Machining fiber-reinforced composites.

Nontraditional machining operations such as laser machining, water jet and abrasive water jetcutting, electrical discharge machining, and ultrasonic-assisted machining have become necessary with the development of new metal-matrix and ceramic-matrix composites.

Machining of fiber-reinforced

composites (FRC) differs significantly from machining of conventional metals and their alloys. In the former, the materials' behavior depends on diverse fiber and matrix properties, fiber orientation, and relative volume of the matrix and the fibers. The tool continuously encounters alternate matrix and fiber materials, whose response to machining can vary greatly. For example, in an aluminum-boron composite, the tool encounters a soft aluminum matrix and hard boron fibers. Similarly, in a glass-epoxy composite, the tool encounters a low-temperature soft epoxy matrix and brittle glass fibers. In the case of aramid fiber-reinforced epoxy, the fibers have to be preloaded in tension and then cut with a shearing action. It is this variation in the requirements of a cutting tool that makes composites difficult to machine.

In view of high tool wear and high costs of tooling with conventional machining, noncontact material-removal processes offer an attractive alternative; they can also minimize dust and noise. In addition, extensive plastic deformation and consequent heat generation associated with conventional machining of fiber-reinforced composites, especially those with an epoxy matrix, can be minimized. These processes include laser machining, water jetcutting (with or without an abrasive), ultrasonic machining, and electrical discharge machining (EDM). Each of these processes has advantages and shortcomings. For example, EDM requires that the composite material be electrically conductive, and laser machining depends on the optical absorption and thermal properties of the composite.

Conventional Machining

Conventional machining of fiber-reinforced composites is difficult due to diverse fiber and matrix properties, fiber orientation, inhomogeneous nature of the material, and the presence of high-volume fraction (volume of fiber over total volume) of hard abrasive fibers in the matrix. Glass-, graphite-, and boron-reinforced composites (even polymer-based) are difficult to machine because of rapid tool wear. Since cemented carbide tools wear rapidly, diamond-impregnated tools may have to be used. Several advances have been made in the development of tool materials, including polycrystalline diamond tools, diamond-plated tools, and diamond-impregnated tools in various forms, such as core drills, milling cutters, drills, and grinding wheels. Although cemented carbide and diamond are the most commonly used materials, high-speed steel (HSS) tools are used in some cases, but at the expense of rapid tool wear.

Boron, for example, is hard and abrasive while titanium is chemically more reactive with most tool materials. This dissimilar combination makes the task of machining a boron-titanium composite challenging. A variety of machining operations are performed on this material including drilling, reaming, countersinking, routing, milling, and sawing using diamond-impregnated or diamond-plated tools. Recommended drilling conditions are a cutting speed between 200 and 600 feet per minute and a feed rate between 0.0005 and 0,005 inch per revolution. To reduce heat buildup in both the tool and the part, cutting fluids are recommended, and protecting the machinery from the abrasive boron dust to prevent wear of machine elements is also highly recommended.

Since aramid-reinforced plastic composite is an inherently tough material, cutting tools should be sharp and clean. Tools should be cleaned frequently to remove buildup of partially cured resins, which can cause loss in cutting action of the tool. The requirements of tools for machining aramid-reinforced plastics are different from those for machining glass or carbon fibers. In many respects, aramid-reinforced polymer resembles wood; its structure is characterized by the presence of highly oriented fibrous material embedded in a matrix. The best results are obtained when machining is processed in such a way that the fibers are preloaded in tension and then cut with a shearing action.

Special cutters were developed to address this problem. For example, Boeing Aircraft Co. (Seattle) developed a four-fluted spiral rotary carbide milling cutter (Figure 1a) with a unidirectional helix throughout much of its length and a reverse-directional helix adjacent to the cutting edge. Chip breakers are arranged along the lands with the notches at alternate lands aligned and the notches of the other lands intermediately aligned. The notches are cut at an angle of 20 degrees from a line perpendicular to the axis of the cutting tool. These cutters with angled chip breakers were designed to cleanly cut the aramid-fiber-reinforced plastic composites. The cutters operate at production speeds with minimal overheating. Figures 1(b) through 1(d) show some of the other designs of cutters specifically made for drilling aramid-fiber-reinforced resin composites.

