Issue 78

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

modification on stress transfer [15]. Developing natural fibre composites involves interacting variables such as fibre loading, fibre treatment, fibre length, processing conditions and material composition. Conventional one-factor-at-a-time approaches are insufficient to capture these interactions efficiently. Design of Experiments provides a systematic framework for studying multiple factors with fewer trials, while also identifying individual and interaction effects relevant to process optimization and material design. Response Surface Methodology, supported by Analysis of Variance and regression modelling, enables prediction, optimization and assessment of statistically significant factors with improved experimental efficiency [16]. Despite continuing progress in natural fibre reinforced polypropylene composites, important gaps remain. Many studies examine individual properties such as tensile strength, flexural behaviour, impact resistance or dielectric response, but fewer studies consider how these properties interact and contribute to overall performance. SEM observations are also often used only as qualitative evidence rather than being directly related to mechanical behaviour and fracture mechanisms. In addition, integrated statistical frameworks that can simultaneously optimize multiple responses and explain the associated microstructural failure mechanisms remain limited. This study addresses these gaps by examining the effect of alkali-treated sisal fibre reinforcement on the multifunctional performance of polypropylene composites through an integrated experimental and statistical approach. Mechanical testing, dielectric evaluation, SEM examination and RSM-based optimization were combined to establish structure–property relationships. The novelty of the work lies in linking multifunctional characterization with statistically validated predictive modelling in a single framework. Rather than reporting isolated property improvements, the study connects processing conditions, microstructural features, fracture mechanisms and composite performance, supporting the development of sustainable lightweight composites for structural and semi-structural applications [17]. Materials ompression-moulding grade polypropylene supplied by Mangal Polyproducts, Mangalore, India, was selected as the matrix material. The certified density and melting temperature of the polypropylene were 0.91 g/cm 3 and 150°C, respectively. Sisal fibre obtained from Shrilaxmi Groups, Cherukapalli, Andhra Pradesh, India, was used as reinforcement. The certified density of the sisal fibre was 1.5 g/cm 3 , and the fibre diameter ranged from 100 to 300 µm. Sisal powder of approximately 300 µm and long fibres of 100 mm were supplied by the same vendor; the long fibres were manually cut to the required lengths. Analytical-grade sodium hydroxide and distilled water were used for fibre treatment and washing, in line with commonly reported material-preparation practices for natural fibre polymer composites [6, 15, 16]. The details of the materials, their specifications and their respective purposes in composite fabrication are provided in Tab. 1. The complete experimental sequence, from material preparation to characterization and optimization, is presented as an editable text-only flowchart in Fig. 1 and is described in the following sections. C M ATERIALS AND EXPERIMENTAL PROCEDURE

Material

Specification

Purpose

Polypropylene (PP)

Commercial-grade thermoplastic polymer

Matrix material Reinforcement

Sisal fibre (Agave sisalana) Sodium hydroxide (NaOH)

Natural fibre reinforcement

Analytical grade Laboratory grade

Alkali treatment of fibres

Distilled water

Washing and rinsing after alkali treatment

Table 1: Constituents and materials used for composite fabrication.

Fibre surface treatment Before composite fabrication, sisal fibres in powder form and fibres of 10 mm and 20 mm length were alkali treated to improve fibre–matrix compatibility and interfacial adhesion. The fibres were soaked in 10% NaOH solution for 72 h and then sun dried for one week to remove residual moisture. Untreated fibres were also sun dried for the same duration so that both fibre categories experienced comparable environmental exposure. The dried fibres were packed separately with silica gel crystals to minimize moisture absorption before composite preparation. Alkali treatment removes hemicellulose, lignin, waxes and other non-cellulosic surface constituents, thereby increasing surface roughness, improving wettability and promoting mechanical interlocking with the polypropylene matrix [6, 7, 15, 16]. The sequence of alkali treatment, washing, drying and storage adopted for sisal fibre modification is illustrated in Fig. 2.

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