Composite materials are made from reinforcing plastics with fibres. The strength of the fibres is much higher than the matrix, and indeed it is estimated that 70-90 % of the load applied to composites structures is carried by those fibres [108]. Indeed, when drilling com- posites the quality of drilled hole depend on more on fibre than the matrix [109]. A critical factor that affects the damage tolerance in composites is the fibre direction. Generally, the energy can be easily passed from one composite layer to the other if they both have same fibre orientation and as a result, the severity of damage is greater than what would be if the fibres in each layer had different orientations, since this will have a transfer of energy between layers and allow it to fail at a higher load. Therefore, from a crack resistance point of view, a [0/90/0/90] stacking sequence is preferred over [0/0/90/90] or [0/90/90/0] [110]. However, from a seperation resistance point of view, the interlaminar interface between lami- nates with different ply orientations (i.e. cross-ply configuration) are mechanically weak and therefore, local separation of the laminate from one another is a common form of damage in such configurations [111]. The most common types of reinforcing fibre and their properties are given in Table 2.11.
Table 2.11: Typical properties of common reinforcing fibres [112]
As mentioned earlier, the machinability of FRPs depends on the fibre orientation with re- spect to the direction of the cutting tool motion [29]. Figure 2.13 shows the fibre orientation definition in drilling unidirectional composites. In addition to fibre orientation, there are other factors which govern the fibres contribution to machining composites such as the me- chanical properties of the fibre, the interaction between the fibre and the resin and the percentage of fibres in the composite commonly known as fibre volume fraction. These four factors can influence the following parameters when machining composites:
• Cutting forces.
• Cutting temperatures.
• Cutting mechanism.
Figure 2.13: Fibre orientation definition in drilling unidirectional composites (a) chisel edge, (b) major cutting (edge Φ : angularposition, θ : f ibreorientation) [29]
Cyclic cutting forces are developed when machining composites due to fibre orientation throughout the thickness of the laminate. The material removal in drilling is performed by the two cutting edges (in the case of a two flute twist drill) and the penetration of the chisel edge. The two cutting edges have identical cutting behaviour and relative fibre orientation angle (see Figure 2.10) [29]. The chip thickness is independent of the angular position of the cutting edge. For example, when the cutting tool edge is parallel to the fibres direction, the cutting velocity vector is perpendicular to the fibres which are oriented at 90◦. The rotation of the cutting edge clockwise decreases the fibre orientation to zero at 90◦ angular position [29]. Previous studies on drilling GFRPs indicated that cutting forces were minimum when drilling at 0◦ or 90◦ degrees fibre orientations [113]. However, in another study on drilling unidirectional GFRP, Enetyew et al. [114]] reported that lowest thrust force occurs around the rotational angles of 135◦ and 315◦. C.Gao et al. [115]] reported that the cutting force was minimum at 45◦ fibre orientation and maximum at 90◦. Ghafarizadeh et al. [116] also reported that maximum and minimum cutting forces occur at 90◦ and 0◦ fibre orientations respectively when milling unidirectional CFRPs. Wang et al. [117] also reported that the surface roughness and cutting forces of machined epoxy reinforced unidirectional carbon fibre were reduced when using positive rake angles 0◦ and 20◦. Drilling at low feed rates and high cutting speeds reduce the cutting forces and delamination, but they could also result in undesirable thermal degradation of the matrix [118]. Previous studies on the effect of fibre orientation on cutting forces when machining epoxy composites reinforced with unidirectional carbon fibre by Wang et al. [117] are shown in Table 2.12. The surface
Table 2.12: Effect of fibre orientation angle on cutting forces [117, 119]
quality of the drilled hole in unidirectional composites will vary around the circumference due to fibre orientation. For example, the fibres tend to be pulled out when the tool cutting edge is parallel to the fibre orientation. The increase in cutting angle within 20◦-45◦ range causes bending and compression action in addition to fibre pulling and worst surface quality is achieved in this range [108]. Shear and bending is observed when the cutting tool edge is perpendicular to the fibre orientation [108]. Gao et al. [115] reported that the fibre orientation angle, depth of cut, and cutting speed are important factors affecting the cutting force and surface roughness. Wang et al. [117] reported that the surface roughness of machined epoxy reinforced unidirectional carbon fibre was minimum between 0◦ to 90◦ fibre orientation and gradually increased with fibre orientations up to 150◦, this was also confirmed by Ghafarizadeh et al. [116]. C.Gao et al. [115] also reported that the surface roughness increased with fibre orientation. For example, the surface roughness measured at 45◦fibre orientation was smoother than at 135◦ due to more surface irregularities in the later one. Ghafarizadeh et al. [116] also reported that the minimum surface roughness is achieved for 45◦and 90◦fibre orientations and maximum at 135◦fibre orientation. Palanikumar [120] also reported that the surface roughness increased with the increase of fibre orientation from 15◦ to 120◦, however, the effect of fibre orientation on surface roughness becomes insignificant when increasing the cutting speed beyond 175 m/min.