Since aramid-reinforced polymers are not particularly hard, HSS or coated (TIN) HSS tools should provide reasonable tool life. Cemented carbide tools on aramid-reinforced polymers provide longer tool life and maintain sharper cutting action. These tools could easily handle single- (aramid-) reinforcing-fiber composites. When hybrid composites containing glass or graphite and aramid have to be machined, tool wear will be high. Tools with geometrically undefined cutting surfaces, such as diamond-impregnated or diamond-plated tools, are not recommended for machining aramid-reinforced polymer composites. To avoid fuzz and delamination, backup support is recommended on both entrance and exit sides. Du Pont has developed guidelines for machining Kevlar aramid composites and should be consulted for details [1].

In view of the high hardness and abrasiveness of glass fibers in glass-fiber-reinforced plastic (GFRP) composites, cemented carbide and preferably diamond tools (single-crystal and polycrystalline) are recommended for machining these materials. While HSS tools can be used to machine GFRPs, they wear rapidly and cutting speeds should be kept low enough to avoid overhearing the tool. Maintaining the sharpness of the tool is a problem for both HSS and cemented carbide tools due to the glass fibers' abrasive action on the cutting tool. The machining of GFRPs can be considered similar to drilling a hole in a resin-bonded grinding wheel except that the abrasive in this case is glass. A dull tool can dissipate considerable heat into the workpiece and damage the resin-based composites. Although alumina-based tools can be used because of their hardness, the possibility of chemical reaction between alumina and glass should not be overlooked. Polycrystalline diamond (PCD) tools are preferred, particularly in the case of GFRP components with a high glass fiber content (about 60 percent), which have to be machined to tight tolerances and with good surface finish. Of course, rigid machine tools are preferred when machining GFRPs with PCD in order to take advantage of PCD's superior cutting capability.

To prevent wear of machine elements in relative motion due to abrasive action of the glass fibers, protective covers must be incorporated in the machine tool. Sometimes it is necessary to dedicate a machine tool for machining this material if enough parts are to be fabricated on a continuing basis. When using PCD tools, it is preferable to clamp rather than braze the insert on the tool holder to avoid softening the braze material. The final choice of the tool material depends on the economics of the machining operation and the part requirements. Appropriate dust-collection and extraction systems should be in place when machining GFRPs. Operator safety should be a prime consideration; use of masks, gloves, and aprons or lab coats is required to minimize the safety risks associated with loose glass fibers and dust. Cutting speeds range from 100 to 150 surface feet per minute for carbides and 500 to 1500 surface feet per minute for PCDs.

Although carbon-fiber-reinforced plastic (CFRP) composites are generally fabricated to near-set-shape, additional machining operations such as drilling holes and trimming edges are needed. High tool wear and delamination of the composites are some of the concerns in machining. Koplev, Lystrup, and Vorn conducted orthogonal machining tests parallel and perpendicular to the fibers using a quick-stop device to freeze the cutting process and obtain the chip root [2].

When cutting parallel to the fibers, Koplev et al. found the surface to have visible fibers. They also found nearly all fibers were fractured perpendicular to their longitudinal direction. When the composite was machined perpendicular to the fibers, they did not find visible fibers on the surface. Instead, they found the whole surface to be coated with a thin layer of the matrix material. Below the surface layer, Koplev et al. found a layer of material with cracks. In addition, they observed a rather sharp notch with no cracks in front when machining perpendicular to the fiber direction. In contrast, a crack was found in front of the notch when machining parallel to the fibers.

Based on these observations, Koplev et al. pointed out that during machining of CFRP perpendicular to the fibers, two separate effects occur near the tool tip. As the tool moves forward, it presses on the composite in front of it causing the composite to fracture and create a chip. At the same time, a downward pressure on the composite below the tool produces fine cracks (about 0.01-inch deep) into the specimen. When the composite is machined parallel to the fibers, the tool applies pressure on the specimen, resulting in chips.

In view of the hard abrasive reinforcing fibers used in most polymer matrix composites, tool wear in conventional machining is a serious problem with most tool materials. Hasegawa, Hanasaki, and Satanaka conducted extensive studies on the characteristics of tool wear when machining GFRPs [3]. They found the tool wear to be predominantly abrasive in nature and proportional to the contact pressure between a glass fiber and the tool under a constant cutting length. They divided the tool wear with cutting speed into three regions. At very low speeds they found the tool wear to be negligible and independent of cutting speed and dependent only on the length of the cut, with the wear increasing linearly with the length of cut. At intermediate speeds the tool wear was found to increase with cutting speed. At higher speeds tool wear was found to increase rapidly and independently of speed. Hasegawa et al. developed a rheological model of tool wear to explain the observed wear phenomenon in the machining of GFRPs.

