Comeld - Damage Control for Composite to Metal Joints

Ewen J C Kellar and Faye Smith

Paper presented at Design & Performance of Composite Materials Conference, The University of Sheffield, 3-6 April 2005

Abstract

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.

1. Introduction

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. ^[1] Correctly designed ^[2] and manufactured adhesively bonded joints do not exhibit failure due to fatigue loading. ^[3] 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, ^[4] 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.

2. Results

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.

3. Conclusions

• 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.

4. References

1. Hart-Smith L J: 'Adhesive bonded single lap joints'. NASA CR 112236. January 1973.
2. Hart-Smith L J: 'Design of adhesively bonded joints'. Joining fibre-reinforced plastics. Edited by F L Matthews. Elsevier Applied Science. 1987, pp.271-311. ISBN 1-85166-019-4.
3. Thrall E W Jr: 'Bonded joints and preparation for bonding'. Proc. AGARD-NATO Lecture Series Number 102. 1979:5-1, 5-89.
4. International Patent Publication Number WO 2004/028731 A1. 'Workpiece Structure Modification.' Applicant: The Welding Institute. Inventors: Bruce Guy Irvine DANCE and Ewen James Crawford KELLAR.

Acknowledgements

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.