In another study, H.Gao et al. [121] also reported that drilling damage at the exit of the hole was more likely to occur at fibre orientations in the range of 0◦ to 90◦ with damage level increasing with the decrease of the fibre orientation, and less in the range of 90◦ to 180◦ with damage increasing with the increase of the fibre orientation when drilling unidirectional epoxy composites. Enetyew et al. [114] reported that when drilling GFRP, the surface roughness is higher at fibre orientations of 135◦and 315◦along the circumference of the hole, they also indicated that fibre pull-out is likely to occur in the region of 135◦to 175◦ and the region of 315◦ to 335◦. A number of fibres content in the composites plays a significant role on the machined surface quality, chip formation and tool wear, Budan et al. [122] reported that the surface roughness, delamination factor and tool wear increased with the increase in fibre content from 30 % to 60 % when drilling GFRPs. The increase in surface roughness
was due to the increase in the fibre pull-out In addition, the increase in fibre content was reflected by the lack of plasticity deformation and higher brittle fracture which produced small segmented chips while long chips were common with lower fibre content. Having high fibre content in the composite can have a serious impact on health when machined [122]. The surface quality of the machined hole is also influenced by the mechanical properties of the fibres. For example, aramid fibres are weak in compression and tend to bend ahead of the cutting edge which causes them to recede into the matrix during machining and later to be frayed on the surface of the hole [108]. While glass and carbon fibres break in a brittle matter ahead of the cutting edge [29]. The thermal properties of the fibre can also influence the machinability of the composite. For example, the thermal conductivity of carbon fibres is higher than in glass or aramid fibres making it more efficient in dissipating heat away from the cutting zone along its length [29]. Therefore, machining at high cutting speeds forms regions of localised heat zones in the cutting area which could affect the machined surface quality. In addition, the mismatch in thermal expansion properties in the fibre and the matrix could lead to dimensional inaccuracies and thermal stresses which could adversely affect the quality of the hole [29].
The fibre orientation plays a significant role in the developed machining temperatures, Zitoune et al. [123] reported that machining at 90◦ fibre orientation results in a maximum temperature due to the multiple bending and shearing cutting mechanisms which increase the cutting forces and friction, the rise in friction is due to chip flows up along the rake face due to shear along the fibre matrix interface [29] which was also confirmed by Ghafarizadeh et al. [116] when milling CFRPs . In addition, the failure stress when cutting at 90◦ can reach up to 1600 MPa compared to 1000 MPa when cutting at 0◦ fibre orientation. The modulus of elasticity in composites can influence the developed machining temperatures in the workpiece. Merino-Perez et al. [124] reported that the type of fibre reinforcement had an effect on the heat dissipation when drilling a hole in CFRP such that composites with higher modulus dissipate heat more than those with higher strength carbon fibres due to the high degree of crystallinity of the high modulus carbon fibres.