Wedge test double-cantilever-beam specimens are more commonly and appropriately used to evaluate the durability of adhesive bonds. By driving a wedge into one end and exposing the specimen to high humidity and temperature, a very aggressive test condition is achieved with the crack tip being exposed directly to moisture while under opening stress. Poor surface treatments fail with one hour compared to months for the tensile button tests just discussed. Good surface preparations can usually be evaluated in three to ten days. Boeing correlated the performance of wedge tests with the service performance of bonded aircraft components .
Sensor electrodes were attached to both adherends of the wedge test specimens. A special non-conducting wedge was used to prevent the two adherends from shorting. Joints tested in the wedge test configuration exhibit similar behavior to the tensile button specimens and coated metals. Initially, the adhesive shows completely capacitive behavior. As moisture is absorbed, the low-frequency region of the spectra becomes resistive in nature with the extent of the resistive region generally increasing to higher frequencies as more moisture is absorbed.
Moisture uptake is governed by the adhesive, there is little dependence on surface preparation although the response of the bond to absorbed moisture is dependent on surface preparation. The influences of adhesive and surface preparation are shown in Figure 20. FM-73 adhesive, a 250°F-curing adhesive, absorbs a relatively large amount of moisture and the low-frequency impedance drops three orders of magnitude for the two surface preparations. In contrast, FM-300 adhesive, a 350°F-curing adhesive, absorbs much less moisture and the low-frequency impedance drops only one and a half orders of magnitude for the same exposure conditions and time.
The response of the bondline to this absorbed moisture is governed by the type of interfacial bonds and the stability of the interface (Figure 20 and Figure 21). For sandblasted specimens, for which the crack propagates interfacially, the initial, rapid crack growth occurs with minimal change in the low-frequency impedance. For these weak interfaces, which rely predominately on secondary forces that are easily disrupted by moisture [25-27], the crack propagates as soon as moisture reaches the interface - before it has a chance to absorb into the bulk adhesive and change the impedance. Over time, this absorption occurs and the impedance decreases, but the crack has arrested or is growing very slowly as it reaches the point of sustainable stress. For the PAA specimens, crack propagation is within the adhesive and the limiting factor governing crack propagation under these conditions and times is not the interface, but rather, the weakening of the adhesive due to moisture absorption. For the FM-73 adhesive, there is a distinct relationship between the low-frequency impedance (absorbed moisture) and crack growth until the crack arrests after approximately one centimeter of propagation. A similar relationship is likely for FM-300 adhesive, but the data are limited in this region. At the arrest point, the adhesive continues to absorb moisture, but without additional crack growth.
The utility of tracking moisture absorption in a bondline as a means of health monitoring an adhesive joint is dependent on the adherend surface preparation and the resulting failure mechanism. The use of EIS or other moisture-sensitive probes is best suited for situations where relatively long exposure to moisture results in joint failure from plasticization or other weakening of the adhesive or from hydration or other corrosion of the adherend surface. PAA adherends are one example in which these conditions are met. For these joints, the impedance spectrum significantly changes in shape and the low-frequency impedance decreases by one to three orders of magnitude well before hydration of the oxide and joint failure. Thus there is ample time to warn of impending bond degradation and preventative action can be taken.
This ability to warn is in contrast to the situation with sandblasted adherends where crack propagation occurs interfacially with the very first ingress of moisture to the interface. The smooth adherends, without a high density of physical bonds, fail as soon as a small amount of moisture reaches the interface. There is no need for the adhesive to absorb moisture and for the moisture to be in contact with the interface for an extended period of time. Fortunately, these types of surface preparations and the resulting bonds are not used where strong, durable joints are required. Structures for which these bonds are acceptable would not be candidates for bondline health monitoring.
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.