To overcome the rapid tool wear experienced in conventional machining of some composites containing hard, abrasive, or refractive constituents, alternative material-removal operations have been adopted. Laser machining, electrical discharge machining, water jetcutting and abrasive water jetcutting, and ultrasonic machining are basically noncontact machining operations involving no cutting tools and, consequently, no cutting forces.

Laser Machining

Laser machining is based on the interaction of the work material with an intense highly directional and coherent monochromatic beam of light. Material is removed predominantly by melting and/or vaporization. In the case of resin matrix material it is also removed by chemical degradation.

The physical processes involved in laser machining are basically thermal in origin. When a laser beam impinges on a work material, several effects arise including reflection, absorption, and conduction of the laser beam. The amount by which the beam is reflected depends on the wavelength of the laser radiation and the condition and properties of the work material such as roughness, degree of oxidation, and its temperature. The amount of laser energy absorbed by the surface of the composite material depends on the optical and thermo-chemical properties of the material.

For efficient lasing action, the percentage of absorption should be as high as possible (or the percentage of reflection as low as possible). Most metals absorb more readily at shorter wavelengths, hence less power is required to machine these materials at these wavelengths. Therefore, Nd:YAG, with a wavelength of 1.06 micrometers, would be more suitable for machining metal-matrix composites than would [CO.sub.2]. In contrast, some of the organic resins and other compounds have a higher percentage of absorption at higher wavelengths close to that of a [CO.sub.2] laser (10.6 micrometers), so that [CO.sub.2] would be more appropriate for machining such materials (e.g., aramid-resin composites). As melting begins or the material begins to interact with its atmosphere, the percentage of absorption may change as the process continues. For example, in drilling, the percentage of absorption in part of the hole drilled could be different from its initial value at the surface.

The type of laser to be used for machining a given composite depends on the work material properties and the following characteristics of the beam: power density; wavelength of emission (type of laser); interaction time [continuous wave (CW) versus pulsed]; polarization of the beam; absorption coefficient at the given wavelength; melting and vaporization temperature; thermal conductivity; heat capacity; diffusivity; and heat of vaporization. The important requirements of lasers for machining include adequate power available at the work (CW or pulsed); controlled focal intensity profile; reproducibility of power, mode, polarization, and stability; reliability; and initial and running costs.

Several types of lasers are used for machining. Chief among them are gas ([CO.sub.2] and excimer) lasers and solid-state (NG:YAG and NG:glass) lasers. These lasers can be operated either in a CW mode or pulsed mode for machining. The important characteristics of laser beams are spatial profile, beam divergence, focusing, temporal behavior, toughness, and power (continuous and peak).

Benefits of laser machining include minimum material waste (kerr width), minimum setup time, no tools (and thus no tool wear or replacement), smooth edge cuts, and low total heat input. As a result, there is low overall distortion or damage of the part, parallel sided cuts are possible, sharp contoured surfaces can be generated independent of workpiece hardness or strength, and cuts can be made without a starter or sloping.

A possible limitation of laser machining is the heat-affected zone (HAZ), where high temperatures imparted to the workpiece at or near the last cut can cause metallurgical changes. This can reduce the fatigue properties of the work material and the quality of holes in deep hole drilling. Blind holes cannot be drilled to precise depths. Lasers are not efficient for use with highly reflective or thermal conducting materials, thick workpieces, or thick nonmetals, which tend to char with reradiation from decomposition products.

Generally, the longer the wavelength of the laser beam, the higher the reflectivity of the metal workpieces. Similarly, the higher the thermal conductivity (thermal diffusivity), the higher the reflectivity. The higher reflectivity of some materials, especially high-conductivity metals such as aluminum and copper, at higher laser wavelengths (for example, 10.6 micrometers for [CO.sub.2]) renders them unsuitable or uneconomical for machining. For wavelengths greater than 5 micrometers most metals reflect more than 90 percent of the incident radiation at low power densities. Consequently, low-wavelength lasers (e.g., NG:YAG with a wavelength of 1.06 micrometers) would be preferable for laser machining of high-conductivity metals, provided there is adequate power available for lasing. In contrast, nonmetals (e.g., plastics, glass, or ceramics) with low thermal conductivity are ideal candidates for [CO.sub.2] laser machining (reflectivity is inversely proportional to the thermal conductivity). The amount of reflectivity can, however, be substantially reduced by modifying the surface conditions on the work materials. For example, the reflectivity of copper at a wavelength of 694.3 nm (ruby laser) can be reduced from 95 percent to less than 20 percent by oxidizing the surface. Similarly, reflectivity can be significantly reduced once the material begins to melt.

