Issue 78

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

contribution of 0.15%; however, its interaction with frequency influenced the stability of dielectric behaviour. At lower frequencies, interfacial polarization and dipole orientation contributed more strongly to the dielectric response. As frequency increased, dipoles had less time to align with the alternating field, resulting in a lower dielectric constant. Alkali treated composites showed more stable dielectric behaviour because improved fibre–matrix adhesion reduced microvoids and interfacial gaps, thereby limiting localized polarization [18, 19].

Source

DF

Adj SS 11235.1

Adj MS 1605.01 582.55 495.22 6834.01 1711.13 436.95 1079.28

F-Value

P-Value

% Contribution

Model

7 1 1 1 1 1 1

119.44

0.000 0.000 0.000 0.000 0.000 0.000 0.000

97.89

Fibre Length Fibre Loading Fibre Treatment

582.6 495.2

43.35 36.85

5.08 4.31

6834.0 1711.9

508.59 127.40

59.55 14.92

Fibre Length × Fibre Length Fibre Loading × Fibre Loading Fibre Loading × Fibre Treatment

437.0

32.52 80.32

3.81

1079.3

11.59

Error Total

18 25

241.9

13.44

— —

— —

— —

11476.9

—

Table 6: ANOVA for impact strength.

Source

DF

Adj SS

Adj MS 4.3487 0.4825 37.4245 0.0985 5.9750 0.3207 0.2069 0.0601

F-Value

P-Value

% Contribution

Model

14

60.8817 0.4825 37.4245 0.0985 17.9250 0.9621 0.6208 5.3533

72.50

0.000 0.006 0.000 0.204 0.000 0.002 0.020

91.92

Fibre Length Fibre Loading Fibre Treatment Log Frequency

1 1 1 3 3 3

8.02

0.73

622.19

56.50

1.64

0.15

99.34

27.06

Fibre Loading × Log Frequency Fibre Treatment × Log Frequency

5.33 3.44

1.45 0.94

Error Total

89

— —

— —

— —

103 66.2350

—

Table 7: ANOVA for dielectric constant. Dielectric measurements were performed on silver-coated specimens under controlled chamber conditions at approximately 10 ⁻⁴ torr. Fibre drying and storage with silica gel helped minimize moisture-related variation during dielectric testing. The results indicate that the dielectric performance must be balanced against mechanical reinforcement. Higher fibre loading can improve stiffness and strength up to an optimum level, but excessive reinforcement increases the probability of fibre agglomeration, voids and microstructural non-uniformity. These defects disturb the insulating continuity of polypropylene and increase interfacial polarization. Therefore, the optimum composite formulation should not be selected on mechanical performance alone; dielectric stability must also be considered when the material is intended for semi structural or electrical insulation applications. Statistical modelling and optimization Response Surface Methodology based on Central Composite Design was used to develop regression models for tensile strength, flexural strength, impact strength and dielectric constant [9, 13, 14]. All models were statistically significant at the 95% confidence level, confirming that the selected factors adequately described the experimental responses within the investigated design space. The tensile model reported R-sq, adjusted R-sq and predicted R-sq values of 97.42%, 96.77% and 95.05%, respectively; the flexural model reported 92.76%, 91.38% and 87.67%; the impact model reported 97.89%, 97.07% and 94.40%; and the dielectric model reported 91.92%, 90.65% and 88.10%. These values demonstrate strong agreement between predicted and experimental values and confirm that the modelling approach was suitable for interpreting the combined influence of fibre length, fibre loading, treatment and frequency [21, 22]. The response surface and interaction trends confirmed the existence of an optimum processing region rather than a monotonic increase in

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