Essay on Vibration Testing

Abstract

Structures commonly fail under considerable static loading, and sometimes they fail under smaller dynamic loading closer to resonance. Dynamic loading, mainly periodic continuous loading, needs to be within particular frequencies and amplitudes to trigger the failure of load-bearing structures. Generally, continuous periodic loading with frequencies close to resonance causes significant displacement and disastrous failures, especially if the loading is unplanned. Vibration testing can therefore aid with the determination of frequencies that result in resonance or the declining structural integrity of a particular. Two devices are widely exploited in the setup for vibration testing, particularly an electrical shaker and digital stroboscope. The electrical shaker is utilized in the vibration tests because it transforms electric current into dynamic mechanical motion; hence it can be a significant source of periodic continuous loading. On the other hand, the digital stroboscope plays a significant role in the observation of the resonance by slowing down periodic motion. The digital stroboscope emits pulses of bright light, which offers a snapshot of moments that makes the periodic motion appear to be at rest. Typically, the experiment was primarily undertaken with the primary objective of learning how to effectively set up and use the vibration tests, use a digital stroboscope to investigate periodic motion, explore natural decay in underdamped structures, and improve an understanding of dynamic structural behavior. The experiment established that the different modes played a significant role in determining the nodes. Moreover, in mode 1, the frequency for resonance falls between 5.8 to 6.2 hertz for the short beam and a frequency between 2.4 to 2.3 hertz for the long beams. The length of the beam played a significant role in the frequency, with the short beam having a higher resonance frequency than the long beams. Therefore, the resonance frequencies were affected by the length of the structure and the mode of vibration. The number of modes affected the number of nodes. The first mode did not have any nodes, the second mode had one node, the three modes had two nodes, and the fourth mode had three nodes.

Introduction

Load-bearing structures have a tendency to fail under extreme static loads. However, these structures can also fail under considerably smaller loads, particularly under the right combination of disturbances. Large displacement and disastrous failures typically occur in structures under continuous periodic loads at frequencies near resonance if the loading is unexpected (Benaroya, Nagurka, & Han, 2022). Therefore, load-bearing structures do not have to be under extreme static loading to fail, as their smaller loads can also have a similar effect when they are close to resonance. Typically, the resonance frequencies are affected by numerous features of the structure, including shape, material, excitation, damping, and attachment, which makes its simulation challenging (Schmitz & Smith, 2012). Various features that define the environment necessary for resonance can be determined and recreated during vibration testing. Vibration testing fundamentally recreates the environment necessary for resonance to establish the structural integrity of the structure (Anderson, 2020). Structural vibration testing primarily aims to empirically determine and explore the conditions that cause a decline in structural integrity and resonance in structures exposed to forced disturbances. In the vibration test, the structure is equipped with various instruments and exposed to quantifiable disturbances, and the dynamic responses are recorded.

Electrodynamic Shaker

The electrodynamic shaker is a device utilized in numerous vibration tests due to its ability to transform electricity into dynamic mechanical force. Typically, the electrodynamic shaker can recreate the three virtual environments, including sine, random, and shock tests. Signal generators aid in setting the required test variables. The vibration test fundamentally validates the integrity, reliability, and stability of structures under dynamic loads (Schmitz & Smith, 2012). In addition to the electrodynamic shaker, the vibration test setup is completed by accelerometers, oscilloscopes, stroboscopes, signal generators, and cooling fans.

Digital Stroboscope

A strobe light is a device that flashes brilliant light repeatedly to aid in studying the motion. In the study of motion, stroboscopic light can take a snapshot of a single moment in motion, which makes periodically moving objects appear to be at rest. The high flash energy discharged by the strobe light within a millisecond makes them appear extremely bright. In most cases, the strobe light is emitted from a xenon gas flash lamp (Ribeiro, Carvalho, Dib, Barbosa, & Wetter, 2023). A high-voltage source triggers a rapid, brilliant flash by passing through the xenon gas. The electric currents excite xenon electrons which emit bright lights when they move to higher energy levels and back.