Energy transfer from a laser to the work material may occur in two ways. At low values of specific power (i.e., below a threshold value), the laser energy is absorbed in a superficial zone of the work material and heat is transmitted into the material by conduction. Above the threshold power, high enough to melt and/or vaporize the material, a vapor column surrounded by molten material forms and energy is absorbed through the entire thickness of the workpiece. The temperature reached by the material produces changes in the mechanical and physical properties near the interaction of the work material and the laser. The natures of these changes and the magnitude of the HAZ depend on the temperature reached in the vapor column and the thermal exchange coefficient between vaporized and solid zones.

Tables 1 and 2 give some of the thermal properties of resin and various reinforcing fibers and unidirectional composites respectively. Note the poor thermal properties of polymer resin, which constitutes 50 to 60 percent of a fiber-reinforced plastic. The properties of aramid fibers are somewhat similar to those of resin with minor differences in magnitudes. In contrast, the properties of carbon and glass fibers are different from those of the resin matrix material. As a result, large differences exist between the thermal properties of resin matrix and glass or graphite fibers, while the difference is negligible with aramid. The energy needed for vaporization of glass or graphite is also very high compared to the matrix. The laser power requirements, therefore, will depend on the fibers used and their volume fraction and not the matrix. However, too high a laser power may vaporize or chemically degrade the polymer matrix.

The vapor column generation mechanism is strongly influenced by the nature of the constituent materials of the composite (fibers and matrix), which may exhibit very different properties. At high specific powers, the time to vaporize the constituents of the composite is very short, but due to their different thermal properties, the fibers and the matrix can exhibit very different values of vaporization times. Tagliaferri calculated the time (t) that will elapse before the vaporization condition is reached on the work material surface under a laser beam source as [4]:

[Mathematical Expression Omitted]

where [kappa] is the thermal conductivity, [T.sub.v] is the vaporization temperature, [F.sub.o] is the specific power, and [kappa] is the thermal diffusivity. Using this equation, it is possible to calculate the minimum values of times needed to vaporize the materials at a given specific power. Figure 2 shows these values for a typical matrix material and three types of fibers. It is possible to observe two limit conditions at constant specific power. The fibers and the matrix exhibit different vaporization times. However, they are closer for aramid fibers in a resin matrix than glass or graphite fibers in a resin matrix. It is evident that fiber-reinforced plastic with aramid fibers responds better with a laser and hence would be a candidate for this material. This is fortuitous since difficulties such as surface delamination and fuzziness are experienced in conventional machining of aramid fiber-reinforced resin-matrix composites.

Some of the defects observed in laser cutting include fiber pullout originating from the matrix surrounded by large zones with loss of matrix material, presence of craters and delamination, uneven kerr width and taper in the cut surface, the presence of charred material, and thermal cracks. The quality parameters include the kerr width at inlet [W.sub.i] and outlet [W.sub.o] of the laser beam; size of the HAZ [W.sub.d], characterized by the presence of fiber debonding from the matrix or matrix recession; and thermal degradation of the fibers and the matrix and the slope of the cut surface [Tan [alpha] from [W.sub.i] to [W.sub.o] of a given thickness (s)].

The quality of laser cutting depends only on the interaction time between the beam and the material and therefore on the translation speed of the beam. As the cutting speed increases, the kerr width ([W.sub.i] and [W.sub.o]) and the slope of the cut surface tend to decrease and reach a steady-state value. The limiting width ([W.sub.i)] is close to the laser beam spot diameter. In contrast, the slope of the cut surface decreases with speed, reaching a minimum value, and increases with further increases in speed. The minimum value depends on the thickness of the sample; the thinner the sample, the higher the minimum value. The HAZ, similar to the kerr width, decreases as speed increases. This may be explained in terms of the interaction time and thermal properties of the work material. The damage diminishes when the energy input is lower, resulting in a shorter interaction time.

