Changes on the surface...

TWI Bulletin, July - August 2005

Radical advances have been made in composite to metal joining....

Faye Smith received a PhD in Composite Materials from Imperial College in London before spending three years as a post-doctoral researcher at Queen Mary College, University of London studing manufacturing, toughening and hot-wet properties of composites. Following a spell in industry developing nickel coated carbon fibre elements for resistance and induction heating/welding applications across all industry sectors, Faye joined TWI in 2002. Faye is currently a Senior Project Leader in the Polymers Section working on the development and commercialisation of Comeld.

Comeld ^TM is a new joining system developed by TWI for creating joints between composite materials and metals. Already it has shown enormous potential in early trials. The technology is now being developed to produce joints with outstanding mechanical properties. As Faye Smith observes producing joints between composite materials and metals may never be the same again.

High strength, low weight, excellent fatigue and corrosion resistance, combined with design and fabrication flexibility have led to the use of composite materials in applications in all industry sectors.

However, to achieve optimum properties from these materials at the point required in a structure, joining them to themselves and other materials in the structure has to be achieved. The three techniques that are currently commonly used to join composite materials to metals are adhesive bonding, mechanical fastening and combinations of the two.

With careful joint design and material lay-up, bolted joints can compete with adhesive bonding in terms of strength. However, adhesive bonding has the advantage over bolting in terms of weight. The presence of the bolt itself dictates that bolted joints are often heavier than bonded ones. Unlike adhesive bonds, bolted joints can be disassembled, which has benefits in terms of inspection and recyclability. In many cases, to address the limitations of both joining techniques, hybrid joints containing both adhesive and bolts are used. This defeats the objective of using composite materials, ie size, weight and cost savings.

Surfi-Sculpt and Comeld

Surfi-Sculpt is a proprietary materials processing technology developed at TWI. The technique allows the surfaces of materials to be reshaped using any power beam. Currently the electron beam process is used. Protrusions or holes can be reproducibly created at either regular or random intervals across the material. The formation of a Comeld joint between a composite material and a metal involves the application of Surfi-Sculpt to the metal before it is joined with the composite material. Examples of Surfi-Sculpted surfaces in various metals are shown in Fig.1 and in high magnification on the front cover of this edition of the Bulletin. Initial research performed at TWI demonstrated that Comeld joints have significant benefits over control joints between composite materials and metals.

Current research

TWI has initiated a programme of work with the following objectives:

• Provide guidelines for Comeld joint manufacture for a range of composite production techniques.
• Demonstrate the difference in mechanical properties between joints with and without Comeld technology.
• Demonstrate that the variation of Comeld parameters can change the failure mode of a composite to metal joint.

The following sections describe the results of work that has been performed in the first year of this three-year programme.

Autoclave processing

Autoclave processing is used to produce composite parts or structures by consolidating layers of fibres pre-impregnated with resin (known as prepreg). This process is commonly used in the aerospace industry as it produces parts with high fibre volume fraction ( ie potentially high strength and stiffness) and low voidage.

This processing technique was used to produce joints between titanium (6Al-4V) and carbon fibre reinforced epoxy (Hexply 8552 from Hexcel laid up in a 0/90 configuration). The first joint made was a double step joint, the dimensions of which are shown in Fig.2. The first task involved machining the titanium samples to the double step shape. Some of these specimens were simply left in the machined state to act as control joints, but for the Comeld joints the Surfi-Sculpt treatment was applied to produce specimens as shown in Fig.3. For both control and Comeld joints a surface preparation technique was used immediately before manufacture of the joints to enhance adhesion between the resin from the composite and the titanium. In the case of control specimens, the metal surface to be joined was also grit blasted before the treatment was applied. The treatment involved washing all metal specimens in a detergent solution, immersion in an etchant ( Table 1) at room temperature, washing in distilled water, before drying in an oven at 75 ±5°C for 10-15 minutes.

