Fatigue fracture of embedded copper conductors in multifunctional composite structures

The fatigue failure mechanisms of copper strips embedded into glass/epoxy were investigated. This combination of materials composes a multifunctional electrical-composite load bearing structure that is essential for systems such as large antennas integrated into aircraft skins. In smart structures applications, bulky and heavy wiring harnesses associated with densely deployed sensors, actuators, and devices can be avoided by using embedded electrical interconnects in a manner analogous to printed circuit boards. Since metals generally exhibit lower fatigue life relative to composites, understanding the failure mechanisms associated with embedded metal conductors is necessary for improving operational life. Specimens having 0.127 mm thick embedded copper strips were used to measure fatigue life as a function of copper strain amplitude. Fracture of the conductor was observed for loading below 75% of the composite ultimate strength, without failure of the composite. The fracture surface morphology was composed of a combination of fatigue crack growth and ductile fracture, with a higher percentage of the latter existing for greater load amplitude. Crack growth in the copper was found to be strongly coupled with debonding between the copper and composite. Prevention of debonding directly influences the fatigue life of the embedded copper strip, much in the same way as composite patches retard crack growth in repaired metal structures.