Caprino and Tagliaferri developed a one-dimensional thermal model to correlate maximum cutting speed ([V.sub.max]), power (P), material thickness (t), and focal spot diameter (d). The maximum cutting speed is given by:

[V.sub.max]=P/Ktd

The performance of the lasers in machining can also be changed by introducing a gas jet. For example, the efficiency of metal machining can often be increased by oxygen-assisted cutting. The technique takes advantage of the additional energy released due to the exothermic chemical reaction of the work material with the oxygen. Depending on the type of work material, laser machining may be assisted by oxygen, inert gas ([N.sub.2] or Ar), or air. For example, oxygen assist (or air and not nitrogen) would be preferable for laser-assisted machining of titanium alloys from energy consideration. However, it may be objectionable from oxidation of titanium, in which case air or inert atmosphere may be preferred. In contrast, nitrogen assist is preferable for machining nickel-based superalloys.

Water Jetcutting and Abrasive Water Jetcutting

High-presSure water jetcutting in unison with fine abrasives is a possible process for machining inhomogeneous materials that are hard and abrasive, such as most polymer-matrix composite materials. Water cools the workpiece and hence minimizes the thermal deformation problems commonly experienced in conventional machining of composites. A narrow kerr, minimum amount of dust and toxic fumes, and practically no delamination effects are some of the salient features of this system. The rapid tool wear commonly experienced in conventional machining of composites is not an issue in water jetcutting or abrasive water jetcutting. Abrasive water jetcutting, however, has some limitations. These include high noise levels (80 to 100 dB) and consequently the need for a catcher at the exit, safety, low removal rates, the inability to machine blind holes or pockets, abrasive particles or high-pressure water jets that can damage the machine elements, and the size of the abrasive water jetcutting system and associated equipment. Removal rates, dimensional accuracy, and finish can also be limitations of abrasive water jetcutting depending on the material and its thickness. When machining thick materials the jet stream tends to angle away from the direction of cutting, resulting in tapered surface. This effect becomes more pronounced as the thickness and/or feed rate increases.

Boron-epoxy, boron-polyester, fiberglass-epoxy, graphite-epoxy, and aramid-epoxy composites are some of the possible materials for water jetcutting. Some metal-matrix and ceramic-matrix composites can also be machined by abrasive water jets, but at reduced cutting rates. Abrasive water jetcutting of composites depends more on the matrix material than on the reinforcement. Straight water jetcutting, with no abrasives, is recommended for materials with a low yield strength (about 10 to 15 ksi).

Electrical Discharge Machining of Composites

EDM can make complex shapes with high precision. It is a slow process, but automation can bring the cost of manufacturing down. The prerequisite for EDM is that the work material be electrically conductive. Organic matrix composites are, therefore, not possible materials for this method of machining. They can be made conductive by being impregnated with metallic fillers (Cu, Al, or Ag powder), but that can defeat the purpose of composites for high-strength and lightweight applications. Metal-matrix composites are ideal candidates for EDM, especially where complicated shapes and high accuracy are required. Only a few ceramic-matrix composites that are electrically conductive can be shaped by EDM. However, recent improvements in the mechanical properties of ceramic-matrix composites-especially the fracture toughness and strength of whisker-reinforced ceramics through better processing technology and starting materials--make them ideally suited for high-temperature and fatigue-resistant applications. For example, the fracture toughness of silicon carbide whisker-reinforced alumina is nearly double that of the material without the fibers. The same is true with silicon nitride-based composites, which are very hard but extremely difficult and costly to machine or grind. If, however, these materials can be made electrically conductive by adding conductive refractory materials such as TiC or TiN without compromising other properties, processing these components by EDM can become an economic possibility. The particle size and percentage of TiC or TiN to be added to the matrix can be adjusted to make it electrically conductive enough to carry out the EDM process without significantly compromising the ultimate properties and performance requirements of the material.

Silicon nitride-based composites can be made electrically conductive by adding TiN, and alumina-based composites by adding TiC. Martin et. at. [5] found that for silicon nitride- based composites with TiN added, EDM can be performed at a conductivity higher than 2 x [10.sup.2] [omega.sup.-1] per centimeter (and preferably 5 x [10.sup.3] [omega.sup.-1] per centimeter), since below this value no electrical arc is produced between the workpiece and the tool. In contrast, for wire EDM of SiC whisker-reinforced zirconia-toughened alumina with TiC added, the minimum conductivity value was found to be 1 [omega.sup.-1] per centimeter. With a 30 volume percent TiC addition to the alumina-based composite, the bend strength was reported to be 125 ksi, while up to 50 volume percent of TiN particles could be added to the [Si.sub.3][N.sub.4] matrix without reducing its fracture toughness. This is one example where difficult-to-machine materials such as ceramic composites can be tamed by making them electrically conductive and able to be processed by EDM.

