Design and Analysis of Composite Overwrapped Pressure Vessel

Transcription

1 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF Design and Analysis of Composite Overwrapped Pressure Vessel B. Varun Chandran, Dr.S.B. Tiwari, Dr.R. Suresh, C.K. Krishnadasan, Dr.B. Sivasubramonian and S. Anoop Kumar Abstract A composite overwrapped pressure vessel (COPV) is a vessel consisting of a thin, metal liner wrapped with a structural fiber composite, designed to hold a fluid under pressure. The combination of composite with higher specific strength and the metal liner with its impermeability will yield tankages with low weight. The hemispheres are created from titanium forgings, and single EB weld at the equator is used to join the two hemispheres. The pressure capability of the gas bottle is the combined capability of the liner and the overwrap. The effective thickness of the Composite is distributed over the liner by suitable number of layers. In the present study, design of liner and composite overwrap is carried out. Because of the symmetry one quarter of the model is created using finite element software ANSYS 13. and analysis is carried out to verify the design. The effect of de-bond location and length of de-bond in the linercomposite interface for weld and transition s is also investigated in this paper. Keywords COPV, Gas Bottle, Carbon/Epoxy, De-bond, ANSYS stresses (compression in the liner and tension in the fibers) before the vessels are in service. This process is known as 'autofrettage' in metal working, while it is termed as 'sizing' in the composite pressure vessel industry [3]. After fabrication, a sizing pressure higher than the operating pressure is applied such that the metal liner is plastically deformed whereas the composite reinforcement is in its elastic range. The elastic unloading of the vessel leaves the liner in compression and the composite reinforcements in tension. By this method, in subsequent loading, the pressure vessel can operate in the enhanced elastic range. The successful development of filament wound pressure tanks with metal liner has provided significant weight savings over the conventional metal pressure tanks. The basic concepts of this design is to use a thin metallic liner designed mainly as permeation barrier with little load carrying capacity capability, while the composite is sized to carry all the pressure loads. Therefore, the weight savings can be derived from the dramatic difference in specific strength between metal and composite. I. INTRODUCTION ILAMENT-wound composite pressure tanks, which utilize F filament winding fabrication technique to form high strength and light weight reinforced plastic parts, are a major type of high pressure vessels and are widely used as fuel tanks and gas bottles in commercial and aerospace industries. The most commonly used materials for liner are aluminum and titanium alloys and for composites are fiber reinforced polymers (FRP), using carbon and Kevlar fibers. Different configurations of COPV are spherical, cylindrical, toroidal etc. Fig 1 shows typical COPV of spherical shape. One of the advantages of combining a load-bearing metal liner with composite over-wrap is that it can introduce internal B. Varun Chandran, M. Tech Student, Dept. of Mechanical Engg., S.C.T College of Engg., Thiruvananthapuram, India. varunmace@yahoo.com Dr.S.B. Tiwari, Scientist / Engineer, V S S C, Thiruvananthapuram, India. sbtiwari77@gmail.com Dr.R. Suresh, Scientist / Engineer, V S S C, Thiruvananthapuram, India. C.K. Krishnadasan, Scientist / Engineer, V S S C, Thiruvananthapuram, India. Dr.B. Sivasubramonian, Scientist / Engineer, V S S C, Thiruvananthapuram, India. S. Anoop Kumar, Associate Professor, Dept. of Mechanical Engg., S.C.T College of Engg., Thiruvananthapuram, India. tvm.anoopks@gmail.com Fig. 1: Filament Wound Spherical Pressure Vessel II. OBJECTIVE The main objectives of this work are: 1. Design of overwrap composite gas bottle. 2. Verification of design through finite element analysis. 3. Study of de-bond at liner composite interface. III. DESIGN Composite gas bottles with contributory metal liner concept is adopted for design. In contributory liner design, the liner is designed to operate plastically for efficient use of the material. During pressurization, the liner goes into the elastoplastic and enters into the compression zone during depressurization (Fig.2). It behaves in an extended elastic, in a tension compression-tension mode during ISBN Bonfring

