Comparative study of jetting machining technologies over laser machining technology for cutting composite materials

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1 Composite Structures 57 (2002) Comparative study of jetting machining technologies over laser machining technology for cutting composite materials D.K. Shanmugam *, F.L. Chen, E. Siores, M. Brandt Industrial Research Institute Swinburne, Swinburne University of Technology, Burwood Road, P.O. Box 218, Hawthorn, Melbourne 3122, Australia Abstract There has been a great interest for improving the machining of composite materials in the aerospace and other industries. This paper focuses on the comparative study of jetting techniques and laser machining technics. This paper concentrates on the machining of composite materials like epoxy pre-impregnated graphite woven fabric and fibre reinforced plastic materials that are used in aerospace industries. While considering machining these materials with the traditional machining there are many disadvantages projected. One of these advantages is that all the traditional machining processes involve the dissipation of heat into the workpiece. This serious shortcoming has been dealt by the jetting technologies, which, contrary to the traditional machining, operate under cold conditions. The two methods in the jetting technologies used for processing materials are water jet machining and abrasive water jet machining. The first of these, water jet machining, has been around for the past 20 years and has paved the way for abrasive water jet technology. Water jet machining and abrasive water jet machining have been used for processing composite materials because of the advantages offered by this technologies as compared to traditional techniques of processing. The high surface and structural integrity required of any technique used for processing composite materials has created an opportunity for abrasive water jet machining. Cutting of composites using laser is also an option, and experiments were also conducted to reveal the extent of using laser technique. Ó 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Manufacturers, designers and engineers recognise the ability of composite materials to produce high-quality, durable, cost-effective products. Composite materials are found in many of the products used in our dayto-day lives from the cars we drive, to the boats, RV s, skis and golf clubs we use on the weekends. Additionally, composites are used in many critical industrial, aerospace and military applications. About 65% of all composites produced use glass fibre and polyester or vinyl ester resin, and are manufactured using the open moulding method. The remaining 35% are produced with high volume manufacturing methods, such as carbon or aramid fibre. Composites are broadly known as reinforced plastics. Specifically, composites are a reinforcing fibre in a polymer matrix. Most commonly, the reinforcing fibre is fibreglass, although high strength * Corresponding author. Tel.: ; fax: address: dshanmugam@swin.edu.au (D.K. Shanmugam). fibres such as aramid and carbon are used in advanced applications. The polymer matrix is a thermoset resin, with polyester, vinyl ester, and epoxy resins most often the matrix of choice. Specialised resins, such as, phenolic, polyurethane and silicone are used for specific applications. Common household plastics, such as polyethylene, acrylic, and polystyrene are known as thermoplastics. These materials may be heated and formed and can be re-heated and returned to the liquid state. Composites typically use thermoset resins, which begin as liquid polymers and are converted to solids during the moulding process. This process, known as cross linking, is irreversible. Because of this, composite materials have increased heat and chemical resistance, higher physical properties and greater structural durability than thermoplastics. The benefits of composite materials have fuelled growth of new applications in markets such as transportation, construction, corrosion-resistance, marine, infrastructure, consumer products, electrical, aircraft and aerospace, appliances and business equipment. The benefits of using composite materials include: /02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S (02)00096-X

2 290 D.K. Shanmugam et al. / Composite Structures 57 (2002) High strength composite materials can be designed to meet the specific strength requirements of an application. A distinct advantage of composites, over other materials, is the ability to use many combinations of resins and reinforcements, and therefore custom tailor the mechanical and physical properties of a structure. Lightweight composites offer materials that can be designed for both lightweight and high strength. In fact, composites are used to produce the highest strength to weight ratio structures known to man. Corrosion-resistance composites provide long-term resistance to severe chemical and temperature environments. Composites are the material of choice for outdoor exposure, chemical handling applications and severe environment service. Durability Composite structures have an exceedingly long life span. Coupled with low maintenance requirements, the longevity of composites is a benefit in critical applications. In a half-century of composites development, well-designed composite structures have yet to wear-out. 2. Techniques used for cutting composites materials 2.1. Water jet In water jet machining, materials are removed by the impingement of a continuous stream of high-energy water beads. The machined chips are flushed away by the water. As in conventional machining tools, the water jet