To explain the experimental procedure behind the cyclic loading this subsection first treats the experimental set-up. Thereafter the procedure to determine the size of the disbond during the cyclic loading is discussed. At set intervals throughout the experiment the stiffness of the panels was also monitored, this is treated last.
Experimental set-up
3 RTA and 5 RTW panels were subjected to 300,000 cycles of compression loading. This amount was chosen to comply with the earlier tests performed on co-cured panels, from which it was also determined that the disbond growth after 300,000 cycles was limited. A frequency of 5Hz was applied, for which the material did not heat up, as was measured using an infra red camera, and the test set-up remained relatively free of vibrations. Originally it was planned to test only 3 RTA and 3 RTW panel. However, 2 additional RTW panels were tested because the outcome of the first 3 was different from the expectations at that moment. The load cycles were roughly between -4.7 kN and -47.5 kN, resulting in a load ratio (R) of 0.1. This amplitude was chosen in such a way that it was just below the maximum load of -50 kN of the set-up. It had proven suitable for earlier tests with co-cured panels. An overview of the test set-up can be seen in figure 3.7.
Load Cell Test rig Load control Data acquisition LVDTs of LVDTs (in rest) system
FIGURE 3.7 – Set-up of the fatigue experiments
Disbond size
Based on the measured disbond growth during the cyclic loading of the aforementioned co-cured panels, it was concluded that disbond growth during the initial cycles was likely to be bigger than the growth during the final fatigue cycles. Therefore, the measurements were more closely spaced in the beginning of the testing than at the end. Resulting in the following cycles after which the disbond length was measured: 0, 1, 500, 5000, 10,000, 30,000, 50,000, 100,000, 150,000, 200,000 and 300,000. The measuring was performed with ultra-sound using an Isonic 2006 produced by Sonotron NDT. As only the disbond size was relevant just the areas near the disbond edges were scanned. Everything in between the edges was assumed to be disbonded because it buckled and everything outside was assumed to be intact.
The panel had to be taken out of the set-up and brought to the Isonic 2006 to be able to carry out the ultra-sound tests. To minimize repeatability errors the panels were kept inside the ends providing clamped boundary conditions. The blades providing the anti- buckling support did have to be re-tied after every measurement. The same procedure, with one operator pressing the blade in place and one tying the bolts, was used throughout the entire testing phase.
According to the Isonic 2006 user manual a scanning accuracy of 1 mm can be achieved. However, during the scanning of the panels it was noticed that sometimes probe location shifted when it was close to the antennas, which is where also the crack fronts were located. This caused the accuracy of the probe to be roughly 2 mm per side.
After scanning, the results were post-processed to obtain the disbond length and dis- bond area. Both parameters only considered the disbonded part underneath the stiffener, the adhesive present on the edges of the stiffener typically cracked in a different way and was not considered relevant. A more thorough explanation of how the disbond length and area were determined based on the outcome of the c-scan can be found in appendix C.3.
Stiffness
To get a sense of the overall damage state of the panel the overall stiffness was de- termined by constructing a load-shortening curve up to -47.5 kN at every other disbond length measurement point. Thus, during the 1st cycle and after 500, 10,000, 50,000, 150,000 and 300,000 cycles. Stiffness losses can be an indication of intralaminar or translaminar damage, both of which are not picked up by the c-scan. During the load- shortening measurement of the RTW panels the out of plane displacement was also mea- sured using a second LVDT. This was done in the middle on the backside of the skin, where the out of plane displacement was expected to be at its maximum.
For clarity, all measurements are summarized in table 3.5.
TABLE 3.5 – Measurements of the panels at set intervals
Number of cycles Disbond length Shortening Out of plane dis-
placement
0 RTA & RTW
1 RTA & RTW RTA & RTW RTW
500 RTA & RTW RTA & RTW RTW
5000 RTA & RTW
10,000 RTA & RTW RTA & RTW RTW
30,000 RTA & RTW
50,000 RTA & RTW RTA & RTW RTW
100,000 RTA & RTW
150,000 RTA & RTW RTA & RTW RTW
200,000 RTA & RTW
4 Experimental results
The experiments described in chapter 3 were divided in three steps: quasi-static com- pression tests until failure on pristine reference panels (ultimate strength), cyclic loading and quasi-static compression tests until failure on the cyclically loaded panels (residual strength). The results of these experiments are treated in that same order in the subse- quent sections.