118 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES containing hardener.This has been partially cured(B-staged)such that the resin does not flow at room temperature,but at the same time it remains tacky(sticky to the touch).B-staged epoxy pre-pregs are normally staged(partially cured)to about 15%of full cure for hand lay-up,and up to 25%for automated lay-up. To protect this material and keep it from sticking to itself,a backing or release film is added to at least one side of the pre-preg before it is rolled up for storage or transport. 5.3.1 Pre-Preg Production A pre-preg can be made incorporating a variety of reinforcement fabrics and fiber types.Although it can be produced by the component fabricator,it is normally purchased from a materials-supply company.The following material forms are available as carbon/epoxy pre-pregs. Woven bi-directional cloth pre-preg is most commonly made from plain weave or satin weave fabrics,0.2-0.4 mm thick and up to 1200 mm wide.One common method of pre-impregnation is to infuse the cloth with matrix resin diluted with solvent to lower its viscosity.The pre-preg then passes through a heating tower to remove the solvent and stage the resin.The newer hot-melt method (See Fig.5.2)involves first continuously casting a B-staged resin film on a non-stick backing film of coated paper or polymer.A doctor blade is used to control the thickness of the resin film applied(the same method used to make adhesive film).The reinforcement is then sandwiched between two of these films as it passes through a pair of heated rollers.This process has an advantage over the solvent process in that it produces lower volatile emissions. Unidirectional pre-preg (warp sheet)is made by spreading and collimating many fiber tows(typically around 10 fibers in each tow)into a uniform sheet of Top Paper Reel Comb Impregnation Roll Pull Rolls Doctor Heat Cool Prepreg 88 Toke-up Reel Chill mpregnatio Filming Plate Plate Bottom Paper Roll Fig.5.2 Schematic illustration of hot-melt film pre-pregging process.Adapted from Ref.2
118 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES containing hardener. 2 This has been partially cured (B-staged) such that the resin does not flow at room temperature, but at the same time it remains tacky (sticky to the touch). B-staged epoxy pre-pregs are normally staged (partially cured) to about 15% of full cure for hand lay-up, and up to 25% for automated lay-up. To protect this material and keep it from sticking to itself, a backing or release film is added to at least one side of the pre-preg before it is rolled up for storage or transport. 5.3.1 Pre-Preg Production A pre-preg can be made incorporating a variety of reinforcement fabrics and fiber types. Although it can be produced by the component fabricator, it is normally purchased from a materials-supply company. The following material forms are available as carbon/epoxy pre-pregs. Woven hi-directional cloth pre-preg is most commonly made from plain weave or satin weave fabrics, 0.2-0.4 mm thick and up to 1200 mm wide. One common method of pre-impregnation is to infuse the cloth with matrix resin diluted with solvent to lower its viscosity. The pre-preg then passes through a heating tower to remove the solvent and stage the resin. The newer hot-melt method (See Fig. 5.2) involves first continuously casting a B-staged resin film on a non-stick backing film of coated paper or polymer. A doctor blade is used to control the thickness of the resin film applied (the same method used to make adhesive film). The reinforcement is then sandwiched between two of these films as it passes through a pair of heated rollers. This process has an advantage over the solvent process in that it produces lower volatile emissions. Unidirectional pre-preg (warp sheet) is made by spreading and collimating many fiber tows (typically around 104 fibers in each tow) into a uniform sheet of ~ Top Poper Reel Rber Rolls Doctor -~ Pr~reg Blade -~ Heat ~ Cool Take-up Reel ~f..~ ~- Filming Plate Plate Fig. 5.2 Schematic illustration of hot-melt film pre-pregging process. Adapted from Ref. 2