exerts machining force on the workpiece during the cutting process. This force is transmitted by the water beads causing the cut. The direction of the force is given predominantly by the attack angle of the water jet and is insignificantly affected by the tail flow beyond the cut Abrasive water jet Abrasive water jet cutting technology uses a jet of high pressure and velocity water and abrasive slurry to cut the target material by means of erosion. The impact of single solid particles is the basic event in the material removal by abrasive water jets (Fig. 1). In previous investigations, it has been found [1] that three cutting zones exist in the processing of ductile and brittle materials under abrasive water jets, that is the primary cutting zone at shallow angles of attack, the primary cutting zone at large angles of attack, and the jet upward deflection zone. The attack angle is defined as the angle between the initial jet direction and the particle cutting direction at the point of attack Laser cutting The three essential components of a laser-cutting machine are laser medium, excitation source and the optical resonator. The excitation source drives the atom, ions or molecules of the laser medium to a situation where there is an excess of those at high energy level over those at a low level. This inversion of the normal thermodynamic population distribution leads to laser action: an excited member of the medium undergoing a transition from high to low energy will emit a photon, which in turn stimulates further emission, perfectly in phase, and at the same wavelength, from the other excited members of the medium. The radiation is thus rapidly amplified; the role of the optical resonator is to direct and control the radiation by allowing an appropriate fraction to be bled off as a near-parallel beam while the remainder is circulated within the cavity to maintain laser action. The output is monochromatic, usually with high spatial and temporal coherence. CO 2 laser was used for cutting composites, the laser action results from electric discharge excitation of a low-pressure gas mixture containing carbon dioxide. The beam is invisible, having a wavelength k lying in a far infrared at k ¼ 10:6 lm. the cutting process is based on location of beam focus at the surface of the (moving) workpiece, and provision of a jet of gas coaxial with the laser beam cuts the composite. 3. Review of previous work Machining of composite materials often poses a tremendous challenge, particularly in machining fine pro- Fig. 1. Mechanisms of material removal by solid-particle erosion [1].

3 D.K. Shanmugam et al. / Composite Structures 57 (2002) files and contours and for hybrid laminates consisting of two or more vastly dissimilar materials. Experiments in the field of composite machining like drilling, grinding, turning and screw thread machining were carried out using conventional and jetting techniques [2 6]. Limited research has been carried out in the field of machining composites using jetting techniques. Wang and Wong [7] conducted studies for machining polymer matrix composites using abrasive water jet. Bear brand phenolic fabric matrix composites, which were non-metallic, laminated sheets made by impregnated layers of fibre reinforcement with resin matrix of mm and 16 mm thick was used. Four different pressures were used by them, and for each level of water pressure four levels of transverse speed (400, 1000, 1600 and 2000 mm/min) were tested at four levels of abrasive flow rate (0.1, 0.2, 0.3 and 0.4 kg/min) and a single level of jet impact angle of tests were conducted by them for straight cuts of 60 mm long with a standoff distance between the nozzle and the workpiece set at 4 mm. For all tests, the other parameters were kept constant using the system standard configuration, i.e., the orifice diameter was 0.33 mm, the mixing tube diameter was 1.27 mm, and the length of mixing tube was 88.9 mm. The abrasives used were almandite garnet sand with a mesh number of 80. Observation by them showed that jets with sufficient energy provided a through cut whereas jets with low pressure causes a non-through cut and at the point where there was a non-through cut a pocket was formed with an irregular shape. Delamination was also observed by them on some specimens which were not cut through by the jet and remarks by him says that there was no obvious reason established between the cutting parameters and delamination, the results again showed that delamination can be avoided if clear through cuts can be achieved by correctly selecting the cutting parameters. Hamatani and Ramulu [8] work concerned with the machining of high temperature composites by abrasive water jet. Two types of composite were chosen in that study, one was a silicon carbide/titaniumdiboride and another one was a metal matrix composite (MMC). It was observed by them that the top of the abrasive water jet cut was damaged and rounded, not knife-edge sharp, since the response of a material to erosion by solid-particle impact depends on the angle of impact governing the material removal mechanisms, namely cutting wear and deformation wear. At the upper section the material was removed due to impact at shallow angles and the deformation wear at the lower part due to impact at large angles. It was also noted that burrs were observed on the bottom surface of the abrasive water jet cuts on the MMC, which implied that plastic deformation might be dominant in the cutting of that type of ductile composite. The performance characteristics of abrasives water jet machining showed by them are widely dependent not only on the workpiece material, but also on the