Issue 78

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

composites offered better resistance to bending because the treated fibre surface promoted stronger adhesion and reduced interfacial debonding during three-point bending [7, 10, 17]. The highest flexural strength was observed for treated fibre composites with 10 mm fibre length and 10 wt.% loading. The flexural model accounted for 92.76% of the total variation, with fibre loading contributing 37.81%, the quadratic effect of fibre length contributing 35.14%, fibre treatment contributing 17.61% and fibre length contributing 1.94%. The R-sq, adjusted R-sq and predicted R-sq values were 92.76%, 91.38% and 87.67%, respectively, indicating acceptable predictive capability within the selected parameter range. Moderate fibre content improved stiffness and crack-bridging ability, while excessive loading reduced matrix continuity and encouraged void formation and stress concentration [5, 11].

Adj MS 32.8364 12.7897 87.2177 7.7227 37.8222 15.6241 0.2175

F-Value

P-Value

% Contribution

DF

Adj SS 164.182 12.790 87.218

Source

Model

5 1 1 1 1 1

150.98

0.000 0.000 0.000 0.000 0.000 0.000

97.4

Fibre Length Fibre Loading Fibre Treatment

— — — — — — —

7.6

51.8

7.723

4.6

22.4

Fibre Length × Fibre Length Fibre Length × Fibre Loading

37.822 15.624

9.3 — —

— —

Error Total

20 25

4.350

168.532

—

Table 4: ANOVA of tensile strength.

Source

DF

Adj SS 376.487

Adj MS 94.122

F-Value

P-Value

% Contribution

Model

4 1 1 1 1

67.28

0.000 0.027 0.000 0.000 0.000

92.76

Fibre Length Fibre Loading Fibre Treatment

7.867

7.867

5.62

1.94

153.460 71.480 142.631 29.738 405.864

153.460 71.480 142.631

109.70

37.81 17.61 35.14

51.10

Fibre Length × Fibre Length

101.96

— —

— —

— —

Error Total

21 25

1.399

—

Table 5: ANOVA for flexural strength.

The ANOVA results for impact strength, showing the dominant influence of fibre treatment and interaction effects, are summarized in Tab. 6. Impact strength was especially sensitive to fibre treatment. Neat polypropylene showed an impact strength of 323 J/m, while the composite impact strength ranged from 110 J/m to 178 J/m. Although all composite values were lower than that of neat polypropylene, treated composites consistently showed higher impact strength than untreated composites at comparable fibre length and loading [7, 10, 11]. ANOVA showed that fibre treatment contributed 59.55% to the impact response, followed by fibre length × fibre length at 14.92%, fibre loading × fibre treatment at 11.59%, fibre length at 5.08%, fibre loading at 4.31% and fibre loading × fibre loading at 3.81%. The model contribution was 97.89%, and the R-sq, adjusted R-sq and predicted R-sq values were 97.89%, 97.07% and 94.40%, respectively. These results show that interfacial bonding strongly controlled energy absorption under sudden loading. Treated fibres resisted premature debonding and supported crack arrest, fibre bridging and energy dissipation, whereas untreated composites favoured fibre pull-out and rapid crack propagation [15, 17, 20]. Dielectric behaviour The ANOVA results for dielectric constant, indicating the dominant roles of fibre loading and frequency, are presented in Tab. 7. The dielectric behaviour of the composites varied with fibre loading, treatment condition, fibre length and frequency. The dielectric constant of neat polypropylene at 10 Hz, 100 Hz, 1000 Hz and 10000 Hz was 2.4, 2.3, 2.3 and 2.2, respectively. The corresponding composite values ranged from 3.0 to 5.8 at 10 Hz, 2.6 to 5.2 at 100 Hz, 2.4 to 5.0 at 1000 Hz and 2.2 to 4.2 at 10000 Hz. Fibre loading was the dominant factor affecting dielectric constant, contributing 56.5% in the ANOVA results, while log frequency contributed 27.06% [4, 18, 19]. Fibre treatment alone had a low direct

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