Hydrogen embrittlement is a critical degradation mechanism affecting composite materials, particularly fiber-reinforced polymers (FRPs) and metal-matrix composites (MMCs). It occurs when hydrogen atoms diffuse into the material, leading to a loss of mechanical integrity through mechanisms such as interfacial debonding, matrix cracking, and hydrogen permeation. These phenomena compromise the structural performance of composites, especially in high-stress or corrosive environments.

In fiber-reinforced polymers, hydrogen embrittlement primarily manifests at the fiber-matrix interface. The interfacial region is susceptible to hydrogen-induced debonding due to the chemical and mechanical incompatibilities between fibers and the polymer matrix. Hydrogen atoms, whether introduced through environmental exposure or electrochemical processes, accumulate at these interfaces, weakening the adhesive bonds. This results in a loss of load transfer efficiency between fibers and matrix, reducing the composite's strength and toughness. The debonding process is exacerbated under cyclic loading, where hydrogen enhances crack initiation and propagation along the interface.

Matrix cracking is another consequence of hydrogen embrittlement in FRPs. The polymer matrix, particularly in epoxy-based systems, can undergo hydrogen-assisted degradation. Hydrogen atoms diffuse into the matrix and react with polymer chains, leading to chain scission or crosslink breaking. This reduces the matrix's cohesive strength, promoting microcrack formation. These microcracks coalesce into larger cracks under mechanical stress, further accelerating material failure. In some cases, hydrogen also plasticizes the matrix, temporarily increasing ductility before embrittlement dominates.

Metal-matrix composites face similar challenges but with distinct mechanisms due to the metallic nature of the matrix. Hydrogen embrittlement in MMCs often involves hydrogen diffusion into the metal lattice, where it interacts with dislocations and grain boundaries. In aluminum or titanium-based MMCs, hydrogen atoms accumulate at reinforcement-matrix interfaces, such as those around silicon carbide or alumina particles. This leads to interfacial decohesion, reducing the composite's load-bearing capacity. Additionally, hydrogen can form brittle hydrides in certain metal matrices, such as magnesium or titanium, further degrading mechanical properties.

Hydrogen permeation is a fundamental driver of embrittlement in both FRPs and MMCs. The permeation process involves three stages: adsorption of hydrogen at the material surface, diffusion through the bulk, and trapping at microstructural defects. In polymers, hydrogen diffusivity is relatively high due to their open molecular structure, but solubility varies with chemical composition. In metals, diffusivity depends on crystal structure, with body-centered cubic (BCC) metals like iron being more susceptible than face-centered cubic (FCC) metals like aluminum. Trapping sites, such as voids, inclusions, or dislocations, act as hydrogen sinks, increasing local concentrations and promoting embrittlement.

The severity of hydrogen embrittlement in composites depends on several factors. Material composition plays a key role; for example, carbon fiber-reinforced polymers are more resistant than glass fiber-reinforced ones due to carbon's lower reactivity with hydrogen. In MMCs, the choice of reinforcement and matrix alloy significantly influences susceptibility. Environmental conditions, such as temperature, pressure, and hydrogen partial pressure, also affect embrittlement kinetics. Elevated temperatures generally increase hydrogen diffusion but may reduce trapping efficiency, while high pressures accelerate hydrogen uptake.

Mitigation strategies for hydrogen embrittlement in composites focus on material design and environmental control. For FRPs, selecting matrices with low hydrogen permeability, such as certain thermoplastics, can reduce embrittlement. Fiber coatings or sizing agents that act as hydrogen barriers are also effective. In MMCs, alloying elements that form stable oxides or carbides can hinder hydrogen diffusion. Surface treatments, such as nitriding or shot peening, introduce compressive residual stresses that retard crack initiation. Environmental controls include using hydrogen scavengers or inhibitors in corrosive media to minimize hydrogen generation.

Understanding hydrogen embrittlement in composites is essential for applications where these materials are exposed to hydrogen-rich environments. Aerospace, automotive, and energy sectors increasingly rely on FRPs and MMCs for lightweight, high-strength components. Hydrogen fuel systems, pressure vessels, and offshore structures are particularly vulnerable. By addressing interfacial debonding, matrix cracking, and hydrogen permeation through advanced materials engineering, the durability and safety of composite structures can be significantly improved.

Research continues to explore novel approaches to combat hydrogen embrittlement. Nanostructured coatings, graphene-based barriers, and self-healing matrices show promise in mitigating hydrogen damage. Computational modeling of hydrogen diffusion and trapping provides deeper insights into failure mechanisms. As composite materials evolve, so must the strategies to protect them from hydrogen-induced degradation, ensuring their reliability in demanding applications.