PSI - Issue 53

R.D.F.S. Costa et al. / Procedia Structural Integrity 53 (2024) 376–385 Costa, R. D. F. S. / Structural Integrity Procedia 00 (2023) 000 – 000

382

7

Feed

1 1 1 3

0.255025 0.000625 0.002025 0.257675

0.255025 0.000625 0.002025

125.94

0.057 0.677

Drilling torque

Cutting speed

0.31

Error Total

Figure 5a presents the torque progression after drilling 345 holes with the same tool (drill 2) in multi-material type I. This is the only factor which had a significant variation, result supported by the ANOVA analysis made before, as the forces in this multi-material and both the forces and torque in type II kept stable values along the whole 345 holes drilling campaign. Figure 5b shows drill 2 after this set of experiments, and it is clear why the torque began to increase. Signs of abrasive wear can be observed in the drill’s outer corner, passing from the main to the secondary cutting edge, which, although not very deep, still contributes to a noticeable change in the drilling results. As the torque represents the force multiplied by the distance from the drill’s centr e, this specific wear was responsible for the increment in the torque values.

a

b

Fig. 5. Torque progression with 345 holes drilling campaign (a) and respective damage detail in the drill ’s flank after the experiments (b).

3.2. Surface defects and roughness analysis There is a consistency between the microscope (Figure 6a) and the x-ray (Figure 6b) analysis, confirming the absence of intradelamination between the CFRP and aluminium layers. Thus, the delamination is just present on the holes’ surface , present specially on the composite layer, as the fibres were removed by the drill’s action . Figure 6c and Figure 6d show another type of surface defects, namely burrs, firstly in the aluminium layer and afterwards in the CFRP one. These defects can be avoided with a correct combination of the drilling conditions.

a

b

c

d

Figure 6. Surface defects on the drilled holes: delamination on multi-material type I under the microscope (a) and x-ray (b), and external (c) and internal (d) burr on multi-material type II under the DinoLite.

The Matlab script used allowed to obtain the binary image from the x-ray one, clearly distinguishing the hole and the damage zone. This resulted in the achievement of several delamination factors, dependent on the chosen

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