2 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF subsequent pressurization. The liner thickness is so sized that its critical buckling stress is marginally higher than the external compressive stresses experienced by the liner under pressure release conditions. This selected thickness is considered for pressure contribution. The distribution of liner thickness and laying of the composite overwrap is done so precisely that the structure will withstand the pressure cycles successfully. Fig. 2: Pressure-Strain curve for metal lined composite pressurant tank A. Design Specification Spherical configuration has been chosen for the design of tank and the specification is given in the Table I. The configuration is shown in the fig. 3. Table I: Specification of Gas Bottle Maximum operating pressure 33MPa Minimum burst pressure 66MPa Proof pressure 49.5MPa Autofrettage pressure 5.5MPa Internal diameter (Liner) 46.5 mm Outer diameter (Liner) 41.5mm sufficient resistance against buckling while de-pressurizing the tank. The study focused on three common materials used in aerospace pressure vessels; Ti, Al and steel alloys. Ti-alloy was selected as the liner material due to the following factors: 1. Good impermeability 2. High strength to weight ratio 3. High specific stiffness Selection of Composite Material Kevlar fiber composite overwrap was successfully employed in the development of light weight tanks earlier for application in the spacecraft propulsion system. The carbon fibers have higher modulus and better weight advantages than Kevlar fiber. So CFRP is selected as composite material. The material properties of liner and composite materials are listed in the table II and III. Table II: Material Properties of Titanium liner (Ti-6Al-4V) Properties Unit Young s Modulus 14 MPa Tensile yield strength MPa Ultimate tensile MPa strength Poisson ratio.31 3 Density 455 kg/m Table III: Material Property of T3Carbon/Epoxy Properties Unit Longitudinal tensile modulus MPa Transverse tensile modulus 925 MPa Poisson s ratio.286 In plane shear Modulus 48 MPa Longitudinal tensile strength 152 MPa Longitudinal compressive 19 MPa strength Transverse tensile strength 54 MPa Transverse compressive strength 223 MPa In plane shear strength 92.9 MPa The main objective of the design is to determine the liner thickness profile, composite thickness at equator and determine the layup sequence. Fig. 3: Metal Lined Composite Pressure Tank B. Material Selection Since minimum weight is a primary design goal, studies were performed to select thin metallic liner and high strength composite overwrap for design. Selection of Liner Material The metallic liner provides high level of gas impermeability, contributes to pressure capability and offers Estimation of Liner Thickness at Polar Opening and Boss to Shell Transition Region The exterior profile of the liner is kept as spherical with radius 25.25mm.Because of the practical winding difficulties and interface requirement, the filament winding is stopped at 6 latitude from the pole. So the liner from the pole to 6 latitude has to withstand the burst pressure load requirement. To meet this requirement, the liner has to be designed for the burst pressure capability of 66bar near to the pole. The thickness at pole can be estimated using the formula = 7.6 mm ISBN Bonfring

