COPV - Technical Brief - April 2013

Technical Brief
April 2013

Validation of Burst Pressure Analysis and Manufacturing Techniques for  Composite Over-wrapped Pressure Vessels

GENOA provides engineers with a validated computational approach to determine residual stresses in composite over-wrapped pressure vessels (COPV). It also identifies leakage and burst locations, as well as associated failure modes under service.

The Challenge:  To accurately determine the onset of crack formation, leakage, slippage of gas between dome and valve assembly, and rupture of COPVs.

Other challenges pertain to accurate determination of the following:

• Ply angle distribution
• Residual stresses due to winding and curing
• Tank reliability due to scatter in manufacturing parameters, defects, and material properties
• Strength allowables for risk reduction
• Rupture due to service load:  Proof, Auto-Frottage (load, unload),  Sustained Fatigue (short, high cycles) pressurization,
•  Slippage/Leakage prediction
• Damage and failure multi-site locations
• Reliability
• Addressing variability in tank  performance considering “As-designed” versus “As-built” and “As-is”
• Tank certification

Another challenge stems from the complexity of modeling and analysis of composite materials. Most finite element analysis (FEA) solvers assess damage at the lamina level whereby damage initiates at lower scale in the fiber, matrix, and interphase. These challenges can be addressed through an integrated capability that couples winding analysis with multi-scale progressive failure analysis, including modeling of defects and uncertainty analysis.

The Solution:  To utilize GENOA’s Filament Winding & Multi-Scale Progressive Failure Analysis

Residual stresses are calculated through GENOA’s Filament Winding (GENOA-FW) module. To calculate residual stresses, GENOA uses design parameters, such as tape tension, internal pressure, and curing effects. The tank finite element model is generated based on inputted tape thickness and width, winding angles, and wrapping circuite location during winding. The winding pattern can be optimized to reduce weight. Tanks with metallic and plastic liners or those without liner are modeled and analyzed with GENOA’s FW module. Residual stresses from autofrettage are also determined with the analysis. To ease the design of the tank, the FW module is capable of generating two distinct FEA models using:         

§         Low Fidelity: 3D thick shell elements that merges all plies into a single element thru thickness

§        High Fidelity: 3D solid where plies thru thickness are modeled with layers of solid elements

With generated FEA model and calculated residual stresses, GENOA’s multi-scale progressive failure analysis (MS-PFA) performs leakage and burst rupture analyses. The code integrates finite element analysis with multi-scale composite mechanics and damage tracking and fracture. Several FEA stress solvers are used within the GENOA environment for evaluating durability and damage tolerance of the tank. These solvers include NASTRAN, ABAQUS, ANSYS, and nodal based MHOST [1].

MS-PFA identifies all stages of damage evolution: damage/delamination initiation and growth, and probabilistic fiber stress rupture fracture initiation and growth to failure. It calculates micro-cracks, stiffness degradation in the matrix, delamination within the plies, and fiber failure in tension and compression including micro-buckling. The COPV tank can be evaluated with MS-PFA subject to static, fatigue, and impact loading conditions to accommodate tank certification process [2]. Reliability evaluation considers the uncertainties in geometry, material, fabrication process and loading [3].  It ranks the effect of considered random variables on leakage, delamination, and rupture of tank and determines the scatter in burst pressure. In addition to uncertainty analysis, a test reduction methodology for determining A- and B-basis allowable design loads is integrated. A-basis design allowables generation can be determined with reduced testing using B-basis values from accepted test methods or can be generated from lamina level uncertainties [4].

Validation: Figure 1 shows a failed composite tank that was modeled and analyzed with the software. Figure 2 shows typical filament winding input requirements supporting helical and hoop winding schemes. Data used as input includes winding schedule, tape width, and tape tension. The tanks analyzed include spherical, and cylindrical with geodesic domes. ASC validated its technical approach for both space and automotive applications and subject to extreme environmental conditions (elevated and cryogenic temperatures).  Typical analysis results are used to determine conditions for leakage, burst location, damage evolution through-the-thickness, and reliability. Filament wound composites over-wrapped tanks were considered in for the application of motor cases for solid rockets motor cases [3] Deterministic evaluation of durability and damage tolerance of tank using GENOA resulted in predicted burst pressures that are within 8.8% of the test for 4 groups of tank design. The pressurized tanks were also evaluated in presence of uncertainties in constituent stiffness and strength and manufacturing variables. The sensitivity analysis indicated that the fiber mis-alignment and ply thickness are the most influential variables that affect the burst pressure as indicated in Figure 3.  Scatter in burst pressure, A Basis Allowable as a result of considered uncertainty is shown in Figure 4. The results show that a reliability 0f 0.999 can be achieved if burst pressure is kept under 3033 psi.  When damage evolution through thickness is of concern, GENOA offers capability to generate high-fidelity solid element FE models. Figure 5 shows the snap shots from automated process to generate high-fidelity solid FE model from previously generated shell element FE model. The high-fidelity model helped account for the out-of-plane shear stresses, which results in delamination, more accurately. Figures 6 to 8 show the damage evolution at the laminate level, ply-by-ply level and final burst predictions compared to the test specimen in Figure 1.

Another validation of the capability done under the NASA space launch initiative effort [5] determines leakage in COPV at cryogenic temperatures. Analysis showed a large percentage of failure in composites initiate as matrix cracking. When matrix cracks it releases energy causing accumulation of additional cracks that continues till a saturation point. GENOA can track this initiation, accumulation and saturation process.

 Conclusion: GENOA provided a reliable and verifiable low as well as high fidelity simulation solution in determining residual stresses, identifying leakage, burst locations, and associated failure modes under service in COPV’s.

References:

1.       S. Nakazawa, “The Mhost Finite Element Program, 3-D Inelastic Method for Hot Structure Components”, Volume 1 Theoretical  Manual,  NASA Technical Contract Report CR-182205, March, 1991.

2.       C. Keddy , “Composite Overwrapped Pressure Vessel Modeling and Analysis”, White Sands Test Facility, Johnson Space Center, July 2008.  

3.       G. Abumeri, F. Abdi, M. Baker, M. Triplet and, J. Griffin “Reliability Based Design of Composite Over-Wrapped Tanks”.  SAE World Congress, 2007, 07M-312, Detroit Mi, April 2007.  

4.       G. Abumeri, F. Abdi, K.S. Raju, J. Housner, R. Bohner and A. McCloskey: “Cost Effective Computational Approach for Generation of Polymeric Composite Material Allowables for Reduced Testing”, Inetch Book publishing, in print for March 2011 (Co-authored with Northrop Grumman and NIAR).

5.       F. Abdi, X. Su “Composite Tank Permeation and Crack Density Prediction and Verification”.  ASME Paper No. IMECE2003-4439 November 2003.

About AlphaSTAR Corporation: AlphaSTAR Corporation is a leading engineering services and software company that provides innovative physics-based simulation technologies for structural modeling and analysis of advanced composite structures in the aerospace, automotive, defense, and energy industries worldwide. As a solution provider, AlphaSTAR partners with Siemens PLM, Altair, ANSYS, MSC Software, DS Simulia, and LSTC. AlphaSTAR is headquartered in Long Beach, California and is the recipient of esteemed industry and technology awards for R&D and software development. Contact marketing@alphastarcorp.com or www.alphastarcorp.com.