# Introduction ilament-wound composite materials have been successfully used in underwater vehicles and ocean structures over the past few years, especially as composite pressure vessels [1][2][3]; the use of composite materials in civil and military aircraft has also expanded considerably over the past few decades due to their light weight and high resistance to salt wat-er corrosion [4]. Particularly, small underwater veh-icles can be manufactured in one piece with composite materials. Both the filament winding and tape lay-up methods can be used to manufacture a small vehicle without sub-assembly [6]. Although decades of R&D in composite materials have focused on aerospace engineering, new applications are opening up in various fields where weight or resistance to corrosion is critical. Particularly, caron composites are considered promising materials for future underwater vehicles and ocean structures due to their corrosion resistance [5,7]. Buckling has become a dominant failure mechanism when compressive stresses generated by the external hydrostatic pressure reach elevated levels for subsea composite pressure vessel. For an underwater vehicle operated in deep sea, hydrostatic pressure-induced buckling tends to dominate structural performance. Furthermore, a cylindrical structure generally experiences unstable buckling, where the loadcarrying capability of the structure decreases after the buckling [7,8]. Generally, high external pressure vessels such as submarine structures have been manufactured of high strength steel, titanium and aluminum alloy. Large buoyancy is required for the structural weight. Accordingly, the weight-sensitive structures are expected to reduce weight for faster and more efficient peformance. It was observed that the use of composite materials for underwater vehicles can reduce their total weight and expand the depth of operation because the reduced weight can allow for greater structural reinforcement [7, 9, and 10]. In the present work, relatively thick-walled composite cylinders (radius-to-thickness ratio, R/t = 18.8) were manufactured by a filament winding process to reduce the material and geometric imperfections for a high depth underwater vehicle [7]. The main objective of this paper is to investigate the buckling, post buckling behavior and failure mode of moderately thickwalled composite cylinders with various winding angles under external hydrostatic pressure for underwater vehicle applications. The helical winding and hoop reinforcement ([±30/90] FW, [±45/90] FW and [±60/90] FW) were used for the composite cylinders. e-mail: kmpandey2001@yahoo.com # Specimen Modeling The specimens were manufactured by a filament winding process using T 700-24 K carbon fiber and Bisphenol A type epoxy resin. All of the cylinders have a 300-mm nominal inner diameter; a 695-mm nominal axial length and an 8-mm nominal thickness (see Fig. 1). The cylinders have three different winding angles: [±30/ 90] FW, [±45/90] FW and [±60/90] FW. The parameters ±30, ±45 and ±60 denote the helical winding angle, while 90 is the hoop winding. For creating the finite element model, ACOS [15], an inhouse program, was used. The carbon composite cylinders were fabricated by a filament winding process and tested in a water pressure chamber. Two commercial software's, MSC.NASTRAN and MSC.MARC, were also used for comparison of the buckling pressure and mode shape. The nominal thickness of the hoop winding is 10% of the total thickness. This value was chosen because the best buckling pressures are obtained when the hoop ratio does not exceed 50% of the total thickness. When the hoop ratio exceeds 50%, the cylinders become very weak with respect to static strength. In this present work the finite element model of composite pressure vessel is made by ANSYS 14.0 APDL, finite element software. Two commercial software, Msc. Nastran and Msc.Marc and Acos, an in-house program were used to create the model. The cylinders have a 300mm nominal inner diameter, 695mm nominal axial length and an 8 mm nominal thickness. The nominal thickness of the hoop winding is 10% of the total Thickness. In ANSYS # Finite Element Analysis Finite element analysis was used to predict not only the buckling loads but also the post buckling behavior. Failure analysis was performed using the inhouse software ACOSwin, which makes possible nonlinear and progressive failure analysis. The commercial programs MSC/NASTRAN (linear analysis) and MSC/ MARC (nonlinear analysis) were used to validate the buckling loads. The theoretical background for ACOSwin is given in [13]. In the finite element models, four node elements, CQUAD4 in MSC.NASTRAN and Element 75 in MSC.MARC, were used. The ACOS program used an 8-node laminate shell element that had 5 degrees of freedom at each node. In Ansys 14.0 APDL laminate shell element 8 node 281 having 6 degree of freedom at each node were used to predict the critical buckling pressure. For non-linear, post buckling behavior, progressive failure analysis was conducted by ACOS using complete unloading as the stiffness degradation method [16,17]. The stacking sequence of different composite laminate with different orientation of fibers has shown in fig. 2. The enlarge view of stacking sequence and different composite laminate with various thickness have been shown in fig. 3. # Simulation The composite structure that used in under water vehicle application, only hydrostatic pressure will consider which can apply redialy