinforced region of the outer skin of the inflatable member, the outer surface of which acts as an engagement surface 1162, in use. The probe portion 1100 is shown in FIG. 13 in a contracted configuration in which the distance between the engagement surfaces 1112, 1162 is less than the distance between the skin 310 and the opposed inner face 319 of the substantially enclosed volume 318.

[0279] A pneumatic supply conduit 1170 (which runs through the umbilical and is connectable to a compressed air supply) extends to the foot portion 1110, and supply tube 1172 branches therefrom to the interior of the inflatable member 1142. The conduit 1172 also runs on through the tubular member 1310 to the second probe arrangement 1200. Also extending to the probe portion 1100 via the umbilical 1300 are electrical supply cables 1174, which supply electrical current to (and enable detection of the connection status of) the electrical contact element 1114. The electrical contact element 1114 has a lower part 1180, and an upper part 1190 that is operatively coupled via platform 1118 of the foot portion 1110, to the inflatable member 1142. The lower part 1180 has a primary lower contact surface 1182 to one side of the probe portion and a secondary lower contact surface 1184 to the opposite side of the probe portion. The upper part 1190 has a primary upper contact surface 1192 to one side of the probe portion and a secondary upper contact surface 1194 to the opposite side of the probe portion. The opposed lower and upper contact surfaces provide a gap therebetween for the inner electrodes.

[0280] The probe portions of the electrical contact apparatus 1000 can be introduced into the volume 318, while in the contracted configuration, and positioned using the drive arrangement 1400, to place the probe portions adjacent to the inner electrodes 3281ai, 3281bi, 3282ai and 3282bi, as shown schematically in FIG. 15(a) for the electrodes of the electrode portions 3221a and 3222a. The inflatable member 1142 is then inflated so that the engagement surface 1162 engages the upper wall 319 of the enclosed volume 318, as shown in FIG. 15(b). The inner electrodes 3281ai and 3282ai are trapped between the upper and lower parts 1180, 1190 and so form a strong electrical connection therewith.

[0281] In this position, as illustrated in FIG. 14, the apparatus 1000 can be used to deliver current via the inner electrodes 3281ai, 3281bi, 3282ai and 3282bi, in the manner generally disclosed above in relation to FIGS. 1 to 8. In conjunction with an apparatus to connect to the outer electrodes, 3281ao, 3281bo, 3282ao and 3282bo, the four spot welds across the electrode portions can be conducted across the electrode portions 3221a and 3221b, and 3222a and 3222b, with the apparatus in the position shown in FIG. 14. In addition, the longitudinal welds between the electrode portions 3221a and 3222a, and between electrode portions 3221b and 3222b can all be completed with the apparatus 1000 in this single position, if required. Alternatively, the inflatable member 1142 may be deflated to disconnect the inner electrodes, before longitudinal welds between the electrode portions 3221a and 3222a are formed using the outer electrodes 3281ao, 3281bo and 3282ao and 3282bo. Still further, the electrical connection apparatus 1000 may be advanced so as to form more than two spot welds, before longitudinal welding is conducted (using the inner or the outer electrodes).

[0282] An alternative electrical connection apparatus 1001 is shown in FIG. 16, having at the end of umbilical 1300 first and second probe portions 1101 and 1201 as previously described. Mounted to the tubular 1311 by which the probe portions are connected, is a thermal imaging camera 1500. The camera 1500 can be used to acquire thermal images from within the enclosed volume 318, when the apparatus 1001 is in situ. This non-destructive testing can be conducted during or immediately after welding steps have been conducted, to verify weld quality (e.g. during cooling), or during a separate step by applying a lower current between electrodes sufficient to heat portions of each of the conductive elements 2301, 3202 to below the melting temperature of the thermoplastic material within the contact areas 340, 350. The apparatus 1000, 1001 and/or the conductive elements disclosed herein can also be used throughout the lifetime of the composite structure formed between the article 310, 312 to perform non-destructive testing (conductivity measurements, resistivity measurements or thermal imaging (in the case of apparatus 1000, from outside of the cavity) and to re-weld or repair any portion of the contact surfaces as required.

[0283] Whilst the examples above relate to thermoplastic carbon fibre composites, the methods and operators may also be applied to other composites, such as fibreglass or composites with a curable matrix material such as epoxy based carbon fibre composites. The faying surfaces may be joined, in such embodiments, by roughening or providing porosity thereto, such that a meltable or softenable material, such as a thermoplastic, provided between the faying surfaces during welding, can infiltrate the porous faying services or conform to the roughened faying services, and join such composite articles together. The meltable of softenable material can also comprise an uncured or partially cured resin or polymer, wherein resistively welding initiates or progresses the curing reaction.