Ultrasonic-assisted drilling involves the use of a rotary tool to which is superimposed an axial vibratory motion at high frequency. A special adapter is required to transmit the vibration from a piezoelectric transducer to the tool. Ultrasonic vibration can reduce friction, break chips, and reduce tool wear. It is a particularly useful technique when the matrix or reinforcing fibers are hard brittle materials. Use of a core drill permits cutting fluids to pass through its center. Ultrasonic machining, though slow, can result in high finish and accuracy of intricate parts. Hence, it is recommended for applications in which intricate shapes of high accuracy and finish are required.

Composites contain fibers that, when machined, can release finer fractions of the fibers into the atmosphere. Also, in the case of polymer-based composites, some of the chemicals released due to heat and thermal damage during machining can be harmful. It is well known that fibrous materials such as asbestos can cause cancer, and that other fibers such as glass are suspected agents. Simultaneous exposure to both inorganic fibers and organic compounds released during machining of polymer-based composites can bring about respiratory and other medical problems. Adequate ventilation and appropriate safety procedures to prevent exposure of personnel to these gases in the laser cutting facility is recommended.

In conventional and nonconventional machining of composites, more than one process can be considered; which process is chosen depends on the following factors: type of machining operation; part geometry and size; accuracy and finish requirements; number and diversity of parts, including materials used; availability of appropriate machine tools, cutting tools, and technology; current machining practice; manufacturing schedule; capital requirements and justification for new equipment; and overall costs.

Of course, in view of limitations on the availability of capital for procuring new machinery or technology, the general tendency is to adapt existing equipment, especially in small manufacturing shops. This may not be the case with large companies such as Boeing, Lockheed, and General Dynamics, where resources-usually augmented by the Defense Department--are normally available. Even a process chosen for machining composites may have to be abandoned later due to changes. For example, the lot size may be increased significantly, necessitating a review of alternative manufacturing processes or a new technology that may be economically attractive.

Although machining of composites differs significantly from machining of conventional metals and their alloys, the intuition is to use conventional machining technology by adapting the existing machine tools. High capital costs and the fact that hard abrasive reinforcing fibers need not be machined are two reasons for the use of conventional machining technology. Also, advances in tool materials and advanced tooling design have facilitated the use of conventional machining technology. Lasers are used extensively in the processing of composites. Applications for finishing with water jet and abrasive water jet technologies are being sought. EDM would be ideal for metal-matrix composites and some ceramic composites if they could be made conductive enough without compromising their properties. It appears that the need to improve performance, reduce manufacturing costs, and improve quality will move manufacturing of composites to more nonconventional methods. This is especially true in a society where today's tools are becoming tomorrow's work materials.

Acknowledgments

The author would like to thank Vinanzo Tagliaferri of Politecnico di Milano, Italy; Yoshio Hasegawa of Osaka Universitv, Japan: Shinsaku Hanasaki of Kumamolo Universilv. Japan; A.K. Ghosh of IIT, Kanpur, India; and Marshall Jones of the GE Corporate Research Center, Schenectady, N. E, for sharing their work and for permission to use it herein. Thanks also to CM. Vissa and Bi Zhang for their assistance in preparing the manuscript and to the Oklahoma Center for Advanced Science and Technology.

References

1. Kevlar Cutting and Machining Handbook, E.I. du Pont de Nemours & Co.

2. Koplev, A., Lystrup, A.. and Vorn, T., Oct. 1983, "The Cutting Process, Chip and Cutting Forces in Machining Composites," Composites. Vol. 14, No. 4, pp. 371-376.

3. Hasegawa, Y., Hanasaki, S.. and Satanaka, S., 1984, "Characteristics of Tool Wear in Cutting of GFRP," Proceedings of the Fifth International Conference on Production Engineering. Tokyo, Japan, pp. 185 190.

4. Tagliaferri, V.. 1990. "Laser Cutting of Reinforced Materials." Handbook of Ceramics and Composites, Vol. 1: Synthesis and Properties, N.P. Cheremisinoff, Ed. New York: Marcel Dekker Inc., pp. 451-467.

