ADHESIVE BONDING OF STRUCTURES

Abstract: A method of forming composite articles from components (12, 14, 16) is described in which the components are joined by adhesive at their interfaces (18). The components have cavities (22) and the adhesive is compressed while it sets by placing magnetically attractable particulate material (20) into the cavities on one side of the interface and placing magnets (26) on the other side of the interface so that the attraction between the magnets and the particulate material compresses the adhesive. The particulate material may be removed from the cavities through apertures (24) after the adhesive has set.

Description: FIELD OF THE INVENTION The present invention relates to the bonding of components to make a composite structure and it especially relates to the bonding of aluminium and aluminium alloys for use in the aircraft industry. BACKGROUND ART Aircraft designers are constantly searching for ways of connecting components together to produce a strong bond while using a minimum of additional material to form the bond in order to keep the weight of the aircraft structure as low as possible. One proposal has been to bond components together using an adhesive and this has proved successful but requires the bond to be compressed while the adhesive sets. When the bonded region is readily accessible, the components can simply be compressed by means of a clamp applied to the bonded region; however, when the bonded region is not accessible, for example because one of the components has an enclosed cavity above the bonded area so that clamps cannot reach the bonded area, the two components must be held in a press to urge the two components together while the adhesive sets. However, not only is it impractical to place large structures in a press but also, since pressure in the press is applied to the whole structure, the pressure can deform parts that are not being bonded and to avoid this it is often necessary to support such parts while the components are in the press. The provision of supports is time-consuming and in many cases the structure being bonded precludes the use of removable internal supports, for example when it is desired to bond two components together to form an enclosed cavity; in the latter case, the support could be built into the enclosed space but this increases the weight of the resulting structure and defeats the primary object of adhesive bonding. The setting of the adhesive can often be accelerated by heat and to achieve simultaneous heating and compression of the adhesive the structure can be placed in an autoclave; however, large autoclaves are expensive. U.S. Patent 3,879,247 describes a method of sealing packages by incorporating a magnetically susceptible strip in one flap of the package, placing a second flap over the first with a layer of adhesive between them, and placing a magnetic coil against the second flap to attract the strip in the first flap and to hold the two flaps together while the adhesive is setting. However, the incorporation of magnetically susceptible strips is disadvantageous, particularly in aircraft where they would add to the weight of the structure and where they could affect the working of instruments. GB 586,978 describes a method of bonding articles together by placing a magnet on one side of an assembly to which adhesive has been applied and an armature on another, so that the adhesive is compressed by the magnetic attraction between the magnet and the armature while it is setting but such an arrangement does not allow the joining of components having enclosed cavities therein. DISCLOSURE OF THE INVENTION We have now developed a new technique for compressing adhesive bonds while they set. According to the present invention, there is provided a method of joining a first component and a second component that are magnetically non-susceptible to form a composite article having an enclosed hollow cavity therein, which cavity is at least partly formed by the first component being joined, which method comprises applying adhesive to the interface between the first and the second components and compressing the adhesive at the interface while it sets, wherein the compression of the adhesive at the interface is achieved by the attraction between magnetically attractable particulate material located within the cavity and a magnet located on the side of the interface remote from the particulate material and wherein the method further includes removing the particulate material from the cavity through an aperture formed in a wall of the cavity. The present invention is primarily directed to bonding components made of aluminium or aluminium alloys in fabricating parts for aircraft. The magnet may be a permanent magnet or an electromagnet. The magnetically attractable material could be small permanently magnetic or ferromagnetic particles, such as small ball bearings, that can be removed from the hollow cavity after bonding by drawing them through the small aperture formed in the structure, the aperture being small enough not to reduce the strength of the composite structure. If required the aperture could be closed after removal of the particulate material. In the method of the present invention, only those parts of a structure are compressed that are being bonded and thus there is no need to support the remaining parts of the structure. The present invention also removes the necessity of using an autoclave to apply heat and pressure to the the adhesive while it sets; thus, according to the present invention, the structure can be heated in a simple oven during bonding, thereby considerably reducing tooling costs. Needless to say, the magnets used in the present invention must be capable of withstanding the temperatures encountered during heating of the adhesive and they should impart a sufficient compressive force on the bond to ensure that the components are adequately bonded together. BRIEF DESCRIPTION OF THE DRAWINGS The method of the present invention is shown schematically by way of example in the attached drawings in which: Figure 1 is a sectional view through a panel during bonding, and Figure 2 is an exploded orthogonal view of a sandwich panel suitable for use in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Two outer aluminium face sheets 12 and 14 are to be bonded to opposed sides of a corrugated aluminium core 16 by means of adhesive (Redux 319A) located at interfaces 18 between the peaks/troughs of the core 16 and the adjacent parts of the sheets 12, 14 to form a composite panel. Ferromagnetic steel ball bearings 20 are passed through apertures 24 in the sheets 12,14 into each hollow cavity 22 formed by the sheets and the core. Electromagnets 26 are placed against the outside of the sheets opposite to the adhesive at 18 and, when energised, attract the ball bearings thereby compressing the adhesive between the core 16 and the sheets 10 and 12. The structure is then placed in an air circulating oven until the adhesive has set. The electromagnets are then de-energised and the ball bearings 20 are removed from cavities 22 through apertures 24. The removal of the ball bearings may be assisted by a magnet (not shown) located opposite the apertures and by inverting the panel. The apertures 24 should be sufficiently small to avoid having an adverse effect on the strength of the structure and they can subsequently be covered over. The corrugated core is preferably waffle-shaped so that the individual peaks and/or troughs of the corrugated core are formed as isolated cells 30 (see Figure 2).

