Results showing the evaluation of entry and exit delamination damage in terms of delamination factor (Fd) for room temperature dry and cryogenic drilling of CFRP plaque are
133
Figure 6.10: Variation of entry and exit delamination damage in terms of delamination factor (Fd = Dmax/Dhole) with the number of holes produced from drilling of CFRP plaque
when machined room temperature dry and with a tool pre-cooled in LN2
6.1.3.1 Entry Delamination
It is shown in Figure 6.10 that entry delamination damage was significantly lower than exit delamination damage for both cryogenic and room temperature dry drilling. The entry Fd was in the range 1.04-1.20, while the exit Fd was in the range 1.26-1.78. This indicates
that exit delamination damage as a result of push-down force is more severe and critical to the quality of drilled holes when drilling CFRPs, which has also been reported by other researchers [3, 4, 7, 9, 117]. Although it was not discussed by other researchers [3, 4, 7, 9, 117], the author suggests that accumulated cutting heat as the drill approaches the exit side of CFRP plaque is the factor contributing to more severe delamination at the exit than at the entry. At the entry, the drill engages and cut only few laminates at the top of the plaque. As the drill approaches the exit of the plaque, it has cut through more laminates resulting in accumulated cutting heat. This accumulated cutting heat would decrease the interlamina fracture strength of the CFRP plaque, which dominates the resistance to delamination damage [4, 5, 8], which will increase the possibility of delamination damage. The effect of heat during the cutting process on the strength of CFRP plaque will be discussed later in Section 6.4. In addition to accumulated heat, the author also suggests that push-down force acting on the last laminates would result in more possibility of delamination damage than peel-up force acting on the first laminates. This is because push-down force is more attributed to indentation by the chisel edge while peel-up force is more attributed to cutting by the cutting edges and peeling up by the flutes. As can be seen in Figures 6.11 and 6.12,
134 exit for the first hole and 300th hole.
Figure 6.11: Optical microscope images showing the entry ((a) and (c)) and exit delamination ((b) and (d)) produced from room temperature dry drilling of the 1st hole and 300th hole in CFRP plaque
Figure 6.12: Optical microscope images showing the entry ((a) and (c)) and exit delamination produced from drilling of the 1st hole and 300th hole in CFRP plaque with a tool pre-cooled in LN2
It is also shown in Figure 6.10 that there was no significant difference between the entry Fd of cryogenic and room temperature dry drilling from the first hole until the 200
th
135 two drilling tests was not sufficient to produce significant difference in entry delamination damage. However, it is shown in Figure 6.10 that cryogenic drilling produced less entry Fd
as compared to room temperature dry drilling from 200th-300th hole at which the cutting tool was severely worn. The drilling experiments should be carried on further to confirm this trend of less entry delamination when drilling with the cryogenically cooled tool.
6.1.3.2 Exit Delamination
As previously discussed, it can be seen in Figures 6.10-6.12 that exit delamination damage was significantly higher and more obvious than entry delamination damage. Therefore, the exit delamination damage is more critical to the quality of the drilled holes when drilling CFRP composites [4, 7, 9, 117].
However, it is shown in Figure 6.10 that there were large variations in the results of exit delamination damage in term of Fd such that the trend of drilling performance could not be
determined. As a consequence, use of the results for exit delamination Fd, which considered
only the maximum diameter of the damaged area, were not considered suitable for evaluation of drilling performance in this research. Due to the anisotropy of the material, the shape of delamination when drilling CFRPs was irregular and, in some cases, consisted of fine cracks from the fibres being pushed down around the edge of the hole [116, 125], Figure 6.11 and 6.12. Consequently, high variations in the results showing the evaluation of delamination damage in terms of Fd could occur as a result of these fine cracks. The example
of possible variations in evaluation of delamination in terms of Fd was previously
demonstrated and discussed in Section 5.1.2.3. Exit delamination damage was, therefore, also evaluated in terms of area-delamiantion factor (Fda), which considered the actual
delaminated area around the edge of the hole. The results showing evaluation of exit delamination in terms of Fda for cryogenic and room temperature dry drilling are presented
136
Figure 6.13: Variation of exit delamination damage in terms of area-delamination factor (Fda = Amax/Ahole) with the number of drilled holes produced from drilling of CFRP
plaque when machined dry at room temperature and with a tool pre-cooled in LN2
It is shown in Figure 6.13 that cryogenic drilling produced less exit Fda from the first
hole to 300th hole as compared to room temperature dry drilling. The average reduction of exit Fda when drilling with cryogenic cooling as compared with room temperature dry
drilling was 0.08 of Fda or 43%, excluding the extreme value of Fda of the first hole for room
temperature dry drilling. This indicates that the application of LN2 pre-cooling of the tool
could improve drilling performance of CFRPs with respect to the quality of the drilled holes by reducing exit delamination damage as compared to room temperature dry machining. The reason for a reduction of exit delamination damage when drilling with cryogenic pre-cooling will be discussed in Section 6.4.
It is suggested that this extreme value of Fda for the first hole drilling when machined
dry at room temperature was resulting from the variations of material properties within the plaque, which was manufactured in-house by WMG. The bonding strength between the laminates would be lower at the area where the first hole was drilled, contributing to more susceptibility to delamination damage. In addition, it is demonstrated in Figure 6.13 that the exit delamination damage for both drilling tests tended to increase as the number of drilled holes increased. This was due to the progressive increase of tool wear and thrust force with the number of drilled holes.
As the cooling time was changed from 10 s to 30 s, it is expected that the reduction of Fda when drilling with cryogenic cooling as compared with room temperature dry drilling
137 temperature of the cooled tool. However, it is shown in Figure 6.13 that the change of cooling time did not result in an increase in reduction of exit Fda. The reduction of exit Fda by
cryogenic drilling reduced from 36% for the 250th hole drilling, for which cooling time was 10 s, to 24% for the 300th hole drilling, for which cooling time was 30 s. This indicates that an increase in effectiveness of cutting heat removal by cryogenic cooling due to longer cooling time was not sufficient to contribute an increase in benefit of exit Fda reduction at the
end of drilling test. It is proposed that this would be because of a more dominant effect of increase in cutting heat due to progressive increase of tool wear and cutting forces at the end of drilling test compared to the increase in effectiveness of cutting heat removal by cryogenic drilling. The more dominant effect of increased cutting heat with the progress of drilling test was previously discussed in Sections 6.1.1.2 and 6.1.2. In addition, it can be seen that the reduction of exit Fda decreased as the drilling process continued. It reduced
from 61% for the 50th hole drilling (excluding the extreme value of Fda of the first hole for
room temperature dry drilling) to 24% for the 300th hole drilling. This indicates that the effect of cryogenic cooling became less dominant with progress of the drilling process. It is proposed that this was also because of an increase in cutting heat being generated as the drilling process continued which reduced the effectiveness in removing cutting heat by cryogenic drilling.