The panel used in these experiments was an 800mm wide by 400mm high aerospace component that had been milled from a single block of 7075 aluminium. A drawing of the panel is included in Appendix A. The panel was suspended by string from a framework mounted to a table. A speckle pattern was applied with paint, first with a base layer of white and then the speckle provided using a sponge to dab on the black. Pictures of the panel in the experimental setup are shown in figure 6.1.
The design of the setup was based in part from the results obtained from the modal analysis. The modal analysis was used to identify a suitable excitation point and frequen- cies for the experiment. It was decided to use the first three frequencies, which included a torsion mode (14Hz), a bending mode (44Hz), and a combination (59Hz). The excitation was provided by an electrodynamic shaker (DataPhysics V100) which was attached to
CHAPTER 6. VALIDATION OF AN AEROSPACE COMPONENT SIMULATION
Figure 6.1: Picture of front of the experimental setup showing the cameras, laser vibrom- eter, and pulsed laser (left) and the back side of the panel showing the attachment to the shaker (right).
Figure 6.2: Front view of the panel showing the stinger attachment and the comparison area for the image decomposition.
the panel with a stinger (M4 300mm stainless steel rod) through a bolted connection. Figure 6.2 shows the front view of the panel and the location of the attachment of the stinger.
A function generator was used to drive the shaker at one of the panel’s first three resonances. The vibration of the panel was controlled by adjusting the output of the function generator, which was in turn connected to an amplifier before being sent to the shaker. The amplitude was measured using a laser vibrometer (OFV-503, Polytec) aimed at the attachment point of the stinger. Table 6.1 shows the amplitude of vibration measured at the excitation point, and the response at the corner of the panel (Point A in figure 6.2).
Table 6.1: The measured excitation displacement at the attachment point of the stinger and the response of the panel at point A.
Amplitude p-p (mm) Excitation Response pt. A
14Hz (1st mode) 0.18 3.2
44Hz (2nd mode) 0.34 1.6
59Hz (3rd mode) 0.10 3.3
Images of the vibrating panel were captured using the PL-DIC system based on a pair of 2 megapixel firewire cameras with 1624x1234 pixel resolution (Stingray F-201B, AVT). The cameras were fitted with a matched pair of 8mm lenses (Cinegon 1.4/8, Schneider) set to an f-stop of 5.6, providing an image magnification of approximately 3.1 pixels/mm. A laser with a 4 nanosecond pulse duration and wavelength of 532 nm was used to freeze the motion of the panel. In addition to providing the driving frequency to the shaker, the function generator also provided a synchronization signal which was routed to the timing box of the DIC system. A diagram of the experimental setup is given in figure 6.3.
The DIC software (Istra4D, Dantec Dynamics) was used to phase shift the image acquisition relative to the driving frequency, permitting the capture of the entire cycle of
CHAPTER 6. VALIDATION OF AN AEROSPACE COMPONENT SIMULATION
Figure 6.3: Diagram showing the arrangement of the experimental apparatus and the attachment of the shaker.
vibration of the panel. A total of 20 images were captured in this fashion at 18 degree phase increments, covering a full 360 degree cycle. The images were processed with the software using a subset size of 49 pixels and an offset of 20 pixels. The subset size was set to this relatively large value to compensate for the quality of the speckle pattern, which was applied by hand and not very uniform. A printed pattern could have potentially been used, although for such a large area there can be issues with bubbling or peeling of the surface.
The pulsed laser was triggered at the same time as the cameras and provided the illumination of the vibrating panel. In order to measure the amplitude of displacement of the excitation, a laser vibrometer was aimed at the front side of the panel where the stinger was attached. Both the velocity and the displacement analog voltage signals from the laser vibrometer were recorded by the data acquisition unit of the DIC system.
6.2.1
Dynamic calibration
A dynamic calibration was performed according to Chapter 4. The procedure was per- formed to assess the minimum measurement uncertainty in the experimental setup, which was then used in comparing the simulation data. A 160mm x 40mm x 4mm thick can- tilever machined from a single aluminum block was excited acoustically while capturing the out-of-plane displacement with the PL-DIC system.
To assess the uncertainty of this particular experimental setup, the cameras were left in the exact same position as they were during the measurements of the aerospace panel. However, this meant that the cantilever that was used was smaller than the desired field of view for the experiments, so it was moved around to six different locations. Figure 6.4 shows the images from both of the cameras at each of the six different locations of the cantilever. Measurements were taken with the cantilever in both a horizontal and
CHAPTER 6. VALIDATION OF AN AEROSPACE COMPONENT SIMULATION vertical positions, to see if there was any difference in the uncertainty.
1
2
3
4
5
6
Figure 6.4: Images captured by the PL-DIC system of the cantilever at six different positions in the field of view. Locations 1-3 are with the cantilever horizontal and 4-6 are vertical.
The results are summarized in table 6.2 and compared to those previously obtained with the High-Speed DIC system. It should be noted that the image magnification was slightly less than half of what was achieved with the high-speed system, as a result of increasing the field of view for the larger aerospace panel. The uncertainties were calculated with the cantilever at the two extrema of deflection, denoted by (−) and (+) in the table.