longitudinal cracks continue to progress very rapidly through the length of the specimen causing it to accelerate swiftly through the completion of collapse. After point C, the cylinder wall contact propagates through the length of the cylinder, resulting in the positive pressure region between C and D. To compare these pressure traces more quantitatively, the minimum and maximum pressures are determined as a percentage of hydrostatic pressure, and the specific impulse of the under and overpressure regions are calculated. The specific impulse is defined as the area under the pressure curve for the time interval of interest. These quantities are compared in Fig. 14.6. Here it is found that carbon/epoxy tubes reach a lower minimum pressure, a higher maximum pressure, and deliver a much greater impulse in the pressure pulse emitted upon collapse. The cause of this increase in pressure and impulse is a result of the increased speed of collapse observed in the high-speed imaging. Due to the catastrophic nature of the collapse of carbon/epoxy tubes, the specimens lose structural stability quite severely early in the implosion event, allowing them to fail at a higher rate. DIC is used to confirm and quantify this difference in collapse speed. The DIC system is first calibrated to collect accurate displacements on the surface of the submerged specimens. The accuracy is then confirmed for each specimen by comparing the outer radius of the specimen calculated by the software to its true value. A finite difference scheme is applied to the measured displacements to calculate Fig. 14.4 Typical dynamic pressure trace about midspan of carbon/epoxy tubes Fig. 14.5 High speed images of implosion event for carbon/epoxy tube 14 Experimental Investigation of Free-Field Implosion of Filament Wound Composite Tubes 113
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