3. Results and Discussion

The graph of the v−G curves obtained from the constant-load SLBT for 75 % r.h. and 81 % r.h. is shown in Figure 2. SLBT specimens were tested at (30, 44, 54, 66, 75, and 81) % r.h.; however, no debonding was observed for samples tested at 66 % r.h. and below. This is because the applied SERR was not large enough to cause debonding. More details about this will be discussed later.

Figure 2. v−G curves obtained from the constant-load SLBT. Symbols are 73 % (□) and 81% (▲) r.h. Note that 100 % r.h. was not tested. Humidity levels of (30, 44, 54, and 66) % r.h. did not exhibit adhesion loss over the experiment's time scale. The error bars are not shown for clarity.

The graphs of the v−G curves obtained from the DCB test for (15, 33, 43, 66, 81 and 100) % r.h. are shown in Figure 3. Figures 2 and 3 are shown on the same y−scale to facilitate comparison. Error bars are not shown on the graphs because the uncertainty in G and v attributable to measuring the debond length is significantly less than is the sample to sample variation. Data sets from all specimens are shown in the graphs.

For the DCB experiments, the SERR once the wedge is introduced or the "initial SERR" is relatively large; in fact, when the wedge is introduced for a debond length of 1 mm, the value of the SERR is on the order of 10⁶ J/m². The initial SERR in the wedge test is much greater than the intrinsic interfacial toughness of the adhesive and therefore a debond will propagate. As a consequence, debonding is measurable at much lower relative humidities than seen with the SLBT. Figure 2 shows a significant difference between the low humidities (15, 33, and 43) % r.h. and the high humidities (66, 81, and 100) % r.h. This is characteristic of a critical relative humidity level for adhesion loss where there is a dramatic loss of adhesion above a critical relative humidity level which is typically between 40 % r.h and 70% r.h. [2, 23] At 81 % and 100 % r.h. the debond behaviors are different. At 81 % r.h., during the early stage of the test, the debond grows rapidly and then arrests suddenly. At a 100 % r.h. the crack grows rapidly and then appears to reverse direction. This behavior is probably caused by swelling of the adhesive and the subsequent change in the stress state of the adhesive from tensile to compressive.

For the SLBT experiments, the initial applied strain energy release rate (SERR) was 200 J/m². Therefore, if the intrinsic interfacial fracture toughness exceeded or met 200 J/m², the debond would not propagate, and the test will not work. Increasing the suspended load would increase the SERR; however, the maximum suspended load and hence the SERR is limited by the strength of the film. Any additional load suspended from the center of the blister resulted in significant plastic yielding and rupture of the film. A 100 % r.h. was not tested because it was suspected that the adhesion level would reach a minimum value above the critical relative humidity level. Only 75 % and 81 % r.h. are shown, because at humidity levels below 75 % r.h. no crack growth was observed over the time frame of the experiment, 4 months. Classic debond behavior was observed, where the debond propagates presumably by a stress-controlled reaction at the crack tip, followed by the debond apparently arresting at a threshold value of the SERR, G_Th. For 75 % and 81 % r.h. G_Th was (95 ± 10) J/m² and (60 ± 10) J/m², respectively. Because no debond growth was observed at low humidity, this suggests that G_Th at these moisture levels is greater than 200 J/m².

Ultimately, this study shows that the results from the two tests are different due to the unusual behavior exhibited by the DCB specimens at high % r.h. and because the debond velocities seen in the DCB are much faster than in the SLBT, for an equal applied SERR. However, it is unclear if these differences are directly attributable to the tests (thin film vs. bending plate, mode mixing, geometry, etc.) or are some artifact of the experiment (moisture concentration, residual and swelling stresses, thickness, etc.).

There are a number of advantages and disadvantages for each test highlighted by these experiments. A serious problem with the SLBT, and common to most peel type tests, is the high stress in the film which can cause yielding in the adhesive. For these SLBT experiments, the maximum suspended load and therefore SERR was limited by the strength of the composite film. Furthermore, the applied loads were large enough to cause visible yielding or creep in the adhesive. This was evident from the permanent set in the delaminated film. This may render the expressions for G derived from linear elasticity invalid, depending if elastic conditions at the crack tip exist or not during debond propagation. Therefore caution must be taken to make sure that creep does not affect the adhesion results. In addition, the DCB test utilizes much smaller samples which can occupy significantly less lab space. The time frame of the DCB test is much less than the SLBT: (1 to 1.5) months and (3 to 4) months, respectively. Although, the SLBT specimen is an open-face geometry and should equilibrate in its environment much sooner than a DCB test specimen (a sandwich-type geometry specimen), the time frame of the experiment suggests that there is no advantage to using the SLBT data to increase the rate of environmental degradation during subcritical testing.