GB2479776A

GB2479776A - Electrical testing of joints between materials

Info

Publication number: GB2479776A
Application number: GB1006731A
Authority: GB
Inventors: Jonathan Meyer
Original assignee: Airbus Group Ltd
Current assignee: Airbus Group Ltd
Priority date: 2010-04-22
Filing date: 2010-04-22
Publication date: 2011-10-26
Anticipated expiration: 2030-04-22
Also published as: GB201006731D0; WO2011131995A1; GB2479776B

Abstract

Strong joints between a metal part 1 and a carbon fibre reinforced plastics component 18 can be made by the "Hypin" technique, where metallic pins 5 protruding from the metal part 1 are embedded in the resin component 18. However, they are difficult to test for fibre distribution, voids etc. The invention proposes making electrical contact to the plastics component 18 via metal terminals 42 inserted or embedded in the component so as to pass into the fibre reinforcement, and measuring the electrical resistance or conductance between the conductive part 1 and the plastics component 18 for comparison with a predetermined value. High resistance values indicate poor contact of fibres with the pins 5 and hence a possibly sub-standard joint.

Description

TESTING JOINTS BETWEEN COMPOSITE PND METAL PARTS

FIELD OF THE INVENTION

The present invention relates to a method of inspecting a joint between two components; it has particular application to a hybrid penetrative joint between a metal component and a fibre-composite component.

BACKGROUND OF THE INVENTION

Joining between composite and metallic or thermoplastic components is currently approached in a number of ways, each with its own limitations.

The use of fasteners is commonplace but tends to result in de-larnination around fastener holes, as well as the ____ associated difficulties of drilling holes in composites such as Carbon Fibre and Aramids such as Keviar.

The bearing strength of laminated composites tends to be low, as does the inter-laminar shear strength. This results in a requirement for significant reinforcement around fastener holes, leading to a large weight increase, which is particularly undesirable in aerospace applications.

Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this. Existing research into the use of surface features to improve the strength of metallic/composite joints is limited.

WO 2008/110835 (Airbus) provides a method of joining a pair of components, in which an array of projections is grown on a bond region of a first (metal) component by stacking a series of layers, to form a kind of spiky mat on the first component. The latter is then urged into a second (OFRP) component so that the projections are embedded in the matrix of the second component. An intimate bond is thus formed.

This is known internally as a Hybrid Penetrative Reinforcement (Hyper) Joint, and the projections are known as Hypins. WO 2008/110835 discusses particular methods of making such pins, but the present invention is not limited to these: another possibility is shown in WO 2004/08731 (TWI), for instance.

The Hyper Joint provides excellent mechanical properties, but, because of its complex geometry and large differential material densities in the interface region, it can be difficult cost-effectively to examine the joint for defects.

The most critical defects are: 1) Lack of fusion between the Hypins and the metallic part, and 2) Lack of fibre consolidation around the Hypins (resin-rich pockets)

SUMMARY OF THE INVENTION

According to the invention there is provided a method of testing the integrity of a joint between a metal or other conductive part and a fibre-composite part, the fibre-composite part being also adequately electrically conductive, in which the resistance between the metal part and a point in the fibre-composite part is measured, and this measurement is compared with a predetermined standard.

Embodiments of the invention use a sensitive ohm-meter or similar device to measure the resistance between a known location in the laminate, such as a pin-hole drilled to expose fibres, or a measurement terminal embedded in the composite, and a known location on the metallic part. The resistance will increase if there are significant resin-rich areas between the Hypins and the conductive fibres in the composite, or a significant lack of fusion between the pins and the base part. The variation can be expected to be in the region of mO or even pc. A method of this type clearly only works for composites utilising electrically conductive fibres (e.g. carbon) and joints utilising conductive materials (e.g. steel, titanium), such materials being referred to for convenience as metals.

Without this method of non-destructive examination (NDE), the alternatives would be limited to X-ray methods such as CT, which are extremely expensive and difficult to implement for large hybrid structures.

Preferably there are several measurement terminals or points. In conjunction with computer modelling, these multiple reference locations can be used in order to build a picture' of where the defects in the joint are located.