COMPONENT FORM AND MANUFACTURE 119 parallel fibers typically 0.125-0.25 mm thick and 300 or 600 mm wide.This is immediately pre-impregnated.Unidirectional pre-preg is the cheapest to make, and it provides laminates with the best mechanical properties.However,it may be difficult to lay into double-curved shapes.Other types of reinforcement architec- ture,such as multi-axial warp knit(also known as non-crimp,knitted,or stitched) fabrics can also be pre-impregnated,but the process becomes increasingly difficult as the fabric becomes thicker. The pre-preg with its non-stick backing films is then inspected for resin content,which is typically between 34%and 42%by weight for carbon pre- pregs,wound onto a roll,and sealed to prevent the absorption of water vapor. Some pre-pregs have up to 15%more resin than is required to form a laminate with the desired fiber/volume fraction.With these pre-pregs,the resin is required to bleed out of the laminate during curing.Low-bleed or non-bleed pre-pregs with a more viscous resin are now more popular. The standard pre-preg thickness for unidirectional materials is of the order of 0.125 mm.More recently,to cut costs,much larger tows are being used,resulting in much thicker pre-pregs;however,because it is more difficult to maintain fiber alignment in thick tows,there is some reduction in mechanical properties of the finished composite. 5.3.2 Pre-Preg Transport and Storage The major disadvantage of pre-preg(apart from the extra cost of creating it from the fiber and resin)is that once the hardener has been added,the resin begins to react.Therefore the material normally only has a limited"shelf'(storage)life and"shop"(usage)life before the resin has reacted sufficiently for the pre-preg to become stiff and intractable for lay-up,or for the quality of the resulting composite to suffer.Most pre-pregs need to be stored in a freezer,typically at around -20C,which halts or at least greatly slows down the curing reaction in the resin.Pre-pregs generally used in aerospace are cured at elevated temperatures,typically 120C or 180C for epoxy resins.Because the resin is designed to react at elevated temperature,the supplier can normally guarantee a shelf(freezer)life of 6 months to a year,and a shop life ("out"life at room temperature)of at least 2 weeks. If the distance from the supplier to the user is long,the pre-preg will need to be shipped in refrigerated shipping containers;or for smaller lots,in insulated packages containing dry ice (frozen carbon dioxide). 5.3.3 Cutting and Kitting When pre-preg is required for use,it is thawed to room temperature before being removed from its bag to avoid picking up condensation.The pre-preg is then moved into the cutting room,which like the lay-up room is maintained as a "clean room,"free of dust and with controlled temperature (around 20C)and
COMPONENT FORM AND MANUFACTURE 119 parallel fibers typically 0.125-0.25 mm thick and 300 or 600 mm wide. This is immediately pre-impregnated. Unidirectional pre-preg is the cheapest to make, and it provides laminates with the best mechanical properties. However, it may be difficult to lay into double-curved shapes. Other types of reinforcement architecture, such as multi-axial warp knit (also known as non-crimp, knitted, or stitched) fabrics can also be pre-impregnated, but the process becomes increasingly difficult as the fabric becomes thicker. The pre-preg with its non-stick backing films is then inspected for resin content, which is typically between 34% and 42% by weight for carbon prepregs, wound onto a roll, and sealed to prevent the absorption of water vapor. Some pre-pregs have up to 15% more resin than is required to form a laminate with the desired fiber/volume fraction. With these pre-pregs, the resin is required to bleed out of the laminate during curing. Low-bleed or non-bleed pre-pregs with a more viscous resin are now more popular. The standard pre-preg thickness for unidirectional materials is of the order of 0.125 mm. More recently, to cut costs, much larger tows are being used, resulting in much thicker pre-pregs; however, because it is more difficult to maintain fiber alignment in thick tows, there is some reduction in mechanical properties of the finished composite. 5.3.2 Pre-Preg Transport and Storage The major disadvantage of pre-preg (apart from the extra cost of creating it from the fiber and resin) is that once the hardener has been added, the resin begins to react. Therefore the material normally only has a limited "shelf' (storage) life and "shop" (usage) life before the resin has reacted sufficiently for the pre-preg to become stiff and intractable for lay-up, or for the quality of the resulting composite to suffer. Most pre-pregs need to be stored in a freezer, typically at around -20°C, which halts or at least greatly slows down the curing reaction in the resin. Pre-pregs generally used in aerospace are cured at elevated temperatures, typically 120°C or 180°C for epoxy resins. Because the resin is designed to react at elevated temperature, the supplier can normally guarantee a shelf (freezer) life of 6 months to a year, and a shop life ("out" life at room temperature) of at least 2 weeks. If the distance from the supplier to the user is long, the pre-preg will need to be shipped in refrigerated shipping containers; or for smaller lots, in insulated packages containing dry ice (frozen carbon dioxide). 5.3.3 Cutting and Kitting When pre-preg is required for use, it is thawed to room temperature before being removed from its bag to avoid picking up condensation. The pre-preg is then moved into the cutting room, which like the lay-up room is maintained as a "clean room," free of dust and with controlled temperature (around 20°C) and
