Two high-speed cameras (Photron SA1, Photron USA, Inc.), offset by 16 are used to capture stereo images of the patterned region of the specimen at 20,000 frames/s for glass/PE tubes, and 36,000 frames/s for carbon/epoxy tubes. The stereo images are analyzed using a commercially available digital image correlation (DIC) software, VIC3D 2012 (Correlated Solutions, Inc., Columbia, SC) to determine real-time, full-field displacements across the viewable surface of the specimen throughout the implosion event with a spacial resolution of 0.5 mm. The digital cameras have a resolution of 1,024 1,024 pixels. The lenses used for each experiment are selected as to image the entire width of the specimen throughout the collapse process, and to capture the full unsupported length of the cylinders. 14.4 Results The glass/PE tubes imploded at a critical pressure of 2.03 MPa, while the carbon/epoxy tubes collapsed at a pressure of 1.61 MPa. These observed buckling pressures are reasonably close to the predictions made based on the methods of Rasheed [15] and Koudela [16]. Both specimen types collapsed completely in a mode 2 buckling shape, and regained some of their original circularity after pressure is removed. Figure 14.2 shows typical pressure histories about the midspan of glass/PE tubes. This trace shows a fairly smooth drop in local pressure, followed by a sharp spike which decays gradually and is followed by many oscillations. This is consistent with previous work done by Turner [7] and Farhat [8] on metallic cylinders and shows that some of the same mechanisms are at play. The drop in pressure between points A and B are a result of the cylinder walls collapsing in toward each other. Surrounding fluid accelerates to follow the walls of the reducing cylindrical volume causing the local pressure to drop. This is confirmed by examining the images corresponding to points A and B in Fig. 14.3. At point C, we see a rapid spike in pressure spike, taking approximately 300 μs rise from the minimum pressure to the maximum pressure. This spike corresponds to the walls of the cylinders contacting each other and arresting their movement, as seen in the matched image in Fig. 14.3. When the walls suddenly stop, so must the fluid accelerated during the previous stage of the collapse, and this rapid change in momentum results in the release of a pressure wave. Following the pressure spike at time C, the pressure drops back down to roughly hydrostatic as the buckled shape spreads axially along the cylinder, causing catastrophic damage to the specimen. In these images, a great deal of damage is clearly visible. The majority of this damage is in matrix cracking and delamination, manifesting in the “ribboning” seen in the high speed images. Upon further post-mortem investigation of the specimens, it is also clear that a good deal of fiber pull-out is present. These features are important to note, as they are energy intensive damage processes which affect the speed of the collapse and thereby the local pressure history. Fig. 14.1 Pressure vessel used for implosion experiments 14 Experimental Investigation of Free-Field Implosion of Filament Wound Composite Tubes 111
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