abrasive water jet system process parameters. For piercing of the ceramic particulate composite, results similar to those of the MMC were observed by them. The taper of the hole produced increased with increasing standoff distance. The one notable exception between the two materials was that while the metal matrix material exhibited a nearly linear increase in hole taper with standoff distance, the variation for the ceramic matrix was clearly non-linear. Based on the preliminary investigation of the machinability of two classes of high temperature composites, they concluded that silicon carbide/titaniumdiboride composite was easily machinable by abrasive water jet and could able to produce good surface finish. The degree of orthogonal accuracy in the cut surface seems to be better under slow cutting conditions. Abrasive water jet machining of the ceramic matrix composite also seemed possible for them and they could able to produce better holes with minimal damage. Caprino and Tagliaferri [9] did experiments to determine the maximum cutting speed for cutting fibre reinforced plastics using laser cutting. Glass fibre reinforced plastic (GFRP), carbon fibre reinforced plastics (CFRP) and aramide fibre reinforced plastics (AFRP) panels were hand laid and press moulded. Different thicknesses, ranging from 2 to 3.5 mm for GFRP, 1.5 to 3.5 mm for CFRP, 2.0 to 4.5 mm for AFRP, were examined by them. In all cases fibre volume content of approximately 50% was achieved. Two CO 2 laser systems were used for the cutting tests a valfivre L500 (0.5 kw power) for low ( kw) and BOC (2.0 kw max power) for high cutting power (0.5 2 kw) for both laser systems the beam energy distribution was Gaussial. During the tests the beam was focussed on the material surface, three focal spot diameters namely 0.25, 0.35 and 0.5 mm were used and the energy densities of W/ cm 2 were used. An inert gas jet, coaxial with the laser beam impinged orthogonally on the sample through a nozzle 2 mm in diameter. The gas flow rate was 80 l/min. It has been shown that the proposed model closely agrees with experimental results obtained by laser machining of polymer matrix composites reinforced with aramide glass and carbon fabric. According to them the model was expected to work well for high power density and feed rates, under these conditions low interaction times are necessary for obtaining through cuts, heat conduction losses was neglected and the cut process was considered quasi-adiabatic. A criterion relying on kerf morphology was applied, a close dependence of the cut quality on the cutting parameters was found by them showing the results in correspondence to maximum cutting speed. They concluded that high power laser system plus high speed feed rates would give best performances, this would permit high quality together with high productivity. However in this case they prohibited the cost of the laser system compared to other cutting systems.

4 292 D.K. Shanmugam et al. / Composite Structures 57 (2002) Tagliaferri et al. [10] conducted experiments in cutting fibre reinforced polyesters using laser cutting and the morphology of the cut surfaces were also examined under scanning electron microscopy. Composites laminates like Kevlar/polyester laminates, glass polyester laminates and graphite/polyester laminates of different thicknesses were cut by CO 2 laser. Micrographic investigations by them showed that the quality of laser cutting depended on the interaction time between the beam and the material. The quality of the cut, in terms of both the uniformity of the surface morphology and the extension of the heat-affected zone, is better for those composites which exhibit low differences between the thermal properties of the constituent materials. The best results according to them have been obtained for aramid fibre composites in which, because of the organic nature of the fibres, the matrix and the fibres have similar properties. Glass fibre/resin and graphite fibre/resin composites have shown poorer results due to the higher vaporization temperature of the fibres and also to the higher thermal conductivity in the case of graphite fibre/resin. The simple thermal model employed here has allowed the experimental results to be analysed and a better understanding of the mechanism of thermal degradation of the composites as a function of cutting parameters and thermal properties of the constituent materials. Finally they concluded that the use of laser cutting therefore seems to be possible for materials such as aramid fibre reinforced composites that normally show some difficulties in being machined by conventional tools. Arola and Ramulu [11] carried out experiments to find out the characteristics of the kerf angle for graphite/ epoxy composite, which was cut using abrasive water jet. Graphite epoxy laminates of 16 and 19 mm thick with stacking sequence and volume fraction of 0.65 were used for cutting tests. All machining was conducted with garnet abrasives. Mesh sizes of garnet #80, 100, and 150 corresponding to mean particle sizes of 180, 150, and 100 mm respectively. Based on their study they summarised the results as follows. Kerf geometry of an AWJ machined Gr/Ep is influenced by the presence of three cutting regions including an initial damage, a smooth cutting and rough cutting regions (Fig. 2). The size of each of these three regions is dependent on the jet energy, which is a function of the cutting parameters and the target material. Material removal micromechanism throughout the kerf depth occurred by brittle fracture in machining Gr/Ep laminates. Kerf width and taper of thin laminates (0 < t < 5 mm) were primarily