A new design method of winding tension considering thick-wall effect for composite overwrapped pressure vessel

Technical Summary: A New Design Method of Winding Tension Considering Thick-Wall Effect for Composite Overwrapped Pressure Vessel

Problem Statement
High-pressure Composite Overwrapped Pressure Vessels (COPVs), particularly those used for hydrogen storage in fuel cell vehicles, increasingly operate at pressures of 70 MPa or higher. As the working pressure rises, the composite wall thickness increases, leading to a "thick-wall effect." In thick-walled COPVs, the stress distribution is not uniform; instead, the fiber-direction stress decreases significantly from the inner-most layer to the outer-most layer. The paper notes that in a 70 MPa COPV, the stress in the inner-most layer can be approximately 30% higher than in the outer-most layer. Consequently, when the inner layers reach their failure criterion, the outer layers remain under-stressed, resulting in a low utilization ratio of fiber strength and a suboptimal burst pressure. Existing design methods, such as the Uniform Residual Stress Method (URM), which aim for uniform residual stress, inadvertently exacerbate this issue by creating a total stress profile that still decreases from the inside out.

Methodology
The study employs a combination of analytical derivation and numerical simulation to address the thick-wall effect:

1. Finite Element Modeling (FEM): A three-dimensional finite element model of a Type III COPV (aluminum liner with carbon fiber/epoxy composite) was established. To capture precise stress gradients, continuum shell elements (SC8R) were used for the composite layers rather than conventional shell elements. The model utilized cyclic symmetry to simulate a 5-degree sector of the vessel under 20 MPa internal pressure.
2. Analytical Derivation: The authors derived an analytical solution for the residual stress generated by winding tension. Based on assumptions of axisymmetry, linear elasticity, and no friction between layers, they formulated the equilibrium equations for the mandrel (liner) and the winding layers. By applying the superposition principle, they established a matrix relationship (Eq. 27) linking the desired residual stress in each layer to the required winding tension for that layer and subsequent layers.
3. Proposed Design Method (TWE): A new design method, termed "Thick-Wall Effect" (TWE), was proposed. Unlike URM, TWE calculates the service stress distribution caused by internal pressure first. It then designs a specific residual stress profile via winding tension that increases from the inner to the outer layers. The goal is to ensure that the total stress (service stress + residual stress) is uniform across the entire thickness.
4. Validation: The TWE method was validated using a numerical simulation in commercial software. The "element birth and death" technique was combined with a thermal expansion analogy (simulating tension via controlled cooling) to mimic the sequential winding process and the resulting residual stresses.

Key Contributions

• Analytical Solution: The paper provides a derived analytical solution for residual stress in thick-walled COPVs that accounts for the interaction between the metal liner and anisotropic composite layers under winding tension.
• Inverse Design Algorithm: It presents a matrix-based method to calculate the specific winding tension required for each layer to achieve a target residual stress distribution, effectively reversing the standard stress calculation process.
• Uniform Total Stress Strategy: The core contribution is the shift in design philosophy from achieving uniform residual stress to achieving uniform total stress. This approach directly targets the mitigation of the thick-wall effect.

Results

• Stress Distribution: The FEM analysis confirmed the thick-wall effect, showing fiber stress dropping from 203 MPa in the inner-most layer to 136 MPa in the outer-most layer under internal pressure alone.
• Comparison with URM: When the TWE method was applied, the resulting total stress distribution was significantly more uniform than that achieved by the URM. In the URM case, the inner-most layer stress remained nearly 20% higher than the outer layers.
• Performance Improvement: The TWE method successfully reduced the stress gradient. While the liner's elastic-plastic behavior prevented perfect uniformity in the simulation, the new method effectively eliminated the observable thick-wall effect in the total stress profile, ensuring all layers approach the failure threshold simultaneously.

Significance and Claims
The paper claims that the proposed TWE method improves the mechanical performance of high-pressure COPVs by fully utilizing the strength of the fiber in the outer layers, which are typically underutilized in conventional designs. By eliminating the stress gradient caused by the thick-wall effect, the method allows for a more efficient use of composite materials, potentially leading to lighter vessels or higher burst pressures for the same material volume. The authors conclude that considering the thick-wall effect is critical for the design of high-pressure COPVs and that their new winding tension design method is an effective solution to this specific engineering challenge.