Issue 78

N. Nayak et alii, Fracture and Structural Integrity, 78 (2026) 378-392; DOI: 10.3221/IGF-ESIS.78.22

composites with balanced mechanical and dielectric properties. The novelty of this study lies in integrating mechanical characterization, dielectric evaluation, SEM-based fracture analysis and Response Surface Methodology within a single multi-response optimization framework. The optimum treated composite exhibited a tensile strength of 19.20 MPa, flexural strength of 36.20 MPa and impact strength of 178 J/m. The developed regression models showed good agreement with experimental observations, with R² values of 97.42%, 92.76%, 97.89% and 91.92% for tensile strength, flexural strength, impact strength and dielectric constant respectively. K EYWORDS . Sisal fibre, Polypropylene composites, Alkali treatment, Response Surface Methodology, Mechanical properties, Fracture surface, Dielectric properties.

I NTRODUCTION

he demand for lightweight, sustainable and environmentally responsible engineering materials has encouraged the wider use of natural fibre reinforced polymer composites as alternatives to conventional synthetic fibre systems. Natural fibres are renewable, biodegradable, low in density and relatively inexpensive, while also offering useful specific mechanical properties. These advantages make them attractive for automotive, construction, consumer, electrical and semi-structural applications [1–3]. They also support sustainable manufacturing and circular-economy objectives by reducing dependence on glass-fibre reinforced plastics. Even so, their broader structural use remains limited by fibre hydrophilicity, natural variability and weak fibre–matrix adhesion, which can reduce stress transfer and lead to premature failure [4, 5]. Among the available natural fibres, sisal has received considerable attention because of its high cellulose content, low density, favourable specific strength, availability, biodegradability and cost effectiveness. Sisal fibres can be used effectively as reinforcement in thermoplastic matrices such as polypropylene. However, surface constituents such as hemicellulose, lignin and waxes often restrict wetting and weaken fibre–matrix bonding. Alkali treatment is therefore commonly applied to remove these non-cellulosic constituents, increase surface roughness and improve mechanical interlocking. These changes help reduce debonding, fibre pull-out and crack initiation during loading [6]. Polypropylene is a widely used thermoplastic matrix for natural fibre composites because it is lightweight, processable, chemically resistant, recyclable, thermally stable and economical. Its compatibility with injection moulding, compression moulding and extrusion also makes it suitable for large-scale production. However, the non-polar and hydrophobic nature of polypropylene limits its compatibility with hydrophilic natural fibres, often resulting in weak interfacial adhesion, void formation and early crack initiation. Improving fibre–matrix adhesion is therefore essential for producing reliable polypropylene-based bio-composites [7–9]. Alkali treatment using sodium hydroxide is an effective and economical method for modifying lignocellulosic fibre surfaces. By removing hemicellulose, lignin, waxes, oils and other amorphous constituents, the treatment exposes cellulose, increases roughness and improves mechanical interlocking with the matrix. These changes enhance stress transfer, reduce fibre pull-out and delay crack propagation. Although many studies report improved tensile, flexural, impact and thermal performance of alkali-treated sisal/polypropylene composites, fewer investigations integrate these improvements with statistical optimization, predictive modelling and SEM-based fracture analysis [10–12]. The performance of sisal fibre reinforced polypropylene composites depends on fibre characteristics, matrix behaviour, interfacial adhesion and processing conditions. Appropriate fibre loading, fibre length and surface modification can improve tensile, flexural and impact properties by promoting stress transfer and reducing fibre pull-out. In contrast, excessive fibre content or poor dispersion may cause agglomeration, voids and stress concentration, reducing structural integrity. Therefore, identifying an optimum balance between reinforcement content and interfacial bonding is critical for high-performance natural fibre composites [13]. Dielectric constant and dielectric loss are influenced by fibre content, moisture absorption, interfacial polarization and microstructural uniformity. Although mechanical and dielectric properties have often been studied separately, limited work has established quantitative relationships among these responses using integrated characterization, statistical modelling and microstructural validation [14]. Composite performance is governed not only by measured mechanical properties but also by damage evolution and fracture mechanisms. Scanning electron microscopy is particularly useful for examining fibre–matrix adhesion, interfacial debonding, fibre pull-out, matrix cracking, fibre fracture, voids and crack paths, thereby clarifying the influence of surface T

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