influenced by standoff distance and both of these features of kerf geometry could be minimised with low standoff distances. Standoff distance should be adjusted between 1.0 and 2.5 mm depending on the selection of the remaining parametric levels. The degree of initial damage width and depth at jet entry might be minimised with a low standoff distance. Extension of the SCR might be accomplished with high jet pressure, larger grit sizes, and lower transverse speeds, which would increase the inherent energy of the jet and therefore inhibit the beginning of waviness patterns. Waviness patterns were formed on the kerf wall as a result of inadequate cutting energy. The degree of waviness might be reduced or even eliminated with combinations of high pressure, large grit sizes, and moderate traverse speed. Models for kerf width, the initial damage width and depth, size of the SCR, and waviness height were developed which predicted those phenomena in AWJ machining of graphite epoxy laminates. Ramulu and Arola [12] conducted a comparative study between water jet and abrasive water jet cutting of unidirectional graphite epoxy composites. Topography and morphology of the machined surfaces were evaluated with surface profilometry and scanning electron microscopy. Cutting experiments were performed with a power jet model water jet, which was driven by a model water jet pump. A unidirectional, bag moulded, 5 mm thick graphite epoxy laminate consisting of resin and IM-6 fibres was used for experimentation. They carried out several cutting experiments for differ- Fig. 2. Schematic diagram of kerf geometry [11].

5 ent orientation with respect to the fibre direction. Based on the experimental study the following conclusions were made by them. The principal material removal mechanism present in WJ machining of unidirectional graphite/epoxy composite was material failure associated with microbending induced fracture and out-of-plane shear. WJ machining exploits the weakness of the composite mechanical properties. AWJ machining was found to be a more feasible machining process for unidirectional graphite/epoxy due to its material removal mechanisms and superior quality surface generation. D.K. Shanmugam et al. / Composite Structures 57 (2002) Fig. 3. Plot of kerf angle against speed. 4. Results and discussion 4.1. Fibre reinforced plastic Kerf angle Composites used in aerospace applications were machined at IRIS, Swinburne University of technology. The intent was to investigate the effects of machining parameters on those composites through microscopic analysis. Several experiments were carried out using fibre reinforced plastic. Tests were conducted to determine the optimum speed at which the minimum kerf angle could be obtained at different intersection angles. Full factorial design was first calculated using MINI- TABversion 13. Three angles were selected (45, 90, 135 ) along with a straight line for measuring kerf angle. Three-level factorial was carried out with three speeds and three techniques. A total of 72 tests were done including one repeat for all the experiments. In all the cutting experiments the material is securely placed over self-sacrificing foam and those together were place on the worktable. Many preliminary trail runs were performed for water jet cutting with different pressure and different speed. Based on these preliminary results with pressure less than 50,000 psi it was observed that the material was not cut through. So the pressure was kept at 50,000 psi and speed was changed to obtain a through cut surface for the material. All experiments were carried out for a length of 20 mm to ensure constant cutting condition. Variations in kerf angles were obtained for various angles at different speeds. After conducting some trials three different speeds (1, 2, 3 mm/s) were taken for each angle. The smallest kerf angle was attained at 1 mm/s for straight-line cutting and the largest kerf angle was attained at 2.7 mm/s for 45 -angle cutting. Fig. 3 gives the plot of kerf angle against the speed for different angles. Trail runs were carried out for determining various speeds using abrasive water jet. With those trail runs it was determined that the minimum pressure could be kept to 35,000 psi and with the speeds of 20, 30, 40 mm/ s. The standoff distance used was 2 mm, the orifice diameter used was 1.33 mm, and the mass flow rate used was 0.32 kg/h. The minimum kerf angle obtained was 0.23 with straight-line cutting and at 20 mm/s, and the maximum kerf angle was 0.99 obtained at 45 -angle cutting at 40 mm/s. As it is shown for example at 30 mm/s the straightline cutting exhibited an angle of 0.3, for 135 -angle the kerf angle was 0.38, for 90 -angle the kerf angle was 0.57 and for 45 -angle the kerf angle was It could be seen that as the speed increases the kerf angle increases and also it was noted that even for the same speed as the angle of turning decreases the kerf angle increases. It was studied that due to the inertia the nozzle was unable to slow down at the turning and causing the jet to lack in energy resulting in very narrow cuts at the bottom. Fig. 4 gives the plot of kerf angle against the speed for different angles. Using laser, experiments were done in determining the possibility of cutting fibre reinforced cutting with variation in speed, power, standoff distance, and with gas pressure. Preliminary tests were conducted to determine the range of speed, and in selecting constant power and constant standoff distance. Increase in power increased the cutting depth but at the same time as the power was increased the heat got intensified and thermal Fig. 4. Plot of kerf angle against speed.

