Strength and Failure Analyses
of Adhesive-Bonded Composite Joints

Composite structures are currently being used extensively in aircraft, space vehicles, marine, and other automotive structures due to their light weight, high stiffness, and large ultimate strength. EWI, partnered with Boeing and Lockheed-Martin in the Composites Affordability Initiative (CAI), investigated the effects of bonding defects on structural integrity and performance by testing thousands of bonded samples. EWI has worked with others in developing applications for metal-composite structure on steel, aluminum, and titanium. From these projects, EWI has developed significant capability in bonded metals and composites methodology, NDE technology, and structural performance evaluation methods.

Recently, EWI developed a finite-element analysis method in an industrially-funded project to assist the design of a bonded joint system for a composite-metal interface. This analysis method allows structural analysts and designers to take advantage of the full potential of composite materials during designing composite adhesive joints. The analysis method has been used to predict stress and strain distributions, joint strength and failure modes of an adhesive-bonded composite joint during loading.

The analysis method was developed based on the commercial finite element software package, ABAQUS. A three-dimensional (3D) model was used in the analysis. The metal and adhesive were meshed with solid elements and the composite was meshed with solid elements and cohesive elements. The number of layers and material property orientations in the composite were considered in the model. Isotropic elastic and plastic material properties were assumed for both the metal and the adhesive. The material properties for the metal were obtained from public literature, while the adhesive material properties were obtained from testing at EWI since they are determined by the bonding process.

Orthotropic elastic material properties were assumed for the composite, which was provided by the composite suppliers. Progressive damage and failure were modeled by defining failure criteria (damage initiation and evolution) to the adhesive and composite. The failure parameters were calibrated with experimental testing results. To validate the analysis method, double lap shear tests were conducted and modeled. The model predictions had good agreement with experimental tested results.

The analysis method was applied to predict the strength and failure mode at room temperature (23°C) and an elevated temperature (60°C) in evaluating adhesive joint designs. The failure at 23°C was interlaminar-shear fracture which occurs in the exterior composite skin, while the failure at 60°C was interfacial-shear fracture (cohesive failure) in the adhesive. In addition, finite element analysis was also used to investigate the effect of adhesive thickness on the joint strength and to monitor the change in failure mechanism with changes in joint dimensions.

This analysis capability provides EWI members benefits in designing adhesive bonded metal-composite joint. For more information on the analysis method, contact yyang@ewi.org or call 614.688.5253 and for more information on metal-composite bonding and testing, contact gritter@ewi.org or call 614.688.5199.

Acknowledgement
This work was funded by Logistics International, Abu Dhabi, Dubai, UAE. The authors sincerely thank the technical support from Logistics International during project execution.