**Appendix B**

1. Binarisation images of composites with different volume density, mass fraction, and thickness

**Figure A1.** (**a**) Volume density 0.139 g/cm2; (**b**) volume density 0.155 g/cm2; (**c**) volume density 0.172 g/cm2; (**d**) volume density 0.188 g/cm2; (**e**) volume density 0.205 g/cm2; (**f**) volume density 0.211 g/cm2; (**g**) kapok fibre 30%; (**h**) kapok fibre 40%; (**I**) kapok fibre 50%; (**j**) kapok fibre 60%; (**k**) thickness of 5 mm; (**l**) thickness of 1.0 cm; (**m**) thickness of 1.5 cm; (**n**) thickness of 2.0 cm; (**o**) thickness of 2.5 cm; (**p**) thickness of 3.0 cm.

2. Calculation results of the fractal dimension of composite materials with different volume density, mass fraction, and thickness.

**Figure A2.** *Cont*.

**Figure A2.** (**a**) Volume density 0.139 g/cm2; (**b**) volume density 0.155 g/cm2; (**c**) volume density 0.172 g/cm2; (**d**) volume density 0.188 g/cm2; (**e**) volume density 0.205 g/cm2; (**f**) volume density 0.211 g/cm2; (**g**) kapok fibre 30%; (**h**) kapok fibre 40%; (**I**) kapok fibre 50%; (**j**) kapok fibre 60%; (**k**) thickness of 5 mm; (**l**) thickness of 1.0 cm; (**m**) thickness of 1.5 cm; (**n**) thickness of 2.0 cm; (**o**) thickness of 2.5 cm; (**p**) thickness of 3.0 cm.
