Exploration of Process Limitations in Fiber Placement and Thickness Stacking Optimization for Thermoplastic Composite Hydrogen Storage Cylinders in Hydrogen-Powered Aircraft

Exploration of Process Limitations in Fiber Placement and Thickness Stacking Optimization for Thermoplastic Composite Hydrogen Storage Cylinders in Hydrogen-Powered Aircraft

Abstract

Hydrogen-powered aircraft represent a cutting-edge exploration in clean energy aviation transportation, imposing higher demands for lightweight hydrogen storage systems. Thermoplastic composites (T700/PEEK), with their high strength, ease of processing, and recyclability, have emerged as ideal materials for hydrogen storage cylinders. However, during the Automated Fiber Placement (AFP) process, fibers in the dome area are prone to buckling and fracture, and significant thickness accumulation occurs at the polar opening. Based on differential geometry theory, this paper establishes a wrinkle-free placement trajectory algorithm, defines the fiber overlap ratio, and optimizes the dome surface geometry by adjusting the ellipsoid ratio. It is found that a prolate ellipsoid structure can effectively reduce thickness stacking at the polar opening, providing theoretical support for the lightweight design of hydrogen storage cylinders in hydrogen-powered aircraft.

Keywords: Hydrogen storage cylinder dome; Thermoplastic prepreg; Fiber placement; Structural optimization

1. Introduction

Hydrogen-powered aircraft are one of the key pathways for the aviation industry to achieve carbon neutrality, requiring hydrogen storage systems that meet both lightweight and high-strength requirements. Thermoplastic composites, such as T700/PEEK, have become the preferred materials for hydrogen storage cylinders due to their high toughness, impact resistance, and rapid molding capabilities. However, during the AFP process (Figure 1), fibers in the dome area are susceptible to buckling due to curvature abrupt changes, and severe thickness accumulation at the polar opening significantly restricts the performance of hydrogen storage cylinders. Existing research has mostly focused on process parameter optimization or placement head design, but in-depth exploration of the synergistic optimization between fiber deformation and surface geometry is still needed. This paper explores the influence of dome surface geometry on fiber overlap by establishing a wrinkle-free placement trajectory algorithm and combining it with ellipsoid ratio parametric design.

Figure 1. Automated Fiber Placement

Figure 2. Wrinkles in thermoplastic prepreg due to inability to deform

2. Theoretical Models and Methods

2.1 Differential Geometry Fundamentals

The dome surface adopts a rotational ellipsoid model, with its parametric equations given by:

where T and M are the semi-axis lengths of the ellipsoid, z is the axial distance, and r is the radial radius. By calculating the geodesic curvature (Kg) and Gaussian curvature (K) of the surface, geometric constraint conditions for fiber placement are established.

2.2 Fiber Deformation Model

Based on beam bending theory, the deformation limit of fiber tows is derived:

where w is the tow width and Rmin is the minimum bending radius. Combined with differential geometry equations, wrinkle-free placement conditions are established:

2.3 Natural Path Algorithm

By discretizing the surface into locally developable regions and connecting the geodesics of each region to form a natural path, fibers can be placed with minimal deformation. This algorithm iteratively generates fiber trajectories to reduce overlap defects at the polar opening.

3. Experimental Design and Results

3.1 Ellipsoid Ratio Parametric Design

Series of prolate (T > M) and oblate (T < M) ellipsoid samples are designed, with ellipsoid ratios m = T / M of 1.0, 1.2, 1.4, 1.6, and 2.0. The polar opening radius is fixed at 75 mm, and fiber placement trajectories are calculated for different ellipsoid ratios.

3.2 Overlap Ratio and Thickness Calculation

The overlap coefficient r is defined as the ratio of the defect area to the single-filament area:

The total thickness accumulation formula is:

where t is the single-filament thickness (0.17 mm).

3.3 Result Analysis

Prolate Ellipsoid: As m increases, the gap at the polar opening decreases from -3.7 mm to -3.0 mm, r decreases from 19.1% to 11.8%, and Tr decreases from 0.202 mm to 0.190 mm (Table 1).

Table 1. Prolate ellipsoid samples with different ellipsoid ratios

Figure 3. Variation of placement angle α along the axial direction for prolate ellipsoids with different ellipsoid ratios

Oblate Ellipsoid: The changes are not significant (Table 2), indicating that oblate ellipsoids have limited optimization effects on thickness accumulation.

Table 2. Oblate ellipsoid samples with different ellipsoid ratios

Figure 4. Variation of placement angle α along the axial direction for oblate ellipsoids with different ellipsoid ratios

Overall, to reduce head thickness accumulation, minimize the occurrence of stress discontinuities and concentrations, and achieve lightweight and high-strength designs, prolate ellipsoids have greater application and research value compared to oblate ellipsoids.

4. Discussion

The prolate ellipsoid structure significantly reduces fiber overlap at the polar opening by increasing the ellipsoid ratio. Its mechanical properties are close to those of hemispherical domes, and it has a shallower axial height, making it suitable for lightweight designs. In contrast, oblate ellipsoids have a smaller effect on overlap improvement due to their gentle curvature changes. Future research needs to combine finite element analysis to verify their strength reliability.

The wrinkle-free placement algorithm and ellipsoid ratio optimization strategy proposed in this paper effectively solve the problems of fiber buckling and thickness accumulation in the dome area of thermoplastic composite hydrogen storage cylinders. The prolate ellipsoid structure performs better in terms of processability and lightweight design, providing new ideas for the design of hydrogen storage systems in hydrogen-powered aircraft.

Original Literature:

Xiaolong Yu, Dajun Huan, Jun Xiao, Yong Li, Exploration of processability limitations of fiber placement and thickness stacking optimization of thermoplastic composite hydrogen storage cylinders for hydrogen-powered aircraft, Aerospace Traffic and Safety, Volume 1, Issues 2–4, 2024, Pages 119-130, ISSN 2950-3388, https://doi.org/10.1016/j.aets.2024.12.002.

(https://www.sciencedirect.com/science/article/pii/S2950338824000226)