RELIABILITY STUDY OF A COMPOSITE OVERWRAPPED PRESSURE VESSEL (COPV)

1. Introduction

Composite Overwrapped Pressure Vessels (COPVs) are widely used in aerospace, automotive, and energy applications due to their high strength-to-weight ratio, corrosion resistance, and superior fatigue performance compared to conventional metallic pressure vessels. A typical COPV consists of a thin metallic liner, primarily responsible for leak tightness, and an external composite overwrap that carries the majority of the pressure-induced loads.

This report presents a detailed finite element–based reliability and failure study of a composite overwrapped pressure vessel using ANSYS Workbench. The analysis focuses on understanding the stress distribution, deformation behavior, and failure margins of the composite laminate and aluminum liner under internal pressure loading.

2. Objectives of the Study

The primary objectives of this study are:

• To understand the structural configuration of a composite overwrapped pressure vessel.
• To perform stress and deformation analysis of a COPV subjected to internal pressure.
• To evaluate the structural reliability using composite failure criteria.
• To identify the most critical layer and governing failure mode.

3. Analysis Tools and Workflow

The simulation was carried out using the following ANSYS Workbench modules:

• ANSYS ACP (Pre): Used for defining composite materials, ply properties, stacking sequence, and laminate thickness.
• ANSYS Static Structural: Used as the solver for stress and deformation analysis.
• ANSYS ACP (Post): Used for composite failure evaluation and Inverse Reserve Factor (IRF) assessment.

The general workflow involved laminate definition in ACP (Pre), structural solution in Static Structural, and post-processing of composite failure results in ACP (Post).

4. Geometry and Structural Configuration

The pressure vessel analyzed is a cylindrical COPV consisting of:

• An inner aluminum liner, responsible for sealing and initial load sharing.
• An outer composite overwrap, responsible for carrying the majority of the hoop and axial stresses.

The composite laminate consists of five plies, including the aluminum liner and four composite plies. Each composite ply has a uniform thickness of 1 mm, resulting in a structurally balanced laminate.

5. Material Properties

5.1 Composite Material

The composite overwrap is modeled using Carbon Fiber Reinforced Epoxy (UD) with a longitudinal modulus of 230 GPa. The laminate is assumed to behave in a linear elastic manner.

Ply orientations used in the study are:

              - 45^o / + 45^o / - 45^o / + 45^o  

This stacking sequence is chosen to provide effective resistance against combined hoop and axial stresses induced by internal pressure.

5.2 Aluminum Liner

The inner liner is modeled as an isotropic aluminum material. It provides a leak-tight barrier and contributes to overall stiffness. A von Mises–based failure indicator is used for the aluminum layer to ensure compatibility with ACP (Post) evaluation.

6. Boundary Conditions and Loading

The vessel is subjected to the following boundary conditions:

• Internal Pressure: A uniform internal pressure of 1 MPa is applied to the inner surface of the vessel.
• Constraints: The mouth of the cylinder is fully constrained in the x, y, and z directions to prevent rigid body motion and simulate a fixed support condition.
• Analysis Type: Static structural analysis with linear elastic assumptions.

These boundary conditions represent a conservative loading scenario, ensuring that the stress results reflect worst-case structural response.

7. Finite Element Modeling

The geometry is meshed in ANSYS Mechanical and assigned a dummy thickness during meshing. The actual laminate thickness and ply definitions are applied through ACP (Pre). This approach ensures accurate through-thickness stress recovery while maintaining computational efficiency.

Layered composite elements are used to capture ply-wise stress behavior and enable detailed failure analysis at the laminate level.

8. Stress Analysis Results

8.1 Hoop Stress

The maximum hoop stress obtained from the analysis is:

• Hoop Stress: 146 MPa

As expected, the hoop stress is the dominant stress component in the pressure vessel due to circumferential loading induced by internal pressure.

8.2 Axial Stress

The axial stress developed along the length of the vessel is:

• Axial Stress: 68 MPa

The axial stress is lower than the hoop stress, consistent with classical thin-walled pressure vessel theory, where hoop stress is approximately twice the axial stress.

8.3 Maximum Principal Stress

The Maximum Principal stress developed in the vessel is:

• Maximum Principal Stress: 182 MPa

8.4 Radial Stress

The radial stress varies through the thickness and is compressive at the inner surface and near zero at the outer surface. Its magnitude is significantly smaller compared to hoop and axial stresses and therefore does not govern failure.

9. Deformation Results

The total deformation of the vessel under internal pressure loading is:

• Maximum Total Deformation: 0.939 mm

The deformation contour indicates smooth and continuous displacement distribution, with no localized deformation concentrations, confirming structural integrity under the applied load.

10. Failure and Reliability Analysis

Composite failure analysis was conducted using the following criteria:

• Maximum Stress
• Maximum Strain
• Von Mises Strain (for aluminum liner)

The key reliability indicator used is the Inverse Reserve Factor (IRF), defined as the ratio of applied stress or strain to the allowable limit. An IRF value less than 1 indicates a safe design.