Objectives

1. Determine how to arrange and utilize the vibration equipment to disturb and test engineering structures.
2. Control the frequency and amplitude of forced vibrations to trigger resonance.
3. Utilize a digital stroboscope to investigate periodic motion
4. Investigate natural decay in underdamped structures.
5. Increase an understanding of dynamic structural behavior and compare empirically witnessed natural frequencies and node locations to model estimates.

Procedure

1. Hot glue the accelerometer onto the shaker piston hole, then attach it to the signal conditioner and then onto the oscilloscope.
2. Switch on the power supply strip, which turns on the oscilloscope and signal conditioner.
3. Put up the oscilloscope by first returning the default settings, utilizing the auto setup, then adjusting the time and voltage settings between 20 mV/div and 50 mv/div to start, then tuning the amplitudes and frequencies later.
4. Choose the short beam sample and quantify its cross-section dimension.
5. Attach the beam sample onto the shaker table piston in a cantilever beam setup using washers, screws, and tube spacers. The four washers are placed on the upper and lower sides of the cantilever with the smoothest edges of the washer in contact with the beam to avoid crack initiation through fatigue. The two tube spacers will serve to elevate the beam sample above the piston.
6. Determine the distance between the beam sample’s free end and the nearest tube spacer and document it as the beam length.
7. Fine-tune the 5530 Power Supply amplifier by turning the channel 1 knob entirely to the left and the channel 2 knob completely to the left.
8. Adjust the sweep sine generator by turning the frequency knobs entirely to the left, switching on the manual mode, and switching on the checked buttons.
9. Switch on the 5530 Power Supply Amplifier by flicking the switches located on the front and back of the amplifier.
10. Regulate the control frequency FINE dial on the sweep sine generator to the midpoint, followed by steadily turning the CORSE dial to trigger structural resonance or a different response. Establish resonance by lightly touching the beam’s free end. Finally, fine-tune by delicately moving the FINE dial while regulating the amplitude as desired, considering that little amplitude is required.

Flashing Light Sensitivity

Considering that some people can be sensitive to flashing lights, such people should be excused from the stroboscope part of the experiment without any punishment or penalty.

1. Switch off the lights in the room and witness resonance with the aid of the pre-adjusted digital stroboscope. It is crucial to note that while the sine generator shows Hertz, the digital stroboscope shows flashes per minute.
2. Track and write down the excitation frequencies as displayed by the oscilloscope, stroboscope, and control frequency setting. Appreciate those low frequencies that might not show on the oscilloscope.
3. Regulate V/div and time/div on the oscilloscope to enable clear viewing of the significant wave period from one peak to another. Major frequency can be obtained from the inversion of the wave period or from the readings from the oscilloscope display if it exists for >10 Hz. Auto setup can be returned if necessary.
4. Create sketches or acquire photographs of the apparently slowed motion and pertinent nodes. Utilize a marker and ruler to outline the node positions. Constantly inspect the screws for their tightness.
5. Replicate the initial procedures for an extended beam sample.
6. Finally, review the real aircraft components that vibrate, such as leading-edge ribs and hat shapes.
7. After completing the experiment, it is crucial to lower the amplitudes and frequencies considerably, then turn the 5530 Power Supply Amplifier off, and the other connected parts of the setup finish with the power supply strip.
8. Ensure the workspace is clean.
9. Copy and distribute the test data, relevant information, and observations made during the experiment.

References

Benaroya, H., Nagurka, M., & Han, S. M. (2022). Mechanical Vibration: Theory and Application. Rutgers University Press.

Ribeiro, I. L. F., Carvalho, G. L., Dib, L. F. G., Barbosa, E. A., & Wetter, N. U. (2023). Vibration amplitude mapping by stroboscopic structured light projection. Optics Communications, 531, 129219.

Schmitz, T. L., & Smith, K. S. (2012). Mechanical vibrations. Modeling and measurement.

Anderson, R. J. (2020). Introduction to mechanical vibrations. John Wiley & Sons.