Issue 78

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

was carried out at 85 bar for 1 h, after which the mould was cooled to room temperature before demoulding. Teflon sheets were placed on the die surfaces to prevent sticking. Two composite plates were prepared for each composition, and two neat polypropylene plates were produced under the same conditions for comparison. Experimental design The experimental design matrix, together with the measured tensile, flexural and impact strength values for each run, is presented in Tab. 2. The experimental programme was developed using Response Surface Methodology based on a Central Composite Design to investigate the combined influence of fibre treatment, fibre length and fibre loading on composite performance [9,13,14]. Fibre treatment was considered a categorical factor with treated and untreated levels, whereas fibre length and fibre loading were considered continuous factors. Fibre length was varied from 0.3 mm to 20 mm, and fibre loading was varied from 10 wt.% to 30 wt.%. For regression modelling, the powder fibre was considered equivalent to a 0.3 mm short fibre. The experimental runs were generated using MINITAB Version 17 and conducted according to the prescribed run order to reduce experimental bias. The principal responses were tensile strength, flexural strength, impact strength and dielectric constant, while SEM fractography was used to support interpretation of the fracture mechanisms [20, 21].

Run order

Treatment

Length of fibre (mm)

Fibre loading (wt.%)

Tensile strength (MPa)

Flexural strength (MPa)

Impact strength (J/m)

1 2 3 4

T T T T T T T T T

10

10

19.20 18.10 17.50 12.10

36.20 32.00 33.10 26.50 28.00 22.50 26.00 29.20 32.56

178.00 169.00 148.74 160.20 131.00 141.93 125.40 135.70 122.66

3

12.9 12.9

17

0.3

20 20 20

5–9

10 20

16.30–16.60

33.20–33.50

166.00–170.00

10 11 12 13 14

14.50

3

27.1 27.1

9.50

17 10 10

14.00 13.80 17.73

30 10

UT UT

15–26

0.3–20

12.9–30

8.55–16.73

20.30–29.93

110.00–132.70

27

Polypropylene

18.60

29.25

323.00

Table 2: Experimental design matrix and measured mechanical properties.

Factor

Type

Levels

Fibre Treatment Fibre Length Fibre Loading

Categorical Continuous Continuous

Untreated, Alkali-treated

0.3-20 mm 10-30 wt.%

Table 3: Factors, types and levels used in experimental design.

Specimen preparation The compression-moulded laminates were machined into standard test specimens according to the relevant ASTM specifications. Two specimens were cut from each plate for tensile and flexural testing, while four specimens were cut from each plate for impact testing. The mechanical test specimens had rectangular cross-sections with a thickness of 3 ± 0.2 mm. Tensile specimens were prepared according to ASTM D638 with a length of 165 ± 2 mm and a width of 12.7 ± 0.2 mm. Flexural specimens were prepared according to ASTM D790 with a length of 127 ± 2 mm and a width of 12.7 ± 0.2 mm. Izod impact specimens were prepared according to ASTM D4812 with a length of 63.5 ± 2 mm and a width of 12.7 ± 0.2 mm. Specimens for dielectric and SEM characterization were also cut from the compression-moulded plates. All specimens were inspected after machining to ensure that visible edge defects, cracks or surface irregularities did not influence the measured properties. The standard tensile, flexural, impact and dielectric test specimens prepared from the compression-moulded laminates are shown in Fig. 3.

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