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Lockheed Rye Canyon Research and Development Center, 1977-1979. This project was the best of fun. I had an idea, or invention, and was free to test it. It worked!
Paul Sandifer and Ray Jusko helped me.
test panel
A flat aluminum panel is supported at eight points within a reaction frame. Two Z-section stringers are riveted to the panel. Two frame elements are attached to the stiffeners by means of special shear clips.

The center portion of the panel is coated with flat black paint. Confectioners' colored beads are spread uniformly over the black strip with aid of a paint brush.

An electrodynamic vibration exciter or shaker is placed beneath the panel. There is a flat, 8-inch diameter head on the shaker. An air gap one-eighth inch thick separates the shaker head from the panel. The shaker is driven in swept sine frequency mode until a panel resonance is detected.

Panel resonance is confirmed by examining the signal from an accelerometer mounted at panel center.
panel test fixture
stringer end support
See at left an aircraft bearing, plain, rod end, spherical, self-lubricating, self-aligning bearing. I used eight of them. Each threaded rod end is screwed into a structural tension load application fixture. I designed a clevis and pin arrangement to attach each bearing to the reaction frame.
test panel image 4
test panel image 5
The channel-section frame is rotated to let you see that each shear clip has three elements. A center element is attached by means of two countersunk bolts to a Z-stiffener outstanding leg or web. At clip top you see that a bolt ties the three elements together.
The two outer elements are attached to the frame by means of rivets.
test panel image 6
Each shear clip is a vibration damping device. The damping mechanism is dry friction or fretting between clip elements in contact.

The invention owes its origin to observation of sonic fatigue tests on panels with Z-stiffeners at Boeing in 1964-65.
test panel image 7

test panel image 8 This is your chance to be a structural test engineer. Spread the confectioners' beads over the panel center portion. Turn on your NCH Tone Generator.

Gradually increase the excitation frequency from 60 to 120 Hz.

You have found the lowest panel resonance frequency!! Observe the bead pattern.
test panel image 9 One band, or node, of beads is hidden beneath the left frame element.
A second node is clearly visible to the right of the right frame element. When you excite the panel at 120 Hz, the beads migrate from the uniform arrangement over the black portion of the panel to occupy these two nodal positions.
test panel image 10 Increase the test frequency. Find resonances with 4, 6, 8, 10, and so on numbers of nodes.
Count the number of nodes. There are 10. Use a test frequency equal to 304 Hz to see the uniformly spread beads migrate into this pattern.
test panel image 11 Now there are 20 nodal lines. Test frequency equals 709 Hz.
This development leaves work for you to accomplish. Select a viscoelastic or elastomeric damping coating to be applied to the layers of each shear clip. Repeat the test sequence.


THINGS FOR YOU TO DO IN THINKING ABOUT CAREER OPTIONS
Use a search engine.

1. Search Boeing and Lockheed Martin for opportunities to work on airframe structural design or testing.
2. Examine employment experience and educaton requirements. See that Finite Element Analysis, mechanical design, and computer aided drawing skills are important.
3. Look for university courses which include analysis of damped thin-wall structures.

crevasses Forensic Acoustics Columbia University sonic fatigue
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