A Review of Mechanical Reinforcement and Piezoresistive Self‐Sensing in Carbon Nanotube Fiber Composites

ABSTRACT Carbon nanotube‐modified fiber composites are often presented as a route to stronger, tougher, and self‐sensing structural materials. The literature, however, is difficult to use directly because the reported gains are drawn from different processing routes, baselines, test scales, and maturity levels. This review examines CNT‐modified carbon fiber/epoxy systems through a processing–structure–property–performance (PSPP) framework coupled with a technology readiness level (TRL) lens. The aim is not only to summarize reported improvements but to show where the evidence is strong, where it is conditional, and where the claims have moved ahead of validation. The review separates four recurring sources of confusion: interfacial‐strength measurements that mix nanoscale and composite‐scale tests, piezoresistive sensing results that trade sensitivity against stability, structural battery claims that sometimes assign the dual‐role function to the wrong phase, and fatigue or aerospace claims that remain trapped between coupon performance and component qualification. This treatment gives the reader a practical map of what CNTs can reasonably add to fiber composites and what still limits their use. The main gaps are not in the existence of CNT effects, but in baseline reporting, processing scalability, long‐term sensing stability, environmental durability, and sub‐element or field‐level validation. Future progress will depend on standardized reporting, durability‐linked calibration protocols, and component‐relevant demonstrations that connect laboratory gains to certifiable structural performance.