Review of State-of-the-art of structural health monitoring in hydrogen composite pressure vessels

Ensuring the safety and durability of composite pressure vessels is critical due to their extensive use in aerospace, automotive, and energy sectors. This review examines recent advances in Structural Health Monitoring (SHM) technologies tailored for Composite Overwrapped Pressure Vessels (COPVs). Special focus is given to flexible strain sensors based on nanofillers such as carbon nanotubes, graphene, MXene, and polymer nanocomposites, which provide high sensitivity, stretchability, and tunable sensing behavior. Key sensing mechanisms including tunneling, piezo-resistivity, and crack propagation and fabrication methods influencing sensor performance and integration are discussed. Shape memory alloy (SMA) filament sensors are also analyzed for their exceptional fatigue resistance, elastic stretchability, and high gauge factors. Case studies demonstrate their practical effectiveness under cyclic pressure loading and burst tests. The review further highlights multifunctional composites integrating self-sensing features for next-generation smart pressure vessels. Challenges related to sensor embedding, environmental impacts, data processing, and scalability are addressed. Future research directions emphasize multi-scale modeling, machine learning for damage detection and prognosis, and fully autonomous SHM systems enabling real-time safety management. These advances are poised to enhance reliability, reduce maintenance costs, and extend the operational life of composite pressure vessels in demanding industrial applications.