Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Arun Shukla

Abstract

Hollow structures such as cylinders in underwater applications can become susceptible to implosion when subjected to crushing hydrostatic pressures as the ocean depth increases. Because of this, the naval and marine continuously seek to improve the survivability and durability of underwater vehicles/structures. Composites have emerged as a promising alternative to metallic structures, offering various advantages such as superior corrosion resistance, increased potential operating depths for submerged structures due to their high strength-to-weight ratio, as well as reduced thermal, magnetic, and acoustic signatures, making them highly desirable for marine applications. In the marine and naval industry, composites are already used in applications such as unmanned underwater vehicles and submersibles. However, recent accidents such as the Titan submersible implosion have exposed the lack of complete understanding of the use of composite in hydrostatic-loaded environments. The focus of this work involves utilizing experimental methods and advanced equipment to characterize the implosion performance of different composite structures and study several implosion energy mitigation strategies. The advances made in this work are expected to directly impact the design of composite structures for underwater applications. This work expanded on existing knowledge and build on previous implosion research conducted at URI.

Chapter 1: An experimental study was performed to analyze the dynamic buckling behavior of stiffened composite tubes. The structures analyzed were filament-wound carbon-fiber/epoxy composite tubes with press-fitted compliant and rigid aluminum ring stiffeners. The ring thicknesses were chosen for a consistent buckling pressure for equally spaced one, two, and three compliant stiffener configurations. The composite structures were submerged inside a pressure vessel and then subjected to increasing hydrostatic pressure until buckle initiation. High-speed photography and Digital Image Correlation (DIC) were used to acquire full-field displacements and velocities of the collapse event. In addition, piezoelectric transducers were used to concurrently record the local dynamic pressure histories along the length of the tube. The results show that increasing the number of stiffeners for the same collapse pressure decreases the overall energy emission from the implosion event by more than 16%. However, the energy mitigation effects are also influenced by the location of the stiffeners in relation to the buckling initiation point.

Chapter 2: This paper utilizes the Rayleigh-Ritz energy method to derive a solution for predicting the critical buckling pressure of ring-stiffened composite cylinders under external hydrostatic loading conditions. The proposed solution is suitable for external stiffeners and isotropic cylinders. The solution agreed with experimental results for internally ring-stiffened filament-wound carbon fiber reinforced composite and aluminum tubes under external hydrostatic loading conditions. A buckling solution for unstiffened cylinders is also presented using the Ritz method. The proposed solutions can guide the safe design of ring-stiffened structures for underwater applications and can be used to validate numerical solutions.

Chapter 3: Experiments were conducted to investigate the dynamic buckling behavior of underwater hybrid composite tubes. The study focused on roll-wrapped hybrid layered glass-carbon fiber epoxy composite shells with a six-layer quasi-isotropic layup configuration. In addition to control specimens consisting of fully glass fiber-reinforced polymer and carbon fiber-reinforced polymer, four different hybrid layup patterns were examined. These specimens fitted with custom endcaps were placed inside a 7-kiloliter pressure vessel and subjected to increasing hydrostatic pressure until dynamic implosion occurred. High-speed cameras captured the failure event, and the resulting images were analyzed using Digital Image Correlation (DIC) techniques to obtain full-field displacement data. Additionally, tourmaline pressure transducers positioned around the specimens recorded local dynamic pressure histories. The results revealed that the contribution of each ply location varied in the overall failure behavior of the structures. The thickness of the internal plies played a dominant role in enhancing the structural performance, while the stiffness of the outer plies greatly influenced the bending stiffness. The energy released during the collapse was highly dependent on the failure mechanism of the internal plies. Specifically, for the considered geometries, tubes with glass fiber internal plies exhibited significantly lower energy emissions compared to carbon fiber inner plies.

Chapter 4: This study explores an optimization system to achieve the highest collapse pressure on glass-carbon hybrid composite cylinders under hydrostatic loading conditions. This work evaluates and validates previously established composite buckling solutions for cylindrical composite structures under hydrostatic pressure with experimental results of hybrid composite shells. It utilizes the validated analytical solution to optimize the buckling pressure by varying layup configuration, optimum layup angle, material content, and thickness of each lamina. The optimization is performed on asymmetric and symmetric layup cases to evaluate the influence of the hybrid layup construction on the buckling performance of the structure. Results show that the thicker glass fiber plies are preferred for inner layers, and the stiffer carbon fiber plies for the outermost layers to achieve maximum buckling collapse pressure for all the optimization cases, as this configuration provides superior flexural rigidity. For hybrid composite structures with asymmetric configurations, the collapse pressure can be higher when most layers are made of glass fiber if the glass layers are at least twice as thick as the carbon layers. Similarly, axial-load-resistant layers (0º) should be located around the laminate’s geometric center with the hoop-load-resistant layers (90º) on or near the outermost layers and shear-resistant layers (45º) between these layers for both symmetric and asymmetric hybrid structures. Moreover, long tubes with small diameters (L/D > 10) favor hoop bending stiffnesses (90º) for all layers in the laminate due to less influence of boundary conditions at endcap locations.

Chapter 5: An experimental study was performed to characterize the effects of Polyvinyl Chloride (PVC) foam fillers on the underwater implosion phenomena of carbon fiber composite structures. The study focused on understanding energy mitigation capabilities and structural improvement properties of different foam geometries when introduced into carbon fiber shells in underwater environments. Four different densities of foams were considered (H60, H100, H130, and H200) to investigate foam density effects, rod and cylindrical foam shapes were considered to understand geometry effects, and the diameter of H100 rod-shaped foams was varied to study diameter effects. The foam-filled composite cylinders, enclosed with custom endcaps, were placed inside a pressure vessel and exposed to increasing hydrostatic pressure until dynamic implosion occurred. 3D Digital Image Correlation technique coupled with high-speed imaging was used to obtain full-field displacements and velocities of the imploding structure. Tourmaline pressure sensors located around the specimen obtained the emitted pressure histories. The pressure and energy emitted decreased with an increase in rod foam density and diameter. The foams significantly decrease the specimen damage with a severity directly proportional to the energy emitted. Cylindrical specimens increased the structure’s collapse pressure by 68% when compared to unfilled shells. Analytical solutions were developed to predict the collapse pressure of both cylindrical filled and unfilled composite shells under hydrostatic pressure loading.

Chapter 6: This work examines the performance of thin carbon fiber composite cylinders under external hydrostatic cyclic loads. It was instigated by the Titan submersible implosion, which had multiple dives of up to 3800m. Two loading conditions were considered to evaluate for both fatigue and creep. The fatigue specimens’ hydrostatic pressure was repeatedly raised to a maximum value (Pmax) and then dropped to a minimum value (Pmin), while the creep specimens had a 90-minute hold at (Pmax). Pmax values ranged up to 90% of the collapse pressure. The pressure of uncycled specimens was noted for strength comparison. The cyclic tests were performed in a designed hydrostatic loading facility attached to a Universal Testing Machine (UTM), after which the specimens were imploded in a 7-kiloliter pressure vessel to evaluate a change in collapse pressure. The 3D Digital Image Correlation (DIC) technique was applied to captured stereo images to obtain full-field displacement data. The results indicate a rise in radial displacements with increased cycles up to a maximum value. However, no significant change in collapse pressure was observed in all cases in this study.

Chapter 7 provides a short summary along with future scope of the work discussed in the dissertation.

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