Machining of natural fiber reinforced polymer composites: Damage mechanisms, fiber–process interactions, and performance implications

In studies addressing damage formation and surface integrity during the machining of natural fiber reinforced polymer composites (NFRPCs), there remains a mechanistic lack of understanding with respect to energy transfer mechanisms. This deficiency primarily arises from the tendency to investigate material removal mechanisms in conventional and non-conventional machining processes in isolation, despite their fundamentally different modes of energy interaction. This review critically evaluates the machining-related literature on NFRPCs within a unified classification framework based on dominant energy transfer mechanisms. The fundamental mechanistic differences between mechanically dominated conventional processes and thermally and erosion-dominated non-conventional processes are systematically elucidated, and the reasons why damage modes observed in these processes cannot be directly compared are explicitly demonstrated. Furthermore, the influence of the chemical and physical characteristics inherent to natural fibers on machining-induced damage mechanisms such as delamination, fiber pull-out, thermal degradation, and loss of surface integrity is examined in detail. Methodological shortcomings in the existing literature, including the limited scope of cross-process comparisons and the lack of standardized experimental conditions, are identified. These deficiencies reveal critical research gaps that hinder the development of physically consistent and performance-oriented machining strategies necessary to support the broader industrial adoption of natural fiber reinforced composites.