Chip formation and cutting mechanisms in composite machining

The chip formation mechanism in composite machining differs from that in conventional metal machining due to their anisotropic properties, low strength and low susceptibility to plastic deformation [29, 39, 40]. The fibre and the matrix in composites contribute to the cutting mechanism final chip formation. The reinforcement fibres in composites are strong, brittle and have poor thermal conductivity like in aramid and glass fibres while the matrix is less stiff and weak with a low tolerance for high temperatures [29]. An earlier study by Koplev et al. [41] revealed that chip formation when machining CFRPs is produced from a series of intermittent fractures without significant plastic deformations usually found in metal chip formations [40]. The brittle nature of fibre/epoxy matrix in composites produces discontinues chips made up of fragments of fibres and matrix materials [40]. Early work by Wang et al. [42] on machining unidirectional composites indicated that chip formation is highly dependent on the fibre orientation. The type of chip formation in composites primarily depend on the cutting parameters, the physical and mechanical properties of the fibre and the matrix and fibre content and orientation [29, 40]. Other important studies by Arola et al. [43] and Hocheng et al. [31] on machining unidirectional composites indicated that several cutting mechanisms can occur and that the formed chip is highly dependent on fibre orientation. Figure 2.8 shows a schematic diagram of the major types of cutting mechanisms in orthogonal machining of CFRP-based on different angles of fibre orientation relative to the cutting direction.

Figure 2.8: Chip formation modes when machining composites using a sharp cutting edge [29, 42]

The formation of chips during machining of unidirectional composites can be classified into the following types based on the fibre orientation and the rake angle of the cutting tool edge [29]:

• Delamination type chip formation (Type I). • Fibre buckling type of chip (Type II).

• Fibre cutting continuous type chips (Type III)

• Fibre cutting type with discontinuous chips (Type IV). • Shearing with a discontinuous chip (Type V).

Machining at θ = 0◦ fibre orientation with positive rake angle induces Mode I propagation along the fibre/matrix interface. Mode I (peeling) loading and bending induced fracture

occurs as the cutting tool edge advances in the workpiece material perpendicular to the fibre direction which applies pressure in the axial direction of the fibres [44]. A crack initiates at the tool point propagating along the fibre matrix interface [29, 45, 46]. The composite layers peels and slides along the rake face of the cutting tool causing them to bend like cantilever beam [29]. The delamination occurs due to Mode I loading (opening) - which is more dominant when cutting with a positive tool rake angle- or due to mode II (in-plane shearing) [46]. The progression of the cutting tool and the continuous peeling and bending cycles acting upon the fibre/matrix produces small segmented (discontinuous) chips which flatten upon separation and attains its original shape due to the absence of plastic deformation [29]. There is a significant fluctuation in cutting forces in this chip type due to the repeated cycles of delamination, bending and fracture [29]. Fibre ruptures with matrix crack propagation ahead of the tool tip is also common in this type when using positive tool rake angle causing the chip to separate due to brittle fracture [47].

Fibre buckling type of chip (Type II) occurs when zero or negative rake angles are used to machine 0◦ fibre orientations. The progression of the cutting tool edge in the workpiece material exerts a compressive load on the fibres along their direction and the negative rake angle of the cutting tool prevents them from separating from the machined surface causing them to buckle, and initiate a crack at the fibre matrix/interface [29, 45, 46] as shown in Figure 2.18.b. The in-plane shearing or Mode II loading is the primary mode of delamination which dominates the chip formation producing small discontinuous chips. While cutting force fluctuations are present in this mode, they are much smaller compared to those produced in delamination type I chip formation mode. Additionally, the machined surface in buckling and delamination types are similar, however, the formed chips in the buckling type II tend to be shorter than those produced from delamination type I [29, 46]. The third form of chip occurs with any cutting tool rake angle when the machined fibres are greater than 0◦ and less than 90◦. Figure 2.18 c and d show the cutting mechanism at 45◦ with positive and negative rake angles. The formed chips may be continuous (Type III) or discontinuous (Type IV) depending on the amount of interlaminar shear when chip slides up the cutting tool rake face [29]. The fibre cutting type chip formation mode is dominated by two distinct fracture modes: First, the fracture occur due to compression-induced shear across the fibres axes which generates cracks in the fibres above (separate from the machined surface) and below (remain on the machined surface and can be seen under microscope) the cutting plane. Second, the interlaminar shear fracture follows next along the fibre/matrix interface as the cutting tool advances in the workpiece material. The chip flows parallel to fibre orientation in a similar way chip passes across the shear plane in metal cutting but with the absence of plastic deformation. The size of the chip here is influenced by fibre

orientation. The interlaminar shear stress at the fibre/matrix interface increase with fibre orientation up to 90◦ which in return decreases the size of the formed discontinuous chip. The resulting machined surface is irregular and fibres protrude out of the surface in various lengths due to the elastic recovery of the fibres after the progression of the cutting tool away from the machined surface and the continuous stretching-induced fractures of the fibres, the protruded fibres are the cause of flank wear [29, 46].

It was previously reported that when the tool edge radius is small enough it can exert concentrated pressure crushing the individual fibres at the point of contact followed by shear failure in the fibre-matrix interface as described earlier [31, 44]. However, if the cutting tool edge radius is larger than the diameter of the fibre, fibre crushing does not take place and a subsurface interfacial failure occurs causing the fibres to separate from the machined surface due to bending failure [31, 44]. Type V chip formation mode occurs when machining at θ = 135◦ or 90◦ < θ < 180◦ [29, 46]. The formed chips are long and discontinuous due to the compressive stress ahead of the cutting tool point leading to cracks in the fibre and the matrix. In addition, delamination and shear fracture along the fibre/matrix interface is present due to severe bending and compression. The fibres are bent then cut along away from the rake face of the cutting tool while the uncut bent fibres beneath the cutting plane remain intact to the machined surface and tend to recover elastically brushing against the clearance face during their recovery [29]. Though the machining technique of metals and the composite material is similar, their machining behaviour is quite different [48]. The inhomogeneous nature of composite material influence its machinability and quality of the machined surface.