10.1 Inverse Reserve Factor Results

• All composite plies exhibit IRF values less than 1.
• The aluminum liner also remains within safe limits under von Mises strain criteria.
• The most critical layer corresponds to the inner region of the laminate, where stress transfer between liner and composite is highest.

These results confirm that the vessel design is structurally safe under the specified loading conditions.

11. Discussion and Interpretation of Results

The results obtained from the finite element analysis not only confirm the structural integrity of the COPV under the applied loading but also allow key design and reliability questions to be addressed seamlessly.

The maximum stress in the pressure vessel is primarily located in the composite overwrap, particularly in the inner composite plies adjacent to the aluminum liner. This occurs because the composite layers are designed to carry the majority of the pressure-induced hoop and axial loads, while the aluminum liner mainly serves as a leak-tight barrier. The liner experiences lower stress levels due to its lower stiffness compared to the high-modulus carbon fibers and due to effective load transfer to the composite overwrap.

The fiber orientation plays a critical role in stress distribution within the composite laminate. The ±45° winding configuration used in this study provides efficient resistance to combined hoop, axial, and shear stresses, leading to a balanced stress state across plies. However, hoop stress remains dominant due to internal pressure loading. Increasing the winding angle toward the hoop direction (for example, ±55° to ±65°) would improve hoop load carrying capacity and could further reduce peak stresses in the composite, potentially increasing the allowable internal pressure before failure.

Based on the failure analysis using Inverse Reserve Factor (IRF), the vessel does not reach its failure limit at an internal pressure of 1 MPa, as all IRF values remain below unity. By linear scaling of stresses, the predicted failure pressure corresponds to the pressure level at which the most critical ply reaches an IRF of 1. This pressure represents the structural failure limit of the vessel according to the selected failure criteria. The governing failure mode is associated with the composite layers rather than the aluminum liner, indicating that the composite strength controls the vessel’s pressure capacity.

The material properties of the aluminum liner and composite layers have a distinct influence on overall strength. The aluminum liner contributes to structural stiffness and ensures leak tightness but has limited strength compared to the composite overwrap. The carbon fiber composite, with its high modulus and tensile strength, dominates the load-bearing behavior and determines the ultimate pressure capacity. Consequently, improvements in composite properties have a more pronounced effect on vessel strength than changes in liner properties.

If the composite material had a higher modulus or tensile strength, the vessel would be capable of sustaining higher internal pressures before failure. A higher modulus would reduce deformation and strain levels in the laminate, while higher tensile strength would directly increase the allowable stress limits, both resulting in an increased failure pressure and improved safety margin.

Several design modifications can be proposed to improve the vessel’s safety margin. First, increasing the composite overwrap thickness would reduce stress levels in each ply and increase the failure pressure. Second, optimizing the fiber winding angle toward a more hoop-dominated orientation would improve circumferential load resistance. Alternative improvements could include using a higher-strength resin system or adopting hybrid laminates to enhance damage tolerance.

Reducing the aluminum liner thickness would significantly decrease the overall weight of the vessel, improving its mass efficiency. However, this would also shift a greater proportion of the load to the composite overwrap, potentially increasing composite stresses. While the liner stress would decrease, careful optimization is required to ensure sufficient leak tightness and avoid excessive strain concentrations at the liner–composite interface.

Overall, the analysis confirms that the COPV design is composite-dominated, with fiber orientation, laminate thickness, and composite material properties being the most influential parameters governing reliability and failure behavior.

12. Assumptions and Limitations

• Linear elastic material behavior assumed.
• Perfect bonding between aluminum liner and composite overwrap.
• No manufacturing defects or residual stresses considered.
• No fatigue, impact, or thermal loading included.
• Static pressure loading only.

Conclusion

This study confirms that the structural behavior and reliability of a Composite Overwrapped Pressure Vessel (COPV) are dominated by the composite laminate rather than the aluminum liner. Finite element analysis showed that internal pressure loading produces a stress state governed primarily by hoop stress, followed by axial stress, in agreement with thin-walled pressure vessel theory. The highest stresses and Inverse Reserve Factor (IRF) values occur in the inner composite plies adjacent to the liner, where load transfer effects are most pronounced.

Fiber orientation and laminate thickness were identified as the most influential design parameters. The ±45° ply configuration provided a balanced response to combined loading, while the results indicate that more hoop-oriented plies and increased composite thickness would further reduce peak stresses and improve failure margins. At an internal pressure of 1 MPa, all plies and the liner remained within safe limits (IRF < 1), with failure predicted to be composite-governed.

Overall, the analysis highlights that COPV performance is design-driven, with laminate architecture playing a critical role in stress reduction, reliability, and pressure capacity.