Structural Design and Experimental Investigation of the Mechanical Performance of Aluminum-Foil- and EVOH-Barrier Unbonded Flexible Pipes for Hydrogen Transport

Abstract

The large-scale deployment of hydrogen energy has created an urgent demand for safe, efficient, and cost-effective hydrogen transportation infrastructure. Reducing transportation costs and improving pipeline reliability are critical for the development of hydrogen supply systems. Owing to their light weight, corrosion resistance, and continuous installation capability, unbonded flexible pipes have emerged as promising alternatives for high-pressure hydrogen transportation. In this study, two DN65 unbonded flexible pipes are designed, with aluminum foil and ethylene–vinyl alcohol copolymer (EVOH) adopted as barrier layers, respectively. Internal-pressure burst tests, 100 h hydrostatic pressure tests, axial tensile tests, and minimum bend radius tests are conducted to evaluate their mechanical performance and service adaptability. The results reveal that the ultimate failure of both pipes is primarily governed by tensile fracture of the reinforcing fibers. Compared with the aluminum-foil-barrier pipe, the EVOH-barrier pipe demonstrates superior pressure resistance and tensile strength, with the average burst pressure increasing from 33.856 MPa to 37.219 MPa, representing a 9.9% improvement, while the peak tensile load rises from 196.4 kN to 292.7 kN, corresponding to a 49% enhancement. In contrast, the aluminum-foil-barrier pipe exhibits better post-bending structural stability and energy absorption capability, as evidenced by the smaller increase in residual ovality from 1.45% and the higher deformation energy sustained before reaching the peak load of 79.2 kJ, compared with 4.16% and 48.2 kJ for the EVOH-barrier pipe. Under the tested conditions, the EVOH-barrier pipe exhibited higher burst resistance and axial tensile capacity, whereas the aluminum-foil-barrier pipe showed better cross-sectional shape retention and high pre-peak deformation work under axial tension. Because the two pipe types also differ in their as-manufactured layer dimensions, these mechanical differences should be interpreted as configuration-level responses rather than isolated effects of the barrier materials. These results indicate that under the operating pressure greater than 10 MPa, the two barrier structures have different mechanical advantages.