Measuring plate vibration using deflectometry: the advantages and limitations of add-on reflective material
Open Access
- Author:
- Rhoades, Gary
- Graduate Program:
- Acoustics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- August 28, 2020
- Committee Members:
- Micah R Shepherd, Thesis Advisor/Co-Advisor
Jeff R Harris, Thesis Advisor/Co-Advisor
Dan Russell, Committee Member
Victor Ward Sparrow, Program Head/Chair - Keywords:
- deflectometry
vibrations
full-field - Abstract:
- The analysis of transient vibrations in structures is no less important than analysis of stationary vibrations, as many structures will be frequently subjected to transient disturbances during normal operation. However, fewer techniques exist that can measure transient vibrations. Deflectometry is a non-contact method of vibration analysis that is able to measure both transient and stationary vibrations, and is the only method to do so with high spatial resolution. In particular, deflectometry is a full field optical method which utilizes the slope fields on the surface of a planar object to track deformations. The use of a high speed camera gives the ability to measure the entire surface instantaneously and with high spatial resolution, providing more knowledge on how structures react to transient excitations. Here, the reflected grid method will be used, as it involves simple geometric calculations of curvature distributions. This method of deflectometry relies on specular reflections to create amplifications of the measured deformations in the test plate. Therefore, this requires the plate under test to have a reflective surface. Application has so far been limited to ideal mirror-finished planar objects, and no testing has been done to quantify the sensitivity of deflectometry regarding low amplitude excitations on the object under study. An experiment was constructed to test the vibration of a flat plate excited by an automatic force hammer. The lower bound of excitement amplitude necessary for effective deflection plots was explored. The deflections of panels subjected to forces of at least 119 milliNewtons or higher, corresponding to 22 microns of maximum deflection, can definitively be captured, with evidence provided for the ability to measure deflections at much lower excitation levels. Additionally, a collection of adhesive tapes, films, and spray were applied to the test object in order to increase the reflectivity of the plate. Tapes, while cheap, are manufactured without strict tolerances on thickness, causing distortions in the reflected image and therefore the deflection plots. Film can be applied with limited success, and spray-on reflective material approaches quality of an ideal mirror, but is only applicable on glass surfaces. A reflective film was then applied to a more complicated structure to highlight the usefulness of deflectometry on a variety of surfaces. The top plate of an acoustic guitar was measured using Mylar film stretched over the surface, while a shaker excited the top plate at each of the two fundamental breathing modes, which have well known deflection shapes. Due to the size of the printed grid, the area which contains specular reflections was limited. Additionally, internal camera noise was strong during the tests. However, expected deflection shapes were still extracted from the data using deflectometry techniques. Thus, the drawback of requiring a reflective surface can be circumvented by modifying non-reflective surfaces with an add-on material. Future tests should be performed with a larger grid and better camera to enhance the results further.