Issue 78

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

calculated from capacitance, specimen thickness, electrode area and permittivity of free space. Three specimens were tested for each composite composition, and the average value was used for analysis. SEM images were obtained from fractured tensile specimens to evaluate fibre dispersion, voids, fibre agglomeration and fracture morphology, including fibre pull-out, fibre breakage, matrix cracking, interfacial debonding and crack propagation features [20, 21]. Thermal characterization Differential Scanning Calorimetry was performed according to ASTM D3418 to evaluate the thermal behaviour of polypropylene and the sisal fibre reinforced polypropylene composites. Approximately 5.6 mg of material was placed in a 30 µL sample pan, while the reference pan was kept empty. The specimen was heated from room temperature to 200°C at 10°C/min and held at 200°C for 3 min to erase previous thermal history. It was then cooled to 30°C at 10°C/min, held at 30°C for 3 min and reheated to 200°C at 10°C/min. The heat absorbed during melting and heat released during crystallization were determined from the area under the corresponding peaks. Three samples were tested for each composite composition, and the average value was considered for analysis. Statistical analysis The experimental data were analysed using Response Surface Methodology in MINITAB Version 17. Fibre treatment was treated as a categorical variable, while fibre length and fibre loading were treated as continuous variables. Second-order polynomial regression models were developed separately for treated and untreated fibre composites to describe the relationships between the selected processing variables and measured responses [13, 14, 21, 22]. Model adequacy was evaluated using ANOVA, coefficient of determination, adjusted coefficient of determination, predicted coefficient of determination and lack-of-fit analysis. Statistical significance was assessed at a 95% confidence level, and the percentage contribution of significant factors was calculated from the ratio of adjusted sum of squares for each factor to the total adjusted sum of squares. Main effects plots, interaction plots and contour plots were generated to interpret the influence of processing variables and identify suitable processing conditions for balanced composite performance [22]. Mechanical behaviour he mechanical and dielectric behaviour of the sisal fibre reinforced polypropylene composites was controlled by the combined effects of fibre treatment, fibre length and fibre loading. The experimental design matrix shows that the highest tensile response was obtained for treated fibres at 10 mm fibre length and 10 wt.% loading, with a tensile strength of 19.20 MPa. The corresponding untreated composite at the same fibre length and loading recorded 17.73 MPa. Treated composites also showed higher flexural and impact strength under comparable conditions, confirming the positive role of alkali treatment in improving fibre–matrix interaction [7, 10, 11]. This improvement is attributed to the removal of hemicellulose, lignin, waxes and other surface impurities, which increased fibre roughness and strengthened mechanical interlocking with the polypropylene matrix [15, 16]. The ANOVA results summarizing the statistically significant factors and percentage contributions for tensile strength are given in Tab. 4. Tensile strength improved when fibre dispersion and interfacial bonding were favourable, as the applied load could be transferred more efficiently from the polypropylene matrix to the sisal fibres [10, 11, 17]. The tensile strength of neat polypropylene was 18.60 MPa, while the composite values ranged from 8.55 MPa to 19.20 MPa. Fibre loading had the strongest influence on tensile behaviour, contributing 51.8% of the total variation. This was followed by the quadratic effect of fibre length at 22.4%, the fibre length–fibre loading interaction at 9.3%, fibre length at 7.6% and fibre treatment at 4.6%. The model contribution was 97.4%, and the reported R-sq, adjusted R-sq and predicted R-sq values were 97.42%, 96.77% and 95.05%, respectively. These values confirm that the regression equation adequately represents tensile behaviour within the investigated design space. At higher fibre loading, incomplete wetting, fibre crowding, void formation and local stress concentration reduced load-bearing capacity, leading to lower tensile strength [2, 5, 17]. The greater improvement in flexural strength compared with tensile strength indicates that the fibres contributed more effectively to resisting crack growth and deformation under bending. In tensile loading, the randomly oriented short fibres were not all aligned with the loading direction, which limited their contribution to direct load transfer. The corresponding ANOVA results for flexural strength, including the contribution of fibre loading, fibre treatment and fibre length effects, are reported in Tab. 5. Flexural strength followed a similar pattern. Neat polypropylene recorded a flexural strength of 29.25 MPa, whereas the composite values ranged from 20.30 MPa to 36.20 MPa. Alkali-treated T R ESULTS AND DISCUSSION

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