Use of an electrochemical probe for predictive assessment of bonded repair

Boron/epoxy-Al double lap shear specimens were prepared and exposed to high humidity and temperature. Half of these was prepared using the grit blast/silane process used for bonded repair of aircraft. The other half was prepared without the silane to reduce durability. Electrochemical impedance spectroscopy (EIS) measurements were made on these specimens with an electrode probe. A set of these specimens was mechanically tested periodically to determine pull strength. The EIS impedance spectra were analysed by means of equivalent circuit analysis. Several circuit parameters exhibited changes upon exposure to moisture. Moisture uptake was calculated from changes in the capacitance of the equivalent circuit. When sensor electrodes are placed in different locations, variations in moisture concentration in the bonded system can be determined. The approach provides advance warning of significant reduction of bond strength and has the potential to monitor adhesively bonded repair patches and indicate the need for preventative maintenance prior to any degradation of bond performance.