120 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES humidity (e.g.,between 50-70%RH).The pre-preg is then unrolled onto the cutting table,with its backing paper still in place.Plies of the required size,shape, and fiber orientation are then cut from the roll;as an example,Figure 5.3 shows a ply stack for a wing rib.This can be done by hand-using a template,or with a die in a roller press;in all but the smallest operations,this is usually done by a numerically controlled flat-bed cutter similar to those used in the textile industry. Cutting is usually achieved using an oscillating blade,but sharp"draw knife" blades as well as lasers or water jets are also used.Some cutters can cut multiple layers of fabric.Some flat-bed cutters can also label the plies automatically.The various ply shapes are then labelled,if necessary,and assembled as part of a kit containing all the plies for a component,which may be delivered directly to the lay-up room or sealed and stored in a plastic bag in the freezer for later use. Abrasive water jet cutting uses a high-pressure water stream,perhaps up to 400 MPa,which is forced through a small sapphire orifice to produce a supersonic jet travelling at speeds up to 900 m s,carrying abrasive particles to form a powerful cutting jet.Most materials can be machined with the water jet's ability to revolve with the robotic end effector.The critical process parameters are speed;stand-off distance;impact angle;water-jet pressure;water flow rate; orifice diameter;abrasive particle shape,hardness and size;and nozzle mixing tube geometry and material.Generally,the impact angle can be optimized to produce the maximum removal rate.The work-piece material should be softer than the abrasive compound.Oscillation of the cutting head can also influence the quality of the cut. Laser cutting can be considered a thermal process as a portion of the beam energy is absorbed by the surface material,and this energy raises the temperature ● Stack..... Center Fig.5.3 Schematic diagram of a typical ply stack for a wing rib
120 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES humidity (e.g., between 50-70% RH). The pre-preg is then unrolled onto the cutting table, with its backing paper still in place. Plies of the required size, shape, and fiber orientation are then cut from the roll; as an example, Figure 5.3 shows a ply stack for a wing rib. This can be done by hand-using a template, or with a die in a roller press; in all but the smallest operations, this is usually done by a numerically controlled flat-bed cutter similar to those used in the textile industry. Cutting is usually achieved using an oscillating blade, but sharp "draw knife" blades as well as lasers or water jets are also used. Some cutters can cut multiple layers of fabric. Some flat-bed cutters can also label the plies automatically. The various ply shapes are then labelled, if necessary, and assembled as part of a kit containing all the plies for a component, which may be delivered directly to the lay-up room or sealed and stored in a plastic bag in the freezer for later use. Abrasive water jet cutting uses a high-pressure water stream, perhaps up to 400 MPa, which is forced through a small sapphire orifice to produce a supersonic jet travelling at speeds up to 900 m s-1, carrying abrasive particles to form a powerful cutting jet. Most materials can be machined with the water jet's ability to revolve with the robotic end effector. The critical process parameters are speed; stand-off distance; impact angle; water-jet pressure; water flow rate; orifice diameter; abrasive particle shape, hardness and size; and nozzle mixing tube geometry and material. Generally, the impact angle can be optimized to produce the maximum removal rate. The work-piece material should be softer than the abrasive compound. Oscillation of the cutting head can also influence the quality of the cut. Laser cutting can be considered a thermal process as a portion of the beam energy is absorbed by the surface material, and this energy raises the temperature oooo o°° Sta~l~" °" Center Fig. 5.3 Schematic diagram of a typical ply stack for a wing rib