Claims: A method of joining a first component (14, 16) and a second component (12) that are magnetically non-susceptible to form a composite article having an enclosed hollow cavity (22) therein, which cavity (22) is at least partly formed by the first component (14, 16) being joined, which method comprises applying adhesive to the interface (18) between the first and the second components (12; 14, 16) and compressing the adhesive at the interface until it has set, characterised in that the compression of the adhesive at the interface (18) while it sets is achieved by the attraction between magnetically attractable particulate material (20) located within the cavity (22) and a magnet (26) located on the side of the interface remote from the particulate material and wherein the method further includes removing the particulate material (20) from the cavity (22) through an aperture (24) formed in a wall of the cavity. A method as claimed in claim 1, characterised in that the first component (14, 16) comprises a first part (14) and a second part (16) that together form the said cavity (22). A method as claimed in claim 2, characterised in that the juxtaposition of the two said components (12; 14,16) forms a further cavity (22) and wherein the first part (14) and the second part (16) are joined together at the same time as the first and the second components are joined to each other, the first and the second parts being joined by applying adhesive to the interface (18) between the first and the second parts (14, 16) and compressing the adhesive while it sets by the magnetic attraction between magnetically attractable particulate material (22) located in the further cavity and a magnet (26) located on the side of the interface remote from the particulate material and wherein the method further includes removing the particulate material from tee further cavity (22) through an aperture (22) formed in a wall of the further cavity. A method as claimed in any one of claims 1 to 3, characterised in that the first and second components (12; 14, 16) are made of aluminium or an aluminium alloy. A method as claimed in claim 1, characterised in that the composite article has a sandwich construction consisting of a corrugated core (16) sandwiched between a first and a second face sheet (12, 14), the core being joined to the face sheets by adhesive at the interfaces (18) between the core and the first and second face sheets, which adhesive is compressed while it sets by means of the magnetic attraction between magnetically attractable particulate material (20) located in the corrugations (22) of the core and corresponding magnets (26) located against the face sheets on the side of the interface remote from the particulate material and wherein the face sheets (12, 14) include apertures (24) therein communicating with the said corrugations (22) whereby the particulate material can be removed after the adhesive has set. A method as claimed in claim 5, characterised in that the corrugated core has a waffle shape comprising peaks and troughs and in which the peaks and/or troughs are formed as isolated cells (30). A method as claimed in claim 5 or claim 6, characterised in that the core and face sheets (12, 14, 16) are made of aluminium or an aluminium alloy. A method as claimed in any one of claims 1 to 7, characterised in that the particulte material (20) is formed by ball bearings. A method as claimed in any one of claims 1 to 8, characterised in that the or each aperture (24) is sealed after removal of the particulate material. A method as claimed in any one of claims 1 to 9, characterised in that the structure is heated in an oven while the adhesive sets.

Inventors: ELLIS, RICHARD KEITH, FINCH, DUNCAN RALPH
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: C09J5
IPC codes: C09J5