6 294 D.K. Shanmugam et al. / Composite Structures 57 (2002) effects affected the side surface and decrease in power resulted in non-cut through surface. For doing the tests the power was kept constant at 2000 W, the standoff distance at 0.5 mm and the gas pressure at 80 kpa. The range of the cutting speed was selected between 300 and 700 mm/min with one speed in between and with one repeat. With lower speeds less than 300 mm/min the laser does not cut through the material. It was observed from the cutting as opposed to the water jet and abrasive water jet the deviation of the kerf angle was on the other way. At the top surface the width was narrow and as the depth progressed the width got broader resulting in the larger width at the bottom. As the speed increases the kerf angle decreases as opposed to both of the cold jet techniques. The smallest kerf angle obtained was 0.16 with the straight-line cutting at a speed of mm/s and the largest angle got was 9.1 with 45 -angle with a speed of 5 mm/s. At lower speeds it gave more time for the material to conduct heat resulting in wider kerf angle at lower speeds. Fig. 5 gives the plot of kerf angle against the speed for different angles (Figs. 6 9). Fig. 7. Typical bottom surface of a 45 -angle graphite epoxy composite using abrasive water jet cutting at 5 magnification. Fig. 8. Typical top surface of a 135 -angle fibre reinforced plastic composite using laser cutting at 5 magnification. Fig. 5. Plot of kerf angle against speed. Fig. 6. Typical top surface of a 45 -angle graphite epoxy composite using abrasive water jet cutting at 5 magnification. Fig. 9. Typical bottom surface of a 135 -angle fibre reinforced plastic composite using laser cutting at 5 magnification.

7 D.K. Shanmugam et al. / Composite Structures 57 (2002) Surface roughness After measuring kerf angles, the samples were cut apart from the whole lot in order to measure their surface roughness (Ra). All straight-line cutting were cut to examine the behaviour of surface roughness. Surface roughness was measured with Form Talysurf Plus measuring machine, which can measure upto 0.1 lm. Since cutting of fibre reinforced plastic using laser burnt the surface on both the sides, the Ra value could not be measured and so only the other two samples was used for measuring. All measurements were made at a distance of 3 mm from the top surface in order that the measurements have to be uniform. The travelling distance of the stylus was kept at 7.2 mm. It was observed that even at very low speeds the plain water jet cutting showed a larger Ra value compared to that of abrasive water jet cutting. The lowest Ra value for water jet cutting was lm and 1 mm/s and the highest Ra value was lm at 3 mm/s. For abrasive water jet cutting the least Ra value was lm at a speed of 20 mm/s and the largest Ra value was 7.29 lm at 40 mm/s. Figs. 10 and 11, shows the plot of Ra in terms of speed for water jet and abrasive water jet cutting Graphite epoxy composite Kerf angle Tests were conducted in determining the use of water jet and abrasive water jet as a way to machine graphite epoxy composites. The results of the investigation show that CO 2 laser cutting was not a viable method for cutting these composites. This was because of the conductive nature of the composites, which increases heat Fig. 12. Plot of kerf angle against speed. transfer to the materials. Forty-eight runs were done including one repeat with three-level factorials. Using plain water jet for cutting the parameters were varied to obtain a cutting range. The pressure was kept at 50,000 psi, and all the tests were carried out for a length of 20 mm, as that was enough to measure the kerf angle and the standoff distance was kept at 2 mm. 4 mm thickness of the graphite epoxy was used and larger thickness resulted in non-through cut. The range of cutting speed was selected from 0.1 to 0.3 mm/s with one centre cutting speed. With the range of speeds the smallest kerf angle obtained was 3.14 at 0.1 mm/s for the straightline cutting. At a 45 -angle cutting the kerf angle was 5.99 at 0.3 mm/s (Fig. 12). Cutting speeds higher than those used for plain water jet were used in experiments conducted using abrasive water jet. These speeds range from 10 to 20 mm/s with one centre speed, more deviation was obtained with sharp angles than the water jet. Upon analysing the results it was found that the speed used was probably higher, those higher speeds were used to see whether it was possible to use such speeds without sacrificing a greater kerf angle but proved wrong. The lowest kerf angle obtained was 2.46 with the straight-line cutting and the largest of the kerf angle 9.09 was obtained with the sharp 45 -angle cutting (Fig. 13). Fig. 10. Ra values for water jet cutting. Fig. 11. Ra values for abrasive water jet. Fig. 13. Plot of kerf angle against speed.