3 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF Estimation of Pressure Capability of Liner The metallic liner attains its maximum pressure capability when it reaches the plastic state and continues to contribute a constant pressure capability. The pressure capability of liner is given by The thickness of the liner t l =1.45 mm Internal radius of the liner R=23.25mm Hence the pressure capability of liner is 1.6 N/mm 2 From 6 latitude onwards, strength of the composite will contribute towards the pressure capability. Near to the equator, the composite contribution will be more. So the liner thickness can be made less at this location. From the above discussion, the requirement of liner thickness is varying from pole to equator as 7.6mm to1.45mm.the liner has to be designed in such a way that it will not give rise to any bending stress. To satisfy this requirement, the liner thickness is designed as follows; the liner thickness is kept uniform as 1.45mm from equator to 3 latitude from pole. Composite Thickness Distribution The designed composite thickness at equator is to be simulated all over the surface of the tank by distributed winding or step back winding to result in quasi-isotropic laminate condition. This construction can be achieved by filament winding using CNC machine. The composite overwrap has to design for the pressure capacity P c = P burst - P = =56 N/mm 2 Effective thickness of Carbon/Epoxy at equator The Quasi isotropic strength of CFRP is σ * =σ c /2 = 152/2=751 N/mm 2 Hence effective thickness of composite at equator is 7.65mm.This thickness has to be built up by N number of layers between α start of 6 to 9 - α start IV. STRUCTURAL ANALYSIS A. Finite Element Model The COPV is a three dimensional structure which requires three dimensional finite element model. But, from the rotational symmetry and loading symmetry, tank is idealized using two dimensional axi-symmetric elements. The axisymmetric element PLANE182 is used for carrying out the material and geometric non-linear analysis. The finite element model is shown in figure.4.the true stress-strain curve of Ti alloy is considered for the material non-linearity of the liner. Geometric non-linear behavior of the vessel is considered because the deformation due to pressure is higher than the thickness of the tank. The bonded and de-bond conditions between liner and composite overwrap is simulated using contact options. The effective modulus corresponding to Quasi-isotropic layup sequence [, 45, 9,-45] is found out and used for the analysis. The symmetric boundary conditions are applied at the equator and pole. The load is applied as internal pressure in the liner. The non-linear static analysis is carried out in three events. Initially, the tank is pressurized to autofrettage pressure and then back to zero and then finally brought to burst pressure. Fig. 4: Finite Element Model of the COPV B. Validation of Finite Element Model Structural analysis of liner alone is carried out considering unit pressure acting on liner surface. Convergence study is conducted to find out the optimum number of elements and eliminate mesh dependent errors in the result. From the study, it is inferred that the radial and axial deformation are converged when the element numbers lies between 25 to 4. So 25 elements are selected for further analysis. The hoop stress value obtained by FEM analysis at the membrane agrees with the theoretical result and is shown in the table IV. Table IV: Comparison of FEM and theoretical result Theory FEM Hoop Stress 7.43 N/mm /mm 2 V. RESULTS AND DISCUSSION A. Bonded Condition Structural analysis is carried out considering the perfect bond between liner and composite for autofrettage cycles. The variation of meridional and hoop stress from equator to pole for the inner and outer surface of the liner and composite is plotted on the graph. Hoop and meridional stress in the liner inner and outer profile for MEOP, PPT and unloading condition is shown in fig. 5 and 6. It has been observed that there is stress raise in the weld and transition due to bending. Also it is ISBN Bonfring

4 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF noted that compressive stress is introduced in the liner during unloading condition. Meridional Stress (MPa) Hoop Stress (MPa) Fig. 5: Meridional stress variation in liner inner and outer Profile MPa-L-OD 49.5MPa-L-OD 5 1 MPa-L-OD 49.5MPa-L-OD 33MPa-L-ID 33MPa-L-OD MPa-L-ID 33MPa-L-OD MPa-L-ID MPa-L-ID shown in fig.7 and 8. It is seen that hoop and meridional stress for operating and proof loading are within the allowable limit.tensile stresses are introduced in the composite during unloading condition. B. Study of De-Bonded Condition Debond between liner and composite interface is assumed at different location such as weld built-up(near equator, to6 from the equator), membrane (constant thickness, 6 to 6 from the equator) and transition (6 to9 from the equator) using a standard contact with coefficient of friction. The effects of length of de-bond viz. 6,1 and 12 in the meridional direction are studied in the weld and transition s. Effect of de-bond locations The variation of Meridional stress from equator to pole for MEOP and PPT condition at different locations in linner OD and composite ID is shown in fig.9 to 14. It is seen that the debond in the membrane has no effect. The effect of debond in the transition and weld is critical. In these the stresses in composite are exceeding the allowable strength for PPT condition. So failure of fiber may occur in this. Fig. 6: Hoop stress variation in liner inner and outer profile Fig.9 : Meridinal Stress variation in the liner outer profile when 6 debond(3 to36 ) is introduced in the membrane Fig. 7: Meridional stress variation in composite inner and outer profile Fig.1 : Meridinal Stress variation in the composite inner profile when 6 debond(3 to36 ) is introduced in the membrane Fig. 8: Hoop stress variation in composite inner and outer profile Hoop and meridional stress in the Composite inner and outer profile for MEOP, PPT and unloading condition is ISBN Bonfring

