Matrix Sic whisker Flexural MMC material volume fraction, strength, toughness ou(MPa)E(MPa)Eu(%) SiaN4 4060 A2124T6(45%B) 45022000081 Al2O3 A6061-76(51%B) 000 5-65 A6061T6(45%Sc)14602000 45-55 Discontinuous-fibre mmc Table 3: Mechanical properties of Sic whisker reinforced A21246(20%sc)65012500240 CMC materials at room temperature A6061T6(20%SiC) 480 120000 5.00 3 MACHINING APPLICATIONS Particle MMC Machining of composite materials differs significantly in many aspects from machining of conventional metals and A2124T6(20%ScC) 55010500700 their alloys [3-5]. In the machining of composites, the A6061-T6(20%SC) 50010500550 material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement No reinforcement and matrix properties, and the volume fraction of matrix A2124F 450700000900 and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to A6061-F machining can be entirely different. Thus, machining of composite materials imposes special demands on the Table 2: Mechanical properties of MMC materials eometry and wear resistance of the cutting tools Accordingly, tool wear mechanisms and developme must be attentively considered to establish correct cutting ool selectio In the following, applications of machining processes to 0. 34 Gr/Mg composite materials are reviewed with reference to FR 04 materials and MMC materials 0.37Gr/A As regards the machining of CMC materials, the very Conventiona small number of contributions received and the scarcity of s Steel,At,Ti,Mg information available in the open literature on this topic did not allow for the preparation of a dedicated section d02 0.60 Gr/Epox 3. 1 Machining of fibre reinforced plastic composites Orthogonal machining of FRP FRP composites with different fibre orientations allowed or the clarification of the cutting mechanisms taking place in FRP(Figure 2). When machining is conducted at an angle of o to the fibre orientation the laminate is Figure 1: Specific strength vs specific stiffness for varie subjected to stresses parallel to the fibres. In addition, the MMC materials. Number in front of the composite is the surface below the cutting edge is compressed. The reinforcement volume fraction [2 material failure occurring in front of the cutting edge is due to delamination. matrix fracture or fibre-matrix interface 2.3 Ceramic matrix com posites(CMC) failure, which is recognizable from the crack in the CMC materials are being developed mainly to improve the composite laminate ahead of the cutting edge. Individual fracture toughness of unreinforced ceramics which already fractures occurring in the fibres and in the matrix below the possess higher specific modulus and mechanical cutting edge are also visible and remain in the machined properties at high temperature superior to those of metals surface. As the angle between cutting direction and fibre Continuous fibres, discontinuous fibres (whiskers)or orientation increases, fibres are compressed and bent in particles can be utilised as reinforcing constituents in the direction opposite to the fibre orientation, ending up in CMC fibre breakage as a result of bending and pressure load The common reinforcement materials used in CMc are This can result in fibre-matrix interface failure which alumina and silicon carbide, a volume fraction V:= 20% of SiC whiskers added to alumina can increase the fracture directions. which are the least favourable for FRP toughness from 25 to 50 MPa. Such an increase in composites particularly at angles between 30 and 60 to toughness of a ceramic cutting tool will enable it to take the fibre direction, is reflected in a poor surface quality In heavy cuts or to perform without fracture in interrupted a composite machined at 90 to the fibre direction,the g. Conventional hot isostatic pressing techniques fibr subjected ng e used to consolidate CMC composites contrast to laminates with 0 fibres. each fibre has to be Other CMc include carbon/carbon composites in which cut separately. The compressive strain normal to the fibres high strength carbon fibres are embedded in a graphite creates problems as interfacial fractures extend into the matrix. The low density of carbon in combination with the unmachined surface, More favourable conditions develop very high strength of carbon fibres offers potential for the for fibre orientation 135. Fibres are subjected to bending development of ultra high specific strength materials and tensile stress and break in bundles problems arise Table 3 reports the main mechanical properties of some however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrixTensile strength, u,, (MPa) MMC material IContinuous-fibre MMC I I Elastic Strain to modulus, failure, E (MPa) E,, (%) IAI2124-T6(45% B) I 1450 I220000 I 0.81 I Matrix material SiqNd IAl6061-T6(51%B) I 1410 I230000 I 0.74 I Sic whisker Flexural Fracture volume fraction, strength, toughness, Vf (%) uf (MPa) k (MPa) 0 400-650 30-45 IAl 6061-T6 (45% Sic) I 1460 I 200000 I 0.89 I Discontinuous-fibre MMC Al 2124-T6 (2O%SiC) Al 6061-T6 (20% Sic) 650 125000 2.40 480 120000 5.00 lparticle MMC I I No reinforcement Al 2124-F Al 6061-F IAl2124-T6(20%SiC) I 550 I 105000 I 7.00 I 450 700000 9.00 310 70000 12.00 IAl 6061-T6 (20% Sic) I 500 I 105000 I 5.50 I Table 2: Mechanical properties of MMC materials 0.6 h E z D 0.4 a 3j I- 3 5 m C u) 0 E 0 8 a v) 0.2 c 0 0.45 B/Al 0 0.34 Gr/Mg SlCfr 0. 0.37 Gr/AI Be 0 Conventional 0 Steel, Al, Ti, Mg 0 0.37 Gr/AI 1 NO.giCw/Al 0.60 IsI Gr/Epoxy 0.50 Gr/Epoxy 0 50 100 150 Specific stiffness (10 Nrn/Kg) Figure 1: Specific strength vs. specific stiffness for various MMC materials. Number in front of the composite is the reinforcement volume fraction [2]. 2.3 Ceramic matrix composites (CMC) CMC materials are being developed mainly to improve the fracture toughness of unreinforced ceramics which already possess higher specific modulus and mechanical properties at high temperature superior to those of metals. Continuous fibres, discontinuous fibres (whiskers) or particles can be utilised as reinforcing constituents in CMC. The common reinforcement materials used in CMC are alumina and silicon carbide. A volume fraction Vf = 20% of Sic whiskers added to alumina can increase the fracture toughness from 25 to 50 MPa. Such an increase in toughness of a ceramic cutting tool will enable it to take heavy cuts or to perform without fracture in interrupted cutting. Conventional hot isostatic pressing techniques can be used to consolidate CMC composites. Other CMC include carbonkarbon composites in which high strength carbon fibres are embedded in a graphite matrix. The low density of carbon in combination with the very high strength of carbon fibres offers potential for the development of ultra high specific strength materials. Table 3 reports the main mechanical properties of some CMC materials. 400-550 40-60 350-500 45-65 400-500 45 20 500-800 45-55 Table 3: Mechanical properties of Sic whisker reinforced CMC materials at room temperature. 3 MACHINING APPLICATIONS Machining of composite materials differs significantly in many aspects from machining of conventional metals and their alloys [3-51. In the machining of composites, the material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement and matrix properties, and the volume fraction of matrix and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to machining can be entirely different. Thus, machining of composite materials imposes special demands on the geometry and wear resistance of the cutting tools. Accordingly, tool wear mechanisms and development must be attentively considered to establish correct cutting tool selection. In the following, applications of machining processes to composite materials are reviewed with reference to FRP materials and MMC materials. As regards the machining of CMC materials, the very small number of contributions received and the scarcity of information available in the open literature on this topic did not allow for the preparation of a dedicated section. 3.1 Orthogonal machining of FRP Investigations carried out in [6] by orthogonal cutting of FRP composites with different fibre orientations allowed for the clarification of the cutting mechanisms taking place in FRP (Figure 2). When machining is conducted at an angle of 0" to the fibre orientation, the laminate is subjected to stresses parallel to the fibres. In addition, the surface below the cutting edge is compressed. The material failure occurring in front of the cutting edge is due to delamination, matrix fracture or fibre-matrix interface failure, which is recognizable from the crack in the composite laminate ahead of the cutting edge. Individual fractures occurring in the fibres and in the matrix below the cutting edge are also visible and remain in the machined surface. As the angle between cutting direction and fibre orientation increases, fibres are compressed and bent in the direction opposite to the fibre orientation, ending up in fibre breakage as a result of bending and pressure load. This can result in fibre-matrix interface failure which extends into the unmachined surface. These load directions, which are the least favourable for FRP composites particularly at angles between 30" and 60" to the fibre direction, is reflected in a poor surface quality. In a composite machined at 90" to the fibre direction, the fibres are subjected to bending and are sheared off. In contrast to laminates with 0" fibres, each fibre has to be cut separately. The compressive strain normal to the fibres creates problems as interfacial fractures extend into the unmachined surface. More favourable conditions develop for fibre orientation 135". Fibres are subjected to bending and tensile stress and break in bundles. Problems arise, however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrix. Machining of fibre reinforced plastic composites