8 296 D.K. Shanmugam et al. / Composite Structures 57 (2002) Fig. 14. Ra values for water jet cutting. materials, carbon composite and fibre reinforced plastic, using abrasive water jet, plain water jet and laser cutting, shows the potential possibility of using those methods. Though using all the methods seemed to be quite possible, Abrasive water jet cutting promises a better cutting compared to the other two. The microstructure of all materials have been examined which reveal both the extent and nature of damage associated with specific machining parameters. Delamination was sometimes observed as a result of inadequate heat dissipation in laser cutting. Using plain water jet also showed delamination, and abrasive water jet cutting of materials showed some delamination for graphite epoxy composites at very high speeds, but on a general it was very low. Though at some occasions better results were obtained from Water Jet cutting, the fact that it uses very low speed would definitely increase the operation cost, and on the other way decreasing the cutting speed for abrasive water jet would decrease the kerf angle and decrease the surface roughness and waviness without sacrificing much productivity. Analysis done for surface roughness for both of their best and worst case showed that abrasive water jet performed lot better than plain water jet. References Surface roughness All measurements were made at a distance of 2 mm from the top surface in order that the measurements have to be uniform. The travelling distance of the stylus was kept at 5.8 mm. Here too it was observed that even at very low speeds the plain water jet cutting showed a larger Ra value compared to that of abrasive water jet cutting. The lowest Ra value for water jet cutting was 11.2 lm and 0.1 mm/s and the highest Ra value was lm at 0.3 mm/s. For abrasive water jet cutting the least Ra value was lm at a speed of 10 mm/s and the largest Ra value was 4.99 lm at 20 mm/s. As observed an increase in speed resulted in an increase in Ra value and vice versa. Figs. 14 and 15, shows the plot of Ra in terms of speed for water jet and abrasive water jet cutting. 5. Conclusion Fig. 15. Ra values for abrasive water jet. The experimental study conducted to study the kerf characteristics and surface roughness of two different [1] Momber KR, Andreas W. In: Principles of abrasive water jet machining, p. 89. [2] Koplev A, Lystrup Aa, Vorm T. The cutting process, chips, and cutting forces in machining CFRP. Composites 1983: [3] Inoue H, Kawaguchi I. Study on the griding mechanism of glass fiber reinforced plastics. J Eng Mater Technol 1989: [4] Ho-Cheng H. A failure analysis of water jet drilling in composite laminates. Int J Mach Tools Manufact 1990: [5] Tagliaferri V, Caprino G, Diterlizzi A. Effect of drilling parameters on the finish and mechanical properties of GFRP composites. Int J Mach Tools Manufact 1990: [6] Sheridan MD, Taggart DG, Kim TJ. Screw thread machining of composite materials using abrasive water jet cutting. Manufact Sci Eng 1994: [7] Wang J, Wong WCK. Machinability study of polymer matrix composites using abrasive water jet cutting technology. J Mater Process Technol 1999:30 5. [8] Hamatani G, Ramulu M. Machinability of high temperature composites by abrasive water jet. J Eng Mater Technol 1990: [9] Caprino G, Tagliaferri V. Maximum cutting speed in laser-cutting of fiber reinforced plastics. Int J Mach Tools Manufact 1988: [10] Tagliaferri V, Di ilio A, Crivelli V. Laser cutting of fibrereinforced polyesters. Composites 1985: [11] Arola D, Ramulu M. A study of kerf characteristics in abrasive water jet machining of graphite/epoxy composite. J Eng Mater Technol 1996: [12] Ramulu M, Arola D. Water jet and abrasive water jet cutting of unidirectional graphite/epoxy composite. Composites 1993: