Evaluation of Tools and Cutting Conditions on Carbon Fibre Reinforced Laminates

The distinctive characteristics of carbon fibre reinforced plastics, like low weight or high specific strength, had broadened their use to new fields. Due to the need of assembly to structures, machining operations like drilling are frequent. In result of composites inhomogeneity, this operation can lead to different damages that reduce mechanical strength of the parts in the connection area. From these damages, delamination is the most severe. A proper choice of tool and cutting parameters can reduce delamination substantially. In this work the results obtained with five different tool geometries are compared. Conclusions show that the choice of an adequate drill can reduce thrust forces, thus delamination damage. Introduction The use of composite laminates, like fibre reinforced plastics, in complex structures has increased significantly for the last decades. Reasons for this can be found in some unique properties like low weight, high strength and stiffness. Nevertheless, there are still some issues when considering the use of composite laminates. Some of these issues are cost-related, but considerations about machining also lead to some difficulties and lack of acceptance for the implementation of these materials. One of the main machining operations needed for part assembly in structures is drilling, when it is necessary to join different parts. Generally, drilling can be carried out using conventional machinery, with adaptations. However, this operation is likely to cause several damages in the laminates, namely in the region around the drilled hole, being delamination the most serious as it can reduce the load carrying capacity of the connection. As a consequence, engineers had to take into account a loss of both static and fatigue strength during project [1]. Other damages are likely to occur, like fibre torn-out or thermal degradation of the matrix material [2]. So, it is clear that any kind of damage is mostly unwanted and has to be reduced as much as possible. Figure 1 – Delamination mechanism. The main mechanism responsible for delamination is the indentation effect caused by the quasistationary drill chisel edge, acting over the uncut plies of the laminate. These plies tend to be pushed away from the plate, causing the separation of two adjacent plies of the laminate [3]. If the thrust force exerted by the drill exceeds the interlaminar fracture toughness of the plies, delaminations take place– Fig.1. This delamination is known as “push-down“ delamination. It can be found at the drill exit side of the laminate, and is very difficult to avoid. Several approaches had been presented in the last years [1, 4, 5, 6, 7, 8, 9]. Generally it is accepted that delamination can be reduced with an adequate combination of feed rate, drill material and geometry [4, 10, 11, 12, 13, 14]. Apart from that, another type can also be identified when drilling laminate composites, the “peel-up“ delamination. This delamination is a consequence of the drill entrance in the upper plies of the plate and can be avoided with the use of low feeds. Another possible option to consider to reduce delamination at exit, is the pilot hole strategy [15, 16]. The use of a pilot hole enables a thrust force reduction by dividing the operation in two, thus reducing the indentation effect of the final drill. In this work, laminates with 4 mm thickness were produced, considering a cross-ply stacking sequence with prepreg carbon/epoxy plies. Experimental drilling tests were carried out on these plates and thrust force was monitored. Five different drill geometries are used for comparison: two twist drills with different point angles (85o and 120o), a Brad drill, a Dagger drill and a customized step drill. When twist drills are used, a pilot hole strategy is applied. After drilling, hole wall roughness was measured and the delamination extent was determined using enhanced radiography. Conclusions from experimental work demonstrate the influence of drill geometry and drilling parameters feed rate or cutting speed in delamination occurrence. Experimental work The experimental work was divided in three steps: drilling of the laminate plates for thrust force monitoring, hole wall roughness measurement and delamination evaluation by enhanced radiography. In order to accomplish this work, a batch of plates using prepreg CC160 ET 443 with a cross-ply stacking sequence and 24 layers were produced. The plate was then cured under 300 kPa pressure and 130 oC for one hour, followed by cooling. Final plate thickness was 4 mm. Then, the plates were cut in test coupons of 165 * 96 mm for drilling experiments. Drilling operation was carried out in a 3,7 kW DENFORD Triac Centre CNC machine. A total of five different drill geometries, all