5. Martin, C., Cales, B., Vivier, P., and Mathieu, P., 1989, "Electrical Discharge Machinable Ceramic Composites," Materials Science and Engineering, pp. 351-356.

Developing Materials to Fill the Void

Composite materials are being developed to fill the voids left by conventional materials, whose limits are continually being challenged by stringent new requirements.

Composite materials form a material system composed of a mixture or combination of two or more macro-constituents that differ in form and chemical composition and are insoluble in each other [1]. The matrix or the reinforcing fibers can be inorganic (ceramic or glass), organic (polymers), or metallic (aluminum or titanium).

Although the term "composite materials" can be applied to any duplex alloy depending on the scale of reference, it is commonly used to describe a material whose components do not form together as an alloy during processing but have been manufactured separately prior to the combining process. However, some polymer-polymer composites are currently being developed with a duplex structure in one process.

Neither composite materials per se nor the concept of engineering composites is new [2]. Wood is a natural composite consisting of cellulose fibers in a matrix of lignin. Cellulose fibers are strong in tension but flexible; lignin cements the fibers and endows the material with stiffness. The introduction of straw in bricks by the Egyptians in the days of the pharaohs, or incorporation of plant fibers into pottery in the days of the Incas and Mayans to prevent premature cracking are examples of ancient engineered composites [3]. Many other composite materials, such as paper and concrete, have been in existence for some time.

The successful development of glass-reinforced plastics in the 1940s gave yet another approach for the development of new materials. The concept of incorporating high-strength fibers or whiskers in a tough or ductile matrix to form a very high-strength composite has opened up an exciting possibility [4]. By combining strong glass fibers in an epoxy matrix, a new composite material with the strength of glass fibers and ductility of an epoxy is obtained. The main outcomes of such a combination include savings in weight, improvement in strength, and a decrease in the cost of materials and fabrication.

In a fiber-reinforced composite, the fibers carry the bulk of the load and the matrix serves as a medium for the transfer of load to the fibers. The matrix can be a metal, polymer, or ceramic. The fibers can be metal, ceramic, glass, or polymers. Some of the advantages of composites include: high specific strength, stiffness, or modulus; good dimensional stability; unusual combination of properties not easily obtainable with alloys; high fracture toughness, oxidation and corrosion resistance; directional properties; and good resistance to heat, cold, and moisture.

Endowed with some of these features, composites are ideal possibilities for a range of applications involving extreme conditions not possible with conventional alloying. Such applications. include high-strength and lightweight uses (space launchers and vehicles), high operating temperatures (gas turbine engine parts), and resistance to severe (destructive) conditions over a limited period of time (rockets and protection devices for vehicles and missiles reentering earth's atmosphere). However, the use of composites is not limited to extreme conditions.

Common fiber materials used in composites are boron, glass, aramid, graphite, SiC, and [Al.sub.2][O.sub.3]. Table 1 gives some of the mechanical properties of these fibers.

The use of composites has grown significantly in the 50 years since their initial introduction. However, the change in use from high-performance defense, space, and aircraft applications, where cost is of secondary concern, to other civilian application, where cost plays a primary role, has been slow. Composites have not yet grown to a level where they can challenge steel or other structural materials. In fact, they have only made a small dent in the auto industry, where they can offer lightweight vehicles with impact-resistant bodies and lower fuel costs. The auto industry predicts it will use a significant volume of composites if the price becomes low enough.

Because consumption of composites is still low, their unit cost is very high. Since composite processing is a batch manufacturing operation, its costs are also quite high. Industry forecasts show that the unit price of material along with its associated fabrication costs will decrease drastically if the volume used increases. Government can play a catalyst role through industrial policies, tax incentives, and support of research, and by incorporating technology-transfer mechanisms.

References

1. Smith. W.F., 1990, Principles of Material Science and Engineering, 2d ed., New York: McGraw-Hill.

2. Kelly, A., 1967, "The Nature of Composite Materials," Materials, San Francisco: W.H. Freeman and Co.

3. Gordon, J.E. 1976, The New Science of Strong Materials or Why You Don't Fall Through the Floor, Princeton, N.J,: Princeton University Press.

4. Forsyth, P.J.E., Kelly, A., Kennedy, A.J., and Smith, G.C., 1966, "Fiber-Strengthened Materials," Composite Malerials, New York: Elsevier, pp. 69-106.