5 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF Meridional Stress(MPa) profile when 6 debond( to6 ) is introduced in the weld. Effect of length of de-bond in weld and transition s The variation of meridional stress in the transition and weld built-up (MEOP and PPT condition) for different de-bond lengths are shown in the fig.15 to 2. As the de-bond length increases, the stress also increases. There is a chance of continuing the de-bond in the nearby location. The summary of results is shown in Table-V Fig.11 : Meridinal Stress variation in the liner outer profile when 6 debond(6 to66 ) is introduced in the transition Meridional Stress(MPa) Fig.15 : Meridinal Stress variation in the liner outer profile when 1 debond(54 to64 ) is introduced in the transition Fig.12 : Meridinal Stress variation in the composite inner profile when 6 debond(6 to66 ) is introduced in the transition Meridional Stress(MPa) Fig.16 : Meridinal Stress variation in the composite inner profile when 1 debond(54 to 64 ) is introduced in the transition Fig.13 : Meridinal Stress variation in the liner outer profile when 6 debond( to6 ) is introduced in the weld. Meridional Stress(MPa) Fig.14 : Meridinal Stress variation in the composite inner ridional Stress(MPa) Fig.17 : Meridinal Stress variation in the liner outer profile when 12 debond(54 to66 ) is introduced in the transition ISBN Bonfring

6 Proceedings of International Conference on Materials for the Future - Innovative Materials, Processes, Products and Applications ICMF Meridional Stress (MPa) Fig.18 : Meridinal Stress variation in the composite inner profile when 12 debond(54 to66 ) is introduced in the transition Meridional Stress(MPa) Fig.19 : Meridinal Stress variation in the liner outer profile when 12 debond( to12 ) is introduced in the weld. dional Stress(MPa) MPa-debond MPa-debond Fig.2 : Meridinal Stress variation in the composite inner profile when 12 debond( to12 ) is introduced in the weld. Length of debond Table V: Summary of Results Meridional Stress in Composite for PPT (MPa) Transition Weld Membrane Bonded De- Bonded De- Bonded De- Bonded Bonded Bonded Allowable Strength : 751 MPa VI. CONCLUSION A Composite overwrapped pressure vessel is designed to meet the specification and the design is verified through finite element analysis. At the discontinuity s, the stress is more due to the bending.the effect of de-bond is not found in the membrane where as it has been seen in the weld and transition. As the length of de-bond increases, the stress on the overwrap also increases. At the debond, stress exceed the allowable value and this may cause further de-bonding to nearby location and likely to cause damage to the wrap.further work is needed to verify effect of excessive yielding at de-bond zone.yielding with cyclic loading is likely to cause low-cycle fatigue failure of liner. Hence, means to avoid debond between liner and wrap is important in fabrication of the COPV. REFERENCES [1] Manu S Das et.al Parametric study to assess the effect of geometrical discontinuities in Titanium liners used for composite overwrapped pressure vessels Proceeding on International conference on Emerging trends in Manufacturing Technology, TocH College of Engineering and Technology, Ernakulam, September 212 [2] Vinay K. Goyal Analysis methodology for assessing delaminations in Composite overwrapped pressure vessels AIAA, structural dynamics and materials conference, April 212 [3] J. C. Thesken et. al Composite Overwrap Pressure Vessels: Mechanics and Stress Rupture Lifing Philosophy 48 th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference,23-26 April 27, Honolulu, Hawaii [4] M. Madhavi, K. V. J. Rao and K. Narayana Rao, Design and Analysis of Filament Wound Composite Pressure Vessel with Integrated-end Domes, Defence Science Journal, Vol. 59, No. 1, 29, pp [5] David L. Gray and David L. Gray Finite Element Analysis of a Composite Overwrapped Pressure Vessel 4th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit July 24, Fort Lauderdale, Florida [6] Gary Kawahara and Stephen F. McCleskey Titanium-lined, carbon composite overwrapped pressure vessel AIAA, ASME, SAE, and ASEE, Joint Propulsion Conference and Exhibit, 32nd, Lake Buena Vista, FL, July 1-3, 1996 [7] Medhavi sinha Design and burst pressure analysis of CFRP composite pressure vessel for various fiber orientations angle international journal of advances in engineering science and technology [8] Dominic V Rosato Filament winding Inter science publishers, 1964 [9] R M Jones Mechanics of composite materials McGraw- Hill, 1975 ISBN Bonfring