inward direction over the outer surface of the body. The equipment can apply pressures up to 10 MPa, which is equal to the pressure at a depth of 1000 meter of water. At the left end of the composite cylinder all degree of freedom can be restricted and at the right end only two degree of freedom has restricted (x direction & y direction), so that the system will undergo only axial deformation. The finite element modeling, meshing and simulation of carbon-epoxy composite filament wound pressure vessel have shown in figure 4. V. # Results and Discussion The buckling analysis has done by Ansys APDL. It has observed that the result for critical buckling is good matched with the existing experimental results. The figures are describing the comparison study of the composite pressure vessels. Table 3 shows the experimental and finite element buckling pressure. The ANSYS 14.0 APDL results as well as the linear and nonlinear analysis results by MSC/NASTRAN, MSC/MARC and ACO Swin are presented. In ANSYS non-linear buckling analysis has been done. # Conclusion The buckling behavior of moderately thick walled, filament-wound, carbon-epoxy cylinders subjected to hydrostatic pressure was investigated. A total 9 no. of composite laminates has been considered for finite element analysis. The different orientation of the composite layers has been taken [±45/90] FW. considering the initial imperfections of the cylinders. The results show that finite element analysis with shell elements can be used to evaluate the buckling load of moderately thick-walled, filament-wound composite cylinders under external hydrostatic pressure. 1![Figure (a)](image-2.png "FigureFigure 1 :") 2![Figure 2 : Layer stacking sequence of composite pressure vessel [±45/0] FW](image-3.png "Figure 2 :") 44![Figure 4 :(a)](image-4.png "Figure 4 Figure 4 :") ![Fig.5 described the different mode shape obtained by MSC/NASTRAN, MSC/MARC, ACOS win and Ansys 14.0 respectively. Here [±45/90] FW specimen was consider for the finite element analysis.](image-5.png "") 45![Figure 4 :(b)](image-6.png "Figure 4 :Figure 5 :") 67![Figure 6 : Comparison of experimental and computational critical buckling pressure obtained by different software's](image-7.png "Figure 6 :Figure 7 :") ![Analyses were conducted using the finite element package ANSYS 14.0 APDL. Three finite element program ACOS win, MSC/NASTRAN and MSC/MARC were used to validate the results. A shell element 8 node 281 was used to create the finite element model. The ANSYS shell element model predicted the buckling pressure with 1.5% deviation from the other three finite element results and experimental results, not Pcr (Mpa)](image-8.png "") Poisson's?120.253-ratio?130.253-?230.421-ShearG124.14GPamodulusG134.14GPaG233.31GPaIII.a) Mechanical PropertiesPropertySymbolRule ofUnitmixtureElasticE1149GPamodulusE210.6GPaE310.6GPa 1RESULT OBTAINEDBUCKLING PRESSUREPERCENTAGE OF ERROR (%)UNIT(MPa)EXPERIMENTAL TEST0.60-ANSYS 14.0 APDL0.5911.5MSC.NASTRAN0.67712.08MSC.MARC0.69115.2ACOSwin0.67111.8 © 2014 Global Journals Inc. (US) * Composite pressure hulls for autonomous underwater vehicles TJOssc TJLee IEEE oceans conf-erence record Vancouver, BC, Canada October 2007 * Filament winding of the navy composite storage module KACorona-Bittick EBaker GLeon JHall SAMPE J 37 2001 * Ultra-deepwater carbon fiber composite pressure vessel development, dualelement buoyancy unit (DEBU) DJackson MDixon BShepherd EKebadze JLummus MCrews SAMPE J 43 2007 * Buckling of thin laminated orthotropic composite rings/long cylinders under external pressure HARasheed OHYousif Int J Struct Stab 1 485 Dyn2001 * The Technical Writer's Handbook. Mill Valley MYoung 1989 CA: University Science * Stability of anisotropic laminated rings and long cylinders subjected to external hydrostatic pressure HARasheed OHYousif J AEROSP 18 Eng2005 * Post buckling of composite cylinders under external hydrostatic pressure SHHur HJSon JHKweon JHChoi Compos Struct 86 2008 * Buckling of filament-wound composite cylinders subjected to hydrostatic pressure for underwatervehicle applications CJMoon IHKim BHChoi JHKweon JHChoi Compos Struct 92 2010 * Buckling analysis and test of composite shells under hydrostatic pressure JYHan HYJung JRCho JHChoi WBBae J Mater Process Technol 201 2008 * A conceptual design of an underwater vehicle CtfRoss Ocean 33 Eng2006 * Design of submersible pressure hulls in composite materials CSSmith MarineStruct 4 1991 * Post buckling of composite cylinders under external hydrostatic pressure SHHur HJSon JHKweon JHChoi Compos Struct 86 2008 * On the influence of laminate stacking on buckling of composite cylindrical shells subjected to axial compression BGeier H-RMeyer-Piening RZimmermann Compos Struct 55 2002 * Influence of winding pattern on the mechanical behavior of filament woun composite cylinders under external pressure HHernandez-Moreno BDouchin FCollombet DChoqueuse PDavies Compos SciTechnol 68 3-4 2008 * Buckling analysis and test of composite shells under hydrostatic pressure JYHan HYJung JRCho JHChoi WBBae J Mater Process Technol 201 2008 * Delamination buckling and post buckling in composite cylindrical shells under external pressure ATafreshi Thin-Walled Struct 42 2004 * Finite element analysis of burst pressure of composite hydrogen storage vessels PXu JYZheng PFLiu Mater Des 30 2009 * Development of a new generation of filament wound composite pressure cylinders JCVelosa JPNunes PJAntunes JFSilva ATMarques Compos SciTechnol 69 2009