Ewen J C Kellar and Faye Smith
Paper presented at Design & Performance of Composite Materials Conference, The University of Sheffield, 3-6 April 2005
The application of a unique surface treatment technique, called Surfi-Sculpt TM , into a new joining system, called Comeld TM , has been shown to improve damage tolerance of joints between composite materials and metals. In particular, the Comeld approach has been shown to prevent sudden bond-line failure occurring as is typically observed for adhesively bonded joints. Comeld TM joints were shown to either provide a more progressive, and therefore detectable, failure mode within the composite or a typical elastic-plastic failure within the metal in the joint. The precise mode of failure is able to be controlled via adjustments to the Surfi-Sculpt TM surface treatment. In addition the Comeld joints were shown to absorb more than twice as much energy before failure than the control joints.
It has been known for many years that adhesively bonded joints, containing composite materials, can be designed such that the adhesive sustains loads greater than the strength of the parent material.  Correctly designed  and manufactured adhesively bonded joints do not exhibit failure due to fatigue loading.  However, despite such benefits, the unpredictable, often catastrophic failure of the joint between composites and metals has lead to a conservative approach resulting in overdesign, the use of fasteners and the under-utilisation of the true mechanical properties that the hybrid structure offers.
A proprietary material surface treatment technique and joining process developed recently at TWI,  offers the potential for joints to be made between fibre reinforced plastic (FRP) composite materials and metals with enhanced performance and design control. It is expected that this joining technique will allow joint design tobe revolutionised by overcoming some of the problems associated with adhesive bonding and mechanical fastening of composites to metals.
Tensile testing was carried out on two different stepped lap geometries with different material combinations as shown in Figure 1.
For each joint type, specimens were machined to the required shape for the joint. Control specimens were grit blasted and chemically etched and others were treated using the proprietary material surface treatment technique Surfi-Sculpt TM , to be used to create Comeld TM joints. An example of the surface created using this new treatment is shown in Figure 2. Vacuum infusion was used to prepare Comeld joints for the SS/GFRP specimens whilst hand-up was used to prepare Comeld joints for the Ti/CFRP specimens.
Specimens for each joint type were loaded in tension and representative load-displacement curves for the joints are shown in Figure 3. Figure 3 Load-displacement curves for a) double step specimens (SS/GFRP) b) single step specimens (Ti/CFRP).
Fig. 3. Load-displacement curves for: a) double step specimens (SS/GFRP); b) single step specimens (Ti/CFRP)
The load displacement curves demonstrate that the Comeld TM joints tested had an equal or greater load carrying capability than the corresponding control joints. The areas under the load-displacement curves correspond to the energy absorbed during failure of the specimens. For both types of joints, the energy absorbed by the ComeldTM joint was more than double that absorbed by the control joint.
For Comeld TM (SS/GFRP) specimens, Figure 3a, the bond-line did not fail at low loads, therefore the joint was able to reach loads at which damage was seen to occur in the composite material. Damage was visible before failure as whitening of the composite caused by matrix cracking. In sharp contrast, the Comeld TM (Ti/CFRP) specimens, Figure 3b, failed within the metallic tongue with little/no failure observed within the composite as shown in the typical area of plastic yield beyond that seen for the control specimen. Subsequent analysis showed that the Surfi-Sculpt TM process had altered the properties within the metal to induce local failure with little detriment to the upper failure load values seen by the reference specimen.
The contrast in failure mechanisms for the specimens described and the resultant load displacement data, demonstrates the potential for Comeld to produce composite to metal joints with designed-in tolerances and reproducible predictable failure properties - key requirements for the practising composites engineer.
- Comeld TM joints had a higher load carrying capability than control joints.
- Comeld TM joints absorbed more than twice as much energy as the control joints before failure.
- Comeld TM joints failure via a more progressive failure mode than the control joints.
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TWI would like to acknowledge the work performed at the Department of Materials at Queen Mary, University of London on vacuum infusion of Comeld TM joints. Thanks also to Bruce Dance for the application of Surfi-Sculpt TM to the stainless steel and titanium specimens.