Study of Drilling-induced hole exit defects for CFRP composites: Delamination, splintering, and fraying

When drilling carbon fiber reinforced polymer (CFRP), defects tend to occur at the outlet of the hole. This article studied into the reasons, based on which, some measurements can be implemented to reduce defects.

Key words: carbon fiber composite (CFRP) delamination splintering drilling parameters

Source: Journal of Mechanical Engineering

Author: ZHANG Houjiang, Chen Wuyi and Chen Dingchang

 

When drilling carbon fiber reinforced polymer (CFRP), defects tend to occur at the outlet of the hole. These defects are usually in the form of delamination and splintering, with the delamination being larger than the splintering. The delamination formation process includes two stages: the thinning acting stage and the main cutting edge acting stage, with the former being the dominant component. Splintering defects appear at the edge of the hole where the surface fiber is cut in a forward direction. The size of the delamination is affected by factors such as the feed speed, feed rate, bit diameter, and axial force. An increase in the bit feed speed leads to an increase in the delamination, while an increase in the bit rotation speed leads to a decrease in the delamination. The ratio of cutting speed to feed speed should be controlled below 3000-4000 to reduce the delamination. Drilling the hole outlet side of the support, such as an aluminum plate, can also reduce the delamination. Controlling the drilling force, especially limiting the axial force of the thinning of the bit to a certain range, is an effective way to reduce the hole outlet defect. The formation process of the hole outlet delamination includes two stages: the thinning action stage and the main cutting edge action stage. The thinning action stage begins with the thinning pushing against the surface material on the exit side and ends with the thinning drilling out of the surface material. When the thinning of the bit is cut from the inside of the CFRP material to the outside, it has a strong pushing effect on the surface material. The surface fiber layer first bulges at the center of the bit axis and then expands along the direction of the outer fiber layup, splitting between the surface fibers and cutting across the edge. The main cutting edge action stage begins when the thinning cuts out the surface material and ends with the main cutting edge completely drilling out of the surface material. The delamination formed by the thinning is not final and is generated during the drilling process. After the cross edge is drilled out of the workpiece surface, the main cutting edge continues to push outward and has a twisting effect due to its own rotation, ultimately leading to the formation of the delamination.

The effect of the drill bit on splintering formation

Fz- the axial (z) force of the bit against the splintering Fm- the twisting force of the main cutting edge against the splintering

Main factors affecting outlet delamination:

(1) Drill (spindle) speed n and feed speed Vf

Whether it is multi-directional CFRP or unidirectional CFRP, the higher the bit speed n, the smaller the delamination. The higher the feed speed, v, the greater the delamination. For multidirectional CFRP, with Vf=66.0 ~ 120.8 mm/min higher feed speed, delamination decreases significantly with the increase of bit speed, especially at the low-speed stage. When Vf=24.0 ~ 44.0 mm/min is low, the delamination decreases gently with the increase of bit speed n, and the delamination value does not exceed 1mm at any bit speed. This phenomenon of unidirectional CFRP also generally exists, but the feed speed at the cut-off point is lower. It can be seen that reducing the delamination size can be taken to reduce the feed speed and improve the bit speed two measures, but reducing the feed speed will reduce the processing efficiency, so the use of high-speed drilling should be the first consideration.

Under the same cutting conditions, the delamination value of unidirectional CFRP is larger than that of multi-directional CFRP, that is to say, unidirectional CFRP is more prone to delamination.

(2) Cutting speed Vc and feed rate f

Whether it is multi-directional CFRP or unidirectional CFRP, the greater the feed f, the greater the delamination. This phenomenon is particularly prominent at lower feed rates. The effect of different cutting speeds on delamination size is very small, almost to the point of being negligible. This phenomenon also exists in terms of drilling force. It has been pointed out earlier that the formation of hole outlet delamination is divided into two acting stages, in these two acting stages are the drilling force (axial force and torque) in action, that is, the drilling force is the root cause of the formation of delamination, so the delamination and drilling force is almost not affected by the cutting speed is natural. Without considering the influence of cutting speed, if the test data points in the linear regression diagram are used, the following empirical formula is obtained:

Multidirectional CFRP: l=0.26+0.11f

Unidirectional CFRP: l=0.34+0.11f

Formula,

l – Delamination value (mm)

f – Feed (um/r)

(3) The ratio of cutting speed to feed speed Vc/Vr

Whether it is multi-directional CFRP or unidirectional CFRP, the experimental data points of the relationship between delamination value and Vc/Vr are very regular at various feed speeds. When Ve/vf~<3 000 “-4 000, the delamination value of Ve/vf increases and decreases rapidly. When Vc/Vr>3000 ~ 4000, Vc/Vr increases the delamination value, and the decrease is very small, it can be said that at this stage, the impact of Vc/Vr is very small. Interestingly, this situation is similar to the Vc/Vr impact on drilling force, and the Vc/Vr “threshold” that forms the dividing point is also similar. This “threshold value” also has guiding significance for the selection of drilling parameters in actual production. As long as the ratio of drilling speed and feed speed is selected in the range higher than the threshold value, the exit delamination of the drilled hole will be small.

(4) Drill diameter

The larger the diameter of the drill bit, the more serious the delamination of the hole outlet formed after drilling. The situation is the same whether the spindle speed is high or low. Because the larger the diameter of the drill bit, the higher the drilling force during drilling. The higher the drilling force, the greater the delamination value.

(5) Drilling force

The effect of drilling axial force on the exit delamination of multi-directional CFRP drilling is that the larger the axial force, the more serious the exit delamination. However, the relationship between the two is not linear, in the process of increasing the axial force from small to large, the delamination value starts to increase slowly, and then gradually accelerates, and the increase speed becomes faster and faster with the increase. The trend is much like a conic curve. If you use it twice more

By fitting these test data, the following empirical formula can be obtained:

L = 0.76 0.04 Fz + 0.001 Fz

Where Fz= axial force

 

conclusion

(1) The delamination formation process of CFRP drilling outlet consists of two acting stages, namely the transverse edge acting stage and the main cutting edge acting stage, in which the transverse edge acting is the dominant component in the delamination formation.

(2) Hole outlet defects in addition to delaminationing, in the surface fiber is “forward” cutting part, there are often fuzz defects. In the surface fiber is “reverse” cut part, generally cleaner.

(3) The higher the drill (spindle) speed n, the smaller the delamination; The greater the feed speed, the greater the delamination.

(4) Cutting speed Vc has almost no effect on delamination size; The effect of feed rate f on delamination is basically linear, and the greater the feed rate, the greater the delamination.

(5) When Vc/Vf≤3000~4000, the Vc/Vf increase delamination value l decreases very fast; When Vc/Vf>3000~4000, the delamination value l decreases slightly with the increase of Vc/Vf. The relationship between Vc/Vf and delamination value l is basically linear in log-log coordinates.

(6) Under the premise of the same cutting conditions, the larger the drill diameter, the more serious the delamination.

(7) The greater the drilling axial force, the greater the delamination, and the two are basically rising conic relationship.

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