This method is very low in cost, and could be applied in an in-service as well as a production environment. For example, it could be combined with permanently embedded sensors to perform a structural-health-monitoring role, detecting damage propagation due to fatigue or in-service incidents. It is also very portable, and can be used on parts of very large scale where other NDT/NDE methods would be difficult to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, embodiments will now be described with reference to the accompanying drawings, in which: FIG. 1 is a cross-sectional view of a Hyper Joint, with measuring apparatus indicated schematically.

DETAILED DESCRIPTION

Figure 1 shows a joint between a metallic part, for instance a floating rib foot 1, and a CFRP part 18 in the form of a sheet, for instance a wing cover. The metallic part comprises a web portion 3 forming an upstanding bracket part and a pair of flanges 2 forming a base. An array of projections or joint pins ("Hypins") 5 extends from the underside of the flanges 2. As can be seen in Figure 1, the projections 5 are distributed more or less evenly over a bond region which extends over most of the area of the flanges 2.

The base 2 lies flush with a CFRP sheet 18, with the projections 5 penetrating the sheet to approximately half its depth. In fact, a sheet of glass ply 19 is inserted between the base 2 and the CFRP part 18 and is penetrated by the projections; this will be described later. To assemble the parts, the projections are pressed into the prepreg, or the layup, so that they are embedded in the laminate, and the assembly is then cured.

The result is a good join. However, it cannot be inspected by eye, and there could be voids around the pins, or resin regions inadequately penetrated by fibres.

To test the joint, therefore, the following procedure is adopted.

1) A component part is provided, consisting of a metallic part or bracket 1 attached to a Carbon-Fibre panel by means of a Hyper joint (as described in WO 2008/110835) . A valuable innovation is the addition of a single ply of glass-fibre material 19 adjacent to the interface with the bracket to act as an electrical insulator.

2) Either: a single hole or a plurality of small holes 40 is drilled in the laminate at a known distance(s) from the metallic part interface, or a single pin 42 or a plurality of small conductive, preferably metallic, pins is embedded in the laminate with their surfaces co-planar with or protruding above the composite surface. In principle, both these measures could be adopted.

The pins or holes could typically be 3-5mm long and 1mm in diameter.

3) A sensitive device 100 for electrical resistance or conductive measurement is connected at one end 102 to a known location on the metallic part, and at the other end 104 to either a pin inserted into the drilled hole or a terminal pin that is made to contact an embedded pin at the laminate surface.

4) The measured resistance between the bracket 1 and the pin or pins is a function of the distance from the interface and the connectivity between the Hypins and the carbon fibres in the laminate. The insulating ply 19 eliminates conduction between the metallic part face 2 and the laminate 18, and so the only conductive path is through the pins.

5) By calibration through test articles with seeded defects and/or simulation using computational numerical methods it is possible to identify the presence and location of a defect by measuring the resistance between multiple locations and comparing the benchmark values.

In general, the principle is that the plurality of current paths represented by the Hypins behave as parallel circuits, and so the anticipated total resistance measurement would be proportional to the sum of the individual resistances at each pin, as follows: lIRtotai = (1/R1) ÷ (11R2) + (11R3) +.... (1IR) This formula is the basis for identifying the effect of a defect, albeit with the complication that there will be some variability due to the distance between the pins and the measuring points. However, it is this variability in distance that makes it possible to pinpoint the location of a defect if multiple measurements are taken between different locations.

The test setup comprises the resistance measurement device, a plurality of pins embedded at known locations around the conductive part, and a testing rig with contacts designed to locate on the embedded pins and the conductive part in known locations. Theoretically a smaller number of contacts could be used, designed to be movable between the embedded pins on the CFRP part. A small microprocessor is used to analyse the incoming currents and calculate whether a defect is present and whether it is in a critical region (typically the edges of the joint are more critical as they carry more load) . This could then simply give a green or red light as an indication to the user as to whether further investigation is required.

In the foregoing, reference is made to a CFRP part, but the invention is applicable to any fibre-reinforced part, provided the fibres have adequate conductivity. Also, the protrusions can be made by any suitable method, not merely that described in the earlier Airbus application. The invention is also applicable to a joint of the kind shown in Figure 10 of WO 2008/110835, in which a metal "mat" of protruding pins is sandwiched between two CFRP layers. Here there would be measurement pins embedded in each of the CFRP layers.