Table 1 Constituents of etchant

Lay-up and processing techniques were devised that allowed the prepreg to integrate with the Surfi-Sculpted surface without air becoming entrapped as shown in Fig.4.

While it was not possible to eliminate fibre waviness completely, variation of processing parameters dictated that it was reduced significantly as shown in Fig.5.

Following the successful production of double step joints between titanium and CFRP, the same processing techniques were applied to the production of scarf joints (see Fig.6a and 6b) between these materials. The protrusions created using the Surfi-Sculpt pattern on the machined titanium were all the same height which meant there were areas where the protrusions stood higher than the base material. Fig.6 shows a scarf joint and demonstrates that the protrusions can be seen protruding through the surface of the CFRP.

Vacuum infusion processing

Vacuum infusion is a processing technique that is known by a variety of acronyms including RIFT, SCRIMP ^TM , FASTRAC, all of which use vacuum to pull resin into a fibrous pre-form. Curing of the resin produces a composite with low voidage. With increasing concerns and legislation against volatile organic compounds (VOCs), the use of vacuum infusion is increasing, as it requires relatively little outlay to convert from hand lay-up techniques to vacuum infusion, and can significantly reduce VOC emission.

The vacuum infusion process was used to prepare double step ( Fig.2) Comeld and control joints between stainless steel (316L) and glass fibre (non-crimp fabrics ELT-850 and EBX-602 from Saint-Gobain Technical Fabrics BTI UK) reinforced vinylester (Reichhold Dion 9102 500). Figure 7 shows stainless steel samples for Comeld joints which have been machined into a double step configuration and Surfi-Sculpted. Before the infusion process the metal specimens for the control joints were grit blasted, all metal samples were then washed using a detergent, etched using an oxalic/sulphuric acid solution ( Table 2), rinsed in distilled water and finally dried in an oven.

Table 2 Constituents of etchant

Figures 8 and 9 which show Comeld and control joints respectively, demonstrate that the vacuum infusion process was successful in producing well consolidated joints with no voidage. In the case of the Comeld specimen, the protrusions can clearly be seen through the thickness of the composite, interacting with the fibres.

During vacuum infusion, the flow of the resin can lead to movement of parts of the preform relative to each other. This is visible in the control joint in Fig.9 where resin rich areas have been created due to flow of the fabric with the resin at the end of the metal section and at the middle section of the step joint. These are potential weak spots in the joint. In the case of the Comeld joints created, resin rich areas were significantly less common as the mechanical interlocking of the protrusions with the fabric meant that the fabric was less likely to move with the flow of resin.

Tensile testing

Following optimisation of production for each type of specimen, Comeld and control specimens were produced and tested at 1mm/minute in tension. The following sections describe the results of this testing.

Ti/CFRP Double step joint performance

The average results of tensile testing of Comeld ( Fig.4) and control double step joints between titanium and CFRP are given in Table 3. It can be seen that the Comeld specimens had a higher load carrying capability and exhibited considerably more extension during loading than the control specimens. These characteristics are shown more clearly in Fig.10. It can be seen that the Comeld and control joints have similar loading characteristics initially although the Comeld joint continued carrying load long after failure of the control joint. After the knee in the Comeld load displacement curve (which is thought to be a characteristic of the joint), three dips can be seen in the curve. These corresponded to damage, in the form of cracking and delamination occurring in the composite, which initiated at the steps in the joint. It can be seen that despite the damage and dip in the curve, the joint continued to carry load.

Table 3 Average tensile test results for double step Ti/CFRP joints

Table 3 and Fig.10 also demonstrate the exceptional energy absorption characteristics of Comeld joints when compared to control joints. The average values indicate that Comeld joints absorbed more than 13 times more energy before failure than the control joints. The contrasting failure modes of the two types of joints explain this difference in absorbed energy. The control joint shown in Fig.11 failed without warning, at the interface between the composite and the metal.

