**2. Materials and Methods**

### *2.1. Materials*

The materials used were presented in 210 × 210 mm2 sheets with thicknesses of 4.5 mm for CFRP and 4.86 mm for UNS A97075-T6 (UNS A97075). Figure 3 shows the different configurations. These materials have been selected for their importance in commercial airships.

The CFRP composite was made using unidirectional prepregs supplied by Hexcel Composite Company (Stamford, Connecticut, United States) referenced under HEXPLY M21/34%/194/T800S-24K (resin/resin content by weight (%)/fibre weight (gsm)/fibre type). The mechanical properties of the prepregs are density 1.28 g/cc, flexural yield strength 147 MPa, flexural modulus 3.50 GPa, flexural strain at yield 5%, glass transition temperature 185 ◦C. The lay-up sequence of the CFRP was (0/90/45/-45/45/-45)S so as to get a quasi-isotropic laminate. Aluminium alloy UNS A97075-T6 (composition Al 88.78%, Cu 1.87%, Mg 2.62%, Mn 0.08%, Zn 6.03%, Ti 0.11%, other 0.02%) was used on the other sheet. The properties of the aluminium alloy are: density 2.81 g/cc, ultimate tensile strength 524 MPa, yield tensile strength 462 MPa, elongation at break 11%, modulus of elasticity 71.7 GPa.

**Figure 3.** Characteristics and configuration of test materials: (**a**) CFRP; (**b**) UNS A97075 and (**c**) stacked CFRP/UNS A97075.

#### *2.2. Tools*

The chosen tool was a helical model from WC-Co without coating, being selected considering the materials that compose the stacks to be drilled, their thickness, the required qualities and cutting conditions. It has a double-angled tip; the section closest to the centre corresponds to the largest tip angle (140◦) and the projection of the outer edges provides a tighter angle (118◦) (Figure 4).

**Figure 4.** Characteristics of the drill bit used.