of them in K20 tungsten carbide, were used: a twist drill with a point angle of 120o, a twist drill with a point angle of 85o, a Brad drill, a Dagger drill and a customized step drill – Fig. 2. All the holes had a diameter of 6 mm and associated with twist drill, a 1,5 mm pilot hole was made for thrust force reduction. Pilot hole diameter was based on previous works [6, 9, 12]. a) b) c) d) Figure 2 – Drills: a) twist (120o); b) Brad; c) Dagger; d) customized step. Twist drill is a standard drill commonly used. Two point angles – 85o and 120o are compared in this work. Brad drill has a specific point geometry causing the fibre tensioning prior to cut thus enabling a “clean cut” of the fibres. In consequence, machined surfaces are smoother. Dagger drill has a small point angle of 30o, reducing the indentation effect but need more space available at the exit side of the plate. Customized step drill has the intention of performing pilot and final hole in one operation only, dividing the thrust force and, consequently, delamination hazard. During drilling, axial thrust forces were monitored with a Kistler 9257B dynamometer associated to an amplifier and a computer for data collection. No sacrificial plates were used – Fig. 3. Figure 3 – Experimental setup. Cutting parameters were selected according to author’s previous experience, published papers from other authors and fabricant recommendations [6, 9, 12]. As it has been already demonstrated the major importance of feed rate when compared with spindle speed in thrust forces development [12], cutting speed was always equal to 53 m/min, corresponding to a spindle speed of 2800 rpm and feed rate had three levels – low, medium and high – table 1. Table 1 – Relevant cutting parameters Feed rate [mm/rev] Cutting speed [m/min] Spindle speed [rpm] Drill geometry 0,02 53 2800 Twist 85o, twist 120o, Brad, Dagger, customized step 0,04 0,06 After drilling, hole wall roughness was measured with a Hommelwerke profilometer, with a total travelled length of 1,5 mm, a cut-off length of 0,25 mm, an evaluation length of 1,25 mm, a sampling length of 0,25 mm and a travel speed of 0,15 mm/s. For this work the roughness parameter considered was Rmáx, corresponding to the maximum peak-to-valley dimension obtained from the five sampling lengths within the evaluation length lm. Three measurements were made for each hole. Finally, plates were inspected by enhanced radiography. With this purpose, plates were prior immersed in di-iodomethane for contrast for one and a half hour. Developed radiographies were scanned for delamination around the hole measurement. Details of the process can be found elsewhere [9]. Results and discussion Thrust forces. Results considered for thrust force are the maximum value observed during drilling. This result is regarded as a good indication of delamination occurrence as, according to published analytical models [17], higher thrust forces normally correspond to higher delamination probability. Due to signal variation along drill rotation, thrust force values were averaged over one spindle revolution. Results are the average of six experiments under identical conditions. Figure 4 – Thrust force results for feed rate and drill geometries. In Figure 4 the results of thrust force variation with feed rate for the five tools used can be observed. For all geometries, an increase of maximum thrust force with feed rate was detected. Based on these results, it can be said that a low feed rate allows for thrust force reduction and, therefore, delamination around the hole, if the drill geometry remains unaltered. However, a reduction in feed rate also has a consequence on temperature build-up increasing the risk of thermal damages. Measurements of drill tip after six holes in a row showed temperatures from 50 oC with higher feeds to almost 80 oC when lower feeds were used. This aspect should be taken into account. From Fig. 4, it can be said that for the lower feed thrust forces do not differ significantly. As feed rates are higher, differences from the five geometries used are observed. It seems as if lower point angles correspond to lower thrust forces. The odd result of twist 85o can be the outcome of the pilot hole that cancels the chisel edge effect. This result should be observed cautiously and deserves another experiment of twist drill drilling without pilot hole for final comparison. Hole surface roughness. There is still some uncertainty on the importance of roughness in mechanical behavior of drilled plates. The result can be influenced by the number and orientation of fibers that are within the stylus evaluation length. In order to reduce this uncertainty, every result is the average of three measurements in three diverse zones of the machined wall. Table 2 – Roughness values for the five drill geometries.