By contrast, damage initiated in the composite section of the Comeld specimen, shown in Fig.12, at the end of the titanium (area A). Cracks and delamination propagated through the composite, absorbing energy but without significantly affecting the load carrying capability. The second stage of damage initiated at the middle step in the titanium sample (area B), again allowing cracks and delaminations to propagate through the composite. Final failure occurred with delamination of the composite and plastic deformation of the protrusions in the areas designated 'C'. All of these damage mechanisms described absorbed energy and gave audible and visual signs that, in a real structure, would allow detection of a problem before failure.

Ti/CFRP scarf joint performance

The average results of tensile testing of control and Comeld scarf joints between titanium and CFRP are shown in Table 4. From these figures, there does not initially seem to be much difference between the control and Comeld specimens, however the major difference between these two sets of results can be seen more clearly by viewing the individual specimen results, shown in Fig.13. This figure demonstrates the large degree of scatter seen in the control test data in comparison to the Comeld data.

Table 4 Average tensile test data for Ti/CFRP scarf joints

The difference in scatter can be explained by the different failure modes for the two types of joint. The failure mode of the control specimens is shown in Fig.14. Failure was mainly at the interface between the composite and the metal, but a variable amount of composite failure was also seen. It is this inconsistency in the adhesion of the composite to the metal, despite the surface preparation performed, that accounts for the high degree of scatter in the results. In all cases, the joint failed completely.

In the case of the Comeld specimens see Fig.15, failure initiated in the composite, at the narrower end of the titanium (designated A in Fig.15) and propagated through the thickness of the composite. In all cases the composite remained adhered to the metal.

While the Comeld scarf joints between titanium and CFRP, unlike the double step joint, did not show any significant increase in energy absorbed or load carrying capability in comparison to the control joints, they provided something that is potentially more important, especially to designers - a higher degree of consistency.

Stainless steel/GFRP double step joint performance

The average results of tensile testing of Comeld ( Fig.8) and control ( Fig.9) double step joints between stainless steel and GFRP are given in Table 5. It can be seen that the Comeld specimens had a higher load carrying capability and exhibited a much greater extension before failure than the control specimens. On average, the Comeld specimens absorbed more than five times as much energy as the control specimens before failure.

Table 5 Average tensile test data for double step, stainless steel/GFRP joints

The loading history for the control specimens and Comeld specimens, (see Fig.16) was identical until the sudden failure of the control specimens. As shown in Fig.17 the failure of the control specimens occurred at the interface between the composite and the metal.

In the case of the Comeld specimens whitening was seen, at load values slightly below the failure load for the control specimens, within the composite section of the joint near the end of the metal section (section 'A' in Fig.18). This whitening was matrix cracking within the composite. The delamination damage that was seen before failure in the Ti/CFRP double step Comeld joints, and which caused dips in the load-displacement curve, was not seen in these joints. It is thought it was prevented by the through-thickness stitching of the non-crimp fabric. Instead, progression of damage in the form of matrix cracking was seen and heard until the failure point when dramatic delamination was accompanied by fibre and matrix fracture.

Benefits of Comeld joints

Having shown that good quality, reproducible Comeld joints can be manufactured, the testing data produced to date have shown:

• Comeld joints prevent failure at the interface in composite to metal joints.
• By forcing failure into the composite material rather than the interface, Comeld double step joints have been shown to increase load carrying capability and energy absorbed significantly before failure compared with control joints.
• Prevention of interfacial failure in scarf joints significantly decreased the scatter in tensile test results.
• The Comeld joints exhibited damage in the composite material that could be seen and heard before failure of the joint. This could be used to detect problems in a structure before ultimate failure.

Future work

Work is continuing on the Core Research Project into factors such as the effect of other forms of loading on Comeld joints, non-destructive testing of Comeld joints and production of Comeld joints between other materials.

A separate project is underway to investigate the effect of fatigue loading on Comeld joints and metal with the Surfi-Sculpt treatment applied.

A Group Sponsored Project was also started in November 2004, which is working to optimise Comeld joints for use in specific applications requested by the sponsor companies.

Glossary of terms