Date of Award

2021

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

The use of composites has attracted attention in underwater marine applications due to the array of advantages offered by these materials. Various combinations between using metals and composites are also being used together, often in close proximity in underwater environments. Composite materials offer alternatives with reduced weight, improved corrosion resistance, and for submerged structures, greater potential operating depths. In addition, these materials provide improved stealth qualities by having very low thermal, acoustic, and magnetic signatures, increasing their appeal for military applications. For these reasons, the presence of composite materials in marine industries is increasing, and are currently used in several naval applications, such as sonar domes, masts, and hull sheathings. One of the biggest obstacles to widespread adaptation of composite materials is a lack of complete understanding and simple design rules for these materials, especially under extreme loading conditions, often arising due to implosion events. Additionally, the need for understanding complex underwater FSI phenomena in catastrophic and dynamic events which occur under the action of hydrostatic conditions is equally relevant. For this reason, the present work looks to expand the current knowledge of complex underwater near-field structures including plate and shell behavior by specifically examining the effect of implosion.

Firstly, an experimental investigation is conducted to study the mechanics of underwater implosion of cylindrical bonded sandwich composite shells. Sandwich structures comprised of concentric carbon-fiber/epoxy shells with PVC foam cores of different densities are imploded in a large-diameter pressure vessel. High-speed photography in conjunction with Digital Image Correlation (DIC) measurements are employed to obtain full-field displacements of the dynamic collapse process. Local dynamic pressure histories are also simultaneously recorded to investigate fluid structure interaction during implosion. Observations of collapse mode, radial displacement and velocity of collapse, interaction between the concentric shells and the foam core and post-buckling failure sequence are made. Increasing foam core shear modulus linearly increases the experimental buckling pressures. Weaker growth of incipient modal deformations is understood to play a pivotal role in obtaining higher critical buckling pressures from bonded sandwich shells than previously studied unbonded sandwich constructions. Three dynamic collapse behaviors as determined by the relative orientation of collapse between the inner and outer shell are observed. Core foam density and imperfections also strongly influence the impulse, energy and pressure pulses released from the implosions.

Secondly, an experimental and analytical investigation is conducted to study the underwater interaction of implosion pressure pulses with large plates. Two plates with stiffnesses significantly apart are investigated experimentally in a large-diameter pressure vessel for their Fluid-Structure Interaction (FSI) phenomena during proximal implosions of thin metallic shells. High-speed photography, in conjunction with 3D Digital Image Correlation (DIC) measurements, is employed to obtain full-field displacements of the plates. Local dynamic pressure histories are also simultaneously recorded to investigate the incident, reflected and, transmitted fluid pressures across the plates during dynamic loading. The lesser stiffness plate showed higher deflection, allowed a weaker reflected pressure pulse, and a stronger transmitted pressure pulse as compared to the higher stiffness plate. The peak deflections of the plates occurred during the underpressure phase of the implosion event. Four analytical modeling iterations with increasing complexities from Taylor’s FSI model are considered to assess the response of water backed plates to dynamic pressure pulse loadings. Each iteration is analyzed individually in an experimental context to understand its role as a building block in a final analytical model. The final model developed is based on the classical plate-bending equation, and fluid velocity corrected for ‘afterflow’ effects and performed better than Taylor’s original model in predicting pressure-time history of the plates’ reflected pressure and transmitted pressure. The plates’ mid-point deflection profiles are also better predicted using this model. Furthermore, the model showed that the response of a plate during a dynamic implosion pressure pulse interaction is weakly dependent on its bending stiffness. Instead, it is observed that for a large plate, its areal mass density is the dominant factor in determining the reflected pressure, the transmitted pressure, and the plate mid-point deflection profiles.

Thirdly, an experimental investigation is conducted to study the dynamic underwater response of a cylindrical composite shell under near critical hydrostatic pressure, to the implosion of another shell in proximity. A primary cylindrical composite shell is imploded in proximity to a secondary shell which is similar in all respects except for the secondary shell having a smaller length. Length differences of 10% and 20% are chosen to simulate variations in collapse pressures occurring in shells from real life manufacturing defects and/or degradation during operational use. The response of the secondary shell is investigated to understand if and how its collapse occurs in addition to studying the general Fluid-Structure Interaction (FSI) phenomena. The pair of shells are subjected to underwater hydrostatic loading using a large pressure vessel suitable for high-speed photography in conjunction with 3D Digital Image Correlation (DIC). 3D-DIC is employed to obtain full-field displacement measurements of both the shells, and local dynamic pressure histories are also simultaneously recorded. The primary shell always imploded first causing a dynamic loading on the secondary shell. In cases of implosion of the secondary shell, although the transient radial deformations occurred in mode 2, the failure itself occurred with a localized failure of the shell walls. It is observed that there exists a critical stand-off distance for the secondary shell to fail catastrophically upon the implosion of the primary shell. A critical stand-off distance is found to exist only in the case of the 10% smaller secondary shell length. If the secondary shell stand-off distance is above the critical distance or when length of the secondary shell is 20% smaller, the secondary shell responds with bending and breathing modes and no visible damage is recorded. When the stand-off distance is close to critical, the relative orientation of the incipient modal shapes of the two shells is the factor governing the collapse of the secondary shell. A novel method is also developed to decouple full-field 3D-DIC measurements into bending and breathing deformation measurements.

Fourthly, the response of a submerged composite plate due to a dynamic underwater implosion pressure pulse is investigated. The pressure pulse is provided by the implosion of a submerged, cylindrical composite tube positioned in front of the submerged plate. The experiments were conducted in a semi-spherical, large-diameter pressure vessel capable of 3-D Digital Image Correlation (DIC) technique, under hydrostatic pressure. High-speed photography, in conjunction with 3D Digital Image Correlation (DIC) measurements, is employed to obtain full-field displacements of the plates. Local dynamic pressure histories are also simultaneously recorded to investigate the incident, reflected and, transmitted fluid pressures across the plates during dynamic loading. Two different diameter tubes were used for comparison of their implosion’s effect on four different composite plate thicknesses. For each case, to see how the plate responded to and interacted with the pressure pulse provided by the composite tube implosion, the incident, reflected and transmitted pressures on and around the plate were measured. These pressures were then compared to an analytical model developed by Kishore et al. which assumes a Gaussian pressure distribution for the incident pressure profile on the plate. The implosion of large diameter shells caused a doubling of plate deflections when compared to small diameter shells. Increasing thickness from 0.64 mm to 2.54 mm, in the case of a composite plate causes notable decrease in peak plate deflections of 25% in large diameter composite shell cases, and 46% in small diameter composite shell cases. The implosion underpressure pulse is found to have a more cylindrical wavefront shape than the overpressure pulse. An updated model is formulated taking into consideration the two-phase fluid velocity formulation for an implosion pressure pulse. Significantly improved plate deflection and pressure profile predictions are achieved compared to the model developed by Kishore et al., due to the incorporation of the two-phase velocity formulation.

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