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Tensile button test specimens were prepared by bonding aluminum stubs to aluminum plates with different surface treatments. Specimens were placed in high humidity or immersed in water. Periodically, the specimens were removed and sensors were attached to the stub and substrate. EIS measurements were then taken. The impedance spectra for an adhesive bond closely resemble those of a coated metal. Initially, the spectra are capacitive in nature with high impedance at low frequencies. As the adhesive absorbs moisture, the low-frequency spectrum de-creases in impedance and becomes resistive. This behavior reflects moisture ingress into the pores of a polymer and formation of pathways of relatively low resistance. The decrease of the low-frequency impedance for a phosphoric acid anodized (PAA) specimen is shown in Figure 18 and reflects the absorption of moisture. This specimen exhibited a fairly steady impedance de-crease. For these measurements, the data are dominated by the region of the specimen with the lowest impedance. In this case, the data represent the impedance of the near-edge region of the bondline where moisture saturation first occurs.
The effect of this moisture absorption on the tensile pull strength strongly depends on the surface treatment. This dependence is illustrated in Figure 19. The initial decrease in pull strengths from approximately 38 MPa to 27 MPa corresponds to a decrease in the cohesive strength of the adhesive as moisture is absorbed. For PAA specimens, the length of time available for this experiment (approximately five months) under these conditions (humidity, followed by immersion) was insufficient to induce hydration. Accordingly, the microcomposite interphase formed by the porous PAA oxide [28] and the adhesive remained stable and stronger than the cohesive strength of the adhesive so that failure was almost entirely within the adhesive. Similar behavior was also observed for grit blasted and FPL-etched specimens, with the exception of an FPL surface at the end of the experiment where the strength decreased by approximately 50% and partial interfacial failure occurred as the result of hydration. In contrast, because the sanded surfaces exhibit no evolved microroughness, their bonds to the epoxy adhesive rely more on secondary bonds, such as van der Waals forces, which are readily disrupted by moisture. The locus of failure in this case gradually shifted to increasingly more interfacial with the interfacial region beginning along one segment of the edge and growing along the circumference and toward the center.
In-Situ Sensor to Detect Moisture Intrusion and Degradation of Coatings, Composites, and Adhesive Bonds, G.D. Davis, C.M. Dacres, and L.A. Krebs, DACCO SCI Inc.