Design, Analysis, Manufacture, and Test of Shallow Water Pressure Vessels Using E-Glass/Epoxy Woven Composite Material for a Semi-Autonomous Underwater Vehicle

Abstract

Six E-glass/Epoxy shallow water composite pressure vessels with effective length of 45.72 cm and inner diameter of 33.02 cm were designed, analyzed, manufactured, and tested for an external hydrostatic design pressure of 1.14 MPa that corresponds to a depth of 91m in ocean. Composite pressure vessels were designed as composite cylinders fabricated by roll-wrapping and enclosed by two flat plug-supported end-caps due to their ease of manufacturing and cost-effectiveness. The plug-supported end-caps had a combination of tapered contour and initial radial clearance to account for bending and shear stresses in composite cylinders at the end-cap locations. Buckling and stress finite element analyses were performed for the design of the pressure vessels. An eigenvalue buckling analysis was performed to determine a bifurcation buckling pressure (2.51MPa) and a modal shape of the structure for a wall-thickness of 7.72 mm based on 32 layers of 0.2413 mm thick each. These results were then used to perform a nonlinear buckling analysis. The nonlinear buckling pressure was determined to be 1.42 MPa yielding a buckling pressure factor of safety of 1.25. Stress analysis was performed to investigate the stress response of the structure with the wall-thickness of 7.72 mm under the design pressure. Maximum stress and strain criteria were used and stress and strain factor of safety of 11.95 and 17.17 were achieved, respectively. The composite pressure vessels were made of plain weave E-glass/Epoxy fabric. A comparative study of various materials property modeling for woven materials was performed and reported in this work. The three-dimensional Crimp model was explained and employed in this investigation to model the effective properties of a woven composite material, and the results were compared with other existing models. In addition, general guidelines to model the effective properties of woven hybrid materials are also provided. Tube roll-wrapping with wet-laying technique was used to fabricate the pressure vessels. This technique consisted of several steps, namely set-up preparation, fabric impregnation, fabric rolling, shrink taping, curing and cooling, and post-processing. The total time of manufacturing was 7 h for each pressure vessel. The final products needed minor machining. The final total length, inner diameter, and thickness of the manufactured pressure vessels were 49.53 cm, 33.02 cm, and 8.23 mm, based on 32 cured layers, respectively. An average fiber volume fraction of 55% and an average porosity content of 5% was achieved. The fiber waviness due to the fiber migration during the manufacturing under the cylinder compaction was in the order of the waviness of the woven material due to its weave undulation. End-caps were designed using Aluminum 6061-T6 employing Von Mises criterion. The end-caps have seven holes, which are used to place the connectors and a vacuum bolt. The stress factor of safety of 4.2 was achieved for the end-caps. Aluminum 6061-T6 end-caps with 2.9 cm thickness were fabricated. An axial washer and two radial O-rings were used to seal the pressure vessel/end-caps interfaces. The pressure vessels and end-caps were assembled using six tie-rods. Six pressure vessels were tested at the external hydrostatic design pressure of 1.14 MPa inside a high-pressure water-filled chamber. The pressure vessels were intact and no leakage was observed. The pressure vessels were strain gauged during the testing to compare the experimental and finite element analysis results of axial and hoop strains at the mid-length as well as the vicinity of the end-caps for composite cylinders, and excellent agreements were achieved.