Interfacial shear strength and debonding mechanisms in guadua fiber–reinforced polyester composites: Effect of alkali and plasma surface treatments

Natural fiber-reinforced polymer composites offer a sustainable alternative for non-structural construction applications, including partition panels and lightweight cladding systems. However, the hydrophilic nature of natural fibers limits their interfacial bonding with polymer matrices, reducing the overall efficiency of the composite. In this study, Guadua angustifolia bamboo fibers were surface-modified prior to composite fabrication using alkali treatment (NaOH solutions) and dry-etching plasma treatment, and their influence on fiber–matrix interfacial mechanics was systematically investigated. Single-fiber pull-out tests were performed to quantitatively determine the interfacial shear strength (IFSS) and analyze debonding mechanisms in polyester-based composites. Plasma-treated fibers exhibited the highest IFSS (34.46 MPa), followed by alkali-treated fibers (25.15 MPa) and untreated fibers (12.60 MPa). Distinct debonding responses were identified: plasma-treated fibers showed stable and progressive debonding associated with enhanced stress-transfer efficiency, whereas alkali-treated fibers displayed a partially stable response, reflecting intermediate adhesion conditions. Macroscopic tensile and bending tests demonstrated that improvements in IFSS were consistent with enhanced composite strength and stiffness, although excessive alkali concentration led to diminished reinforcement efficiency. Scanning electron microscopy (SEM) of fracture surfaces corroborated the pull-out results, revealing increased matrix adherence and reduced fiber pull-out in plasma-treated specimens. The results demonstrate that controlled surface engineering directly regulates interfacial shear performance and debonding stability, providing mechanistic insight into interface optimization in bamboo fiber-reinforced polymer composites.