It is possible to appreciably improve the strength of joints between composite sheets and metals by using metal studs to join uncured composite to metal and then curing it. This means the composite fibres are not severed by any process that involves drilling through cured material.

The process has been developed by Edmund Semmes at NASA's Marshall Space Flight Centre in Hunstville, Alabama in the USA. The aim is to improve the equipping of structures such as polymer matrix composite tanks and ceramic matrix composite liners for jet and rocket engine nozzles as well as the attachment of metal objects to composite aircraft fuselages and wings.

Present practice is either to drill holes in composite material and screw or rivet it to the metal, insert thread inserts in the uncured material and attach the metal part using screws, or simply to bond the two together. Drilling holes in cured material severs fibres and provides opportunities for cracks to form and the potential for delamination.

Using threaded inserts works quite well but complicates assembly. Relying on bonding alone leaves the assembly at risk of peel failure. Ideally, it was found some years ago, joints should combine both bonding and mechanical fasteners to avoid the start of peel. There is also a problem, if bonding is relied on alone, that failure can arise as a result of the difference in thermal coefficients of expansions between composites and metals.

In the method investigated by Edmund Semmes, the metal component is fabricated to include multiple studs projecting from its surface. In preparation for joining, holes are formed in the uncured composite. The two are then brought together so that the studs are firmly seated in the holes. Alternatively, Semmes suggests Z-direction fasteners could join the uncured composite and metallic parts. These are stepped clamps attached to the metal part by a screw that can be tightened to secure the edge of the composite sheet.

Whereas securing a ceramic matrix composite may require techniques such as chemical vapour infiltration, or adding a fusible transitional metal layer. This is opposed to just putting the polymer matrix composite and metal attachments in an oven. The end result is much the same, and is similar in philosophy to TWI's Comeld process. The latter involves laying up uncured composite on a metallic surface that has been deliberately sculpted. The difference is that the NASA process joins sheet on sheet, while the Comeld process allows attachment of metal and composite sheets or rods at the edges or ends.

As a test, Semmes joined an aluminium plate, 102x76x13mm to a 51x203x6mm plate made of carbon fibre in epoxy, toughened by thermoplastic. Sixteen holes were drilled in the aluminium plate to accept thread tapping screws and serve as substitutes for studs. The array of holes was used as a template to drill 16 corresponding holes in the uncured composite.

The screws were inserted through the holes in the uncured composite and tightened. The resulting assembly was cured at 177ºC and then cooled very slowly at between 5.6 and 8.3ºC per hour to avoid excessive thermal stress. The resulting combined structure was then subjected to a tensile/bending test. When the test was terminated at a load of 20.8kN, the composite panel had bent significantly but there was no sign of joint failure.

For more information: Email sammy.a.nabors@nasa.gov quoting ref MFS-31813-1

Pointers

* Method involves attaching uncured composite sheet to metal sheet using studs and then curing the combination

* The process has been successfully used to join carbon fibre epoxy sheet to aluminium, but could potentially also be applied to join ceramic matrix composites to metals