Electrochemical impedance evaluation of aluminum/boron–epoxy composite bonds

Al/boron-epoxy double lap-shear specimens with five different surface preparations were exposed to hot, humid conditions for up to 3 years. Electrochemical impedance spectroscopy (EIS) measurements were taken using an in situ sensor throughout the exposure period. Periodically, some specimens were pulled to determine bond strength degradation. Aside from a set of deliberately poorly prepared specimens, all specimens retained at least 80% of their initial bond strength with the best specimens showing only a little degradation of strength. The low-frequency EIS impedance for measurements taken with sensor electrodes on the same composite surface decreased exponentially by two orders of magnitude during the exposures as the composite absorbed moisture. This decrease happens well before significant bond degradation, thus allowing the measurements to be used as a warning of moisture intrusion before the moisture has weakened the interface and resulted in a reduction of bond properties. One application of this bond health monitoring technology would be in bonded repair patches.