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

380

5

Drill 1 (N-type) Drill 2 (W-type)

0.05

300 150

22.5

45

Type II (Al/CFRP/Al)

0.2

48

345

3.1. Thrust force/drilling torque signatures and analysis All the curves present in Figure 3 were extracted from holes drilled with the combination of parameters present in Table 1. Likewise, the parameters in multi-material type II were different, resulting in different test times for both drills, so a normalized time was calculated for all cases, dividing the test time for each drill in a scale up to 1 second, so that they could be more easily compared. This way, the analysis of the thrust force and torque signatures can be correctly performed, comparing both drills and types of multi-material.

a

b

c

d

Fig. 3. Thrust force/diameter (a, b) and drilling torque/diameter (c, d) curves of multi-material type I and II, respectively, comparing drill 1 and drill 2 signatures.

The presented graphs allow to observe several aspects, being the main one that the force signature of both the forces and torques is representative of the stages that the drill is passing in the multi-material over time, i.e., the different layers. In the first case, for type I, initially there is a rise in the force, when the drill starts to penetrate the first layer (CFRP) with the first cutting edge and it increases until the second cutting edge is engaged and the layer is being cut with the drill’s entire diameter. Then, a peak occurs, as the second layer (aluminium) can better withstand the compressive and shear loads imposed on the multi-material during the drilling process than the previous one, due to its ductility and ability to undergo plastic deformation, allowing higher thrust forces. Afterwards, in the third layer, composite once more, the values decrease to the level of the first, as this material has a brittle nature, tending to fracture when subjected to significant compressive and shearing forces. On the other hand, the opposite occurs for type II, where there are two peaks, corresponding to both aluminium layers, being the lower forces registered in the middle layer, the CFRP. In addition, drill 2 achieves higher forces and torques than drill 1 due to its higher diameter and, thus, higher area of contact, which, for the same specific cutting pressure, results in higher forces. This is also evidenced by d rill 2’s higher slope curves also in contrast to their counterpart. From the data obtained during the whole set of drilling experiments, several different conclusions can be drawn. By comparing the maximum thrust force and drilling torque obtained from the different combinations of parameters