Date of Award
Master of Science
David K. Hsu
Palaniappa A. Molian
This thesis discusses many existing methods of nondestructive evaluation used on honeycomb structures ranging from ultrasonic transduction to various low frequency techniques. The focus, however, is given to a newly developed technique based on hysteresis effects in force-displacement curves. The area enclosed by the hysteresis loop represents the amount of energy absorbed by the sample during the loading and unloading phases. It is believed that the cause of the energy absorption is due to increased internal frictional loses which occur when the sample is damaged. The loop area was found to correlate well with the level of damage sustained by the structure. This technique is centered on the deduction of a force-displacement curve from an accelerometer tap, which allows the force-displacement curves to be generated from a single tap on one surface of the structure.
Traditionally a force-displacement curve, the equivalent of a stress-strain curve, is produced using a mechanical testing machine. However, this is not a suitable method to be used to attain a force-displacement curve while a structure is in-service because it requires access to both the front and back surfaces of the structure. The deduction of a force-displacement curve from an accelerometer tap proved to be an effect solution to this problem. The great advantage of this method is that it only requires access to one surface of the structure to generate a force-displacement curve. This method also takes much less time to generate the hysteresis loops. A mechanical testing machine could take up to 15 minutes to produce a single force-displacement curve, this method will produce the same curve in seconds. Much of this research was devoted to the testing and development of the techniques used to deduce a force-displacement curve from an accelerometer tap.
This thesis also takes a look at the benefits of two-dimensional Fourier Transforms. During the course of this research, many C-scans of honeycomb composite structures were generated using air-couple ultrasonics. These C-scans were used as baseline images to compare with the results of the newly developed mechanical hysteresis technique. The honeycomb structure can cause very distracting hexagonal patterns in ultrasonic images. The Fourier transform and the processing associated with it is able to suppress these distracting patterns while leaving the rest of the image relatively unaffected. In some cases, not only can the patterns be suppressed, but the flaws can also be drawn out. A program was written to perform and filter the two-dimensional Fourier Transforms to suppress the patterns from the C-scan images.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Foreman, Cory, "Nondestructive detection and characterization of damages in honeycomb composite structures" (2008). Retrospective Theses and Dissertations. 224.