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Browsing by Author "Mollenhauer, David Hilton"

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    Induced strain of actuation of surface bonded and embedded piezoceramic patches
    Mollenhauer, David Hilton (Virginia Tech, 1992)
    The strain induced into isotropic flat plates by surface bonded and embedded piezoceramic patches was investigated. The free piezoceramic patch voltage/strain response was characterized using moire interferometry and Twyman-Green interferometry techniques. Moire interferometry experiments were conducted to investigate the induced strain of actuation in a flat plate with two surface bonded piezoceramic patches and a flat plate with one embedded patch. Three-dimensional finite element models of these two specimens were constructed and analyzed. The measured in-plane voltage/strain response of a free piezoceramic patch was found to be nearly linear and of a much higher magnitude than the published data. Similarly, the out-of-plane voltage/strain response was found to be much higher in magnitude than the published data as well as being non-linear. Correlation between the moire experiment concerning the surface bonded patch specimen and the corresponding finite element model was good, indicating the validity of the modeling technique. Possible depoling damage of the embedded patch caused the correlation between the experiment and analysis of this specimen to be poor. Both finite element models predicted stresses that are not statically significant but may represent problems over many loading cycles.
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    Interlaminar Deformation at a Hole in Laminated Composites: A Detailed Experimental Investigation Using Moire Interferometry
    Mollenhauer, David Hilton (Virginia Tech, 1997-08-20)
    The deformation on cylindrical surfaces of holes in tensile loaded laminated composite specimens was measured using new moire interferometry techniques. These new techniques were developed and evaluated using a 7075-T6 aluminum control specimen. Grating replication techniques were developed for replicating high quality diffraction gratings onto the cylindrical surfaces of holes. Replicas of the cylindrical specimen gratings (undeformed and deformed) were fabricated onto circular steel sectors. Narrow angular regions of these sector gratings were directly evaluated in a moire interferometer. This moire interferometry approach eliminated potential sources of error associated with other moire interferometry approaches. Two composite tensile specimens, fabricated from IM7/5250-4 pre-preg with ply layups of [0₄/90₄]3s and [+30₂/-30₂/90₄]3s, were examined using the newly developed moire interferometry techniques. Circumferential and thickness direction displacement fringe patterns (each 3 degrees wide) were assembled into 90 degrees wide mosaics around the hole periphery for both composite specimens. Distributions of strain were calculated with high confidence on a sub-ply basis at select angular locations. Measured strain behavior was complex and displayed ply-by-ply trends. Large ply related variations in the circumferential strain were observed at certain angular locations around the periphery of the holes in both composites. Extremely large ply-by-ply variations of the shear strain were also documented in both composites. Peak values of shear strain approached 30 times the applied far-field axial strain. Post-loaded viscoelastic shearing strains were recorded that were associated with the regions of large load-induced shearing strains. Large ply-grouping related variations in the thickness direction strain were observed in the [+30₂/-30₂/90₄]3s specimen. An important large-scale trend was observed where the thickness direction strain tended to be more tensile near the outside faces of the laminate than near the mid-ply region. The measured strains were compared with the three-dimensional analysis technique known as Spline Variational Elastic Laminate Technology (SVELT), resulting in a very close match and corroborating the usefulness of SVELT.