Impedance-based Non-Destructive Evaluation for Additive Manufacturing (INDEAM) is rooted in the field of Structural Health Monitoring (SHM). INDEAM generalizes the structure-to-itself comparisons characteristic of the SHM process through introduction of inter-part comparisons: instead of comparing a structure to itself over time, potentially-damaged structures are compared to known-healthy reference structures.
The purpose of INDEAM is to provide an alternative to conventional nondestructive evaluation (NDE) techniques for additively manufactured (AM) parts. In essence, the geometrical complexity characteristic of AM processes combined with a phase-change of the feedstock during fabrication complicate the application of conventional NDE techniques by limiting direct access for measurement probes to surfaces and permitting the introduction of internal defects that are not present in the feedstock, respectively. NDE approaches that are capable of surmounting these challenges are typically highly expensive.
In the first portion of this work, the procedure for impedance-based NDE is examined in the context of INDEAM. In consideration of the additional variability inherent in inter-part comparisons - as opposed to part-to-itself comparisons - the metrics used to quantify damage or change to a structure are evaluated. Novel methods of assessing damage through impedance-based evaluation are proposed and compared to existing techniques. In the second portion of this work, the INDEAM process is applied to a wide variety of test objects. This portion considers how the sensitivity of the INDEAM process is affected by defect type, defect size, defect location, part material, and excitation frequency. Additionally, a procedure for studying the variance introduced during the process of instrumenting a structure is presented and demonstrated. / Doctor of Philosophy / Impedance-based Non-Destructive Evaluation for Additive Manufacturing (INDEAM) is a quality control approach for detecting defects in structures. As indicated by the name, impedance-based evaluation is discussed in this work in the context of qualifying additively manufactured (3D printed) structures.
INDEAM fills a niche in the wider world of nondestructive evaluation techniques by providing a less expensive means to qualify structures with complex geometry. Complex geometry complicates inspection by preventing direct, physical access to all the surfaces of a part. Inspection approaches for parts with complex geometry suffuse a structure with energy and measure how the energy propagates through the structure. A prominent technique in this space is CT scanning, which measures how a structure attentuates x-rays passing through it.
INDEAM uses piezoelectric materials to both vibrate a structure and measure its response, not unlike listening for the dull tone of a cracked bell. By applying voltage across a piezoelectric patch glued to a structure, the piezoelectric deforms itself and the bonded structure. By monitoring the electrical current needed to produce that voltage, the ratio of applied voltage to current draw---impedance---can be calculated, which can be thought of as a measure of how a system stores and dissipates energy. When the applied voltage oscillates near a resonant frequency of a structure (the pitch of a rung bell, for example) the structure vibrates much more intensely, and that additional movement dissipates more energy due to viscosity, friction, and transmitting sound into the air. This phenomenon is reflected in the measured impedance, so by calculating the impedance value over a large range of frequencies, it is possible to identify many resonances of the structure. So, the impedance value is tied to the vibrational properties of the structure, and the vibration of the structure is tied to its geometry and material properties.
One application of this relationship is called impedance-based structural health monitoring: taking measurements of a structure when it is first built as a reference, then measuring it again later to watch for changes that indicate emerging damage. In this work, the reference measurement is established by measuring a group of control structures that are known to be free of defects. Then, every time a new part is fabricated, its impedance measurements will be compared to the reference. If it matches closely enough, it is assumed good. In both cases, impedance values don't indicate what the change is, just that there was a change.
A large portion of this work is devoted to determining the types and sizes of defects that can be reliably detected through INDEAM, what effect the part material plays, and how and where the piezoelectric should be mounted to the part. The remainder of this work discusses new methods for conducting impedance-based evaluation. In particular, overcoming the extra uncertainty introduced by moving from part-to-itself structural health monitoring comparisons to the part-to-part quality control comparisons discussed in this work. A new method for mathematically comparing impedance values is introduced which involves extracting the resonant properties of the structure rather than using statistical tools on the raw impedance values. Additionally, a new method for assessing the influence of piezoelectric mounting conditions on the measured impedance values is demonstrated.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/99966 |
| Date | 15 September 2020 |
| Creators | Tenney, Charles M. |
| Contributors | Mechanical Engineering, Tarazaga, Pablo Alberto, Williams, Christopher B., Camelio, Jaime A., Kong, Zhenyu, Kurdila, Andrew J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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