COMPONENT FORM AND MANUFACTURE 121 of the material.A sufficient amount of such energy will cause local decomposition of the material.Some compromise is required when focussing the laser beam as minimum spot size(a result of using short focal length lenses)is achieved at the expense of depth of field.The creation of thermal energy during cutting can produce problems in the course of dealing with standard epoxy pre- preg systems producing local cure and toxic vapors. All methods of cutting for complex geometry flat shapes must be capable of operation with either a standard robot or gantry-type equipment. 5.3.4 Lay-Up Most aerospace components are still laid-up by skilled labor,although considerable efforts are being made to automate or mechanize the process,as described in the subsequent sections.Hand lay-up is very versatile because human hands make excellent grippers,eyes marvellous sensors,and the brain a powerful process control and quality control unit!Any residual dust or resin from previous use is cleaned off before a thin layer of release agent is applied to the surface,where necessary.The mold will then be moved into the lay-up clean room. The pre-preg plies are then applied to the mold in the correct position, orientation,and sequence according to a set of instructions sometimes called a ply book;these instructions may be viewed on a computer screen.The ply is located on the surface by reference to markings on the mold or with the aid of a rigid or flexible template.Many companies now have lay-up stations where an overhead projector rapidly scans a low-power laser beam to"draw"the outline of each ply on the mold surface.These machines can also project instructions for ply lay-up onto the mold. Typically,the lower backing paper is removed by the operator before lay-up, and the upper one after positioning and consolidating using rollers or other simple tools.For larger plies,two or more operators may be required to handle and position the tacky pre-preg.Where the mold surface is doubly-curved,the pre- preg needs to be further distorted,enabling it to fit the surface. Different types of material may be combined in the same lay-up as long as the materials are compatible.For instance,in sandwich structures,aluminium or Nomex honeycomb and adhesive films will normally be combined with carbon- epoxy pre-preg to form the structure.Different fibers such as glass and carbon may be combined to form hybrid lay-ups,and different reinforcement arrangements such as unidirectional tape and woven fabric may be combined. 5.3.5 Automated Forming of Pre-Preg Stacks To reduce lay-up times and consequently labor costs,automated or semi- automated methods have recently been introduced to aircraft component production lines
COMPONENT FORM AND MANUFACTURE 121 of the material. A sufficient amount of such energy will cause local decomposition of the material. Some compromise is required when focussing the laser beam as minimum spot size (a result of using short focal length lenses) is achieved at the expense of depth of field. The creation of thermal energy during cutting can produce problems in the course of dealing with standard epoxy prepreg systems producing local cure and toxic vapors. All methods of cutting for complex geometry flat shapes must be capable of operation with either a standard robot or gantry-type equipment. 5.3.4 Lay-Up Most aerospace components are still laid-up by skilled labor, although considerable efforts are being made to automate or mechanize the process, as described in the subsequent sections. Hand lay-up is very versatile because human hands make excellent grippers, eyes marvellous sensors, and the brain a powerful process control and quality control unit! Any residual dust or resin from previous use is cleaned off before a thin layer of release agent is applied to the surface, where necessary. The mold will then be moved into the lay-up clean room. The pre-preg plies are then applied to the mold in the correct position, orientation, and sequence according to a set of instructions sometimes called a ply book; these instructions may be viewed on a computer screen. The ply is located on the surface by reference to markings on the mold or with the aid of a rigid or flexible template. Many companies now have lay-up stations where an overhead projector rapidly scans a low-power laser beam to "draw" the outline of each ply on the mold surface. These machines can also project instructions for ply lay-up onto the mold. Typically, the lower backing paper is removed by the operator before lay-up, and the upper one after positioning and consolidating using rollers or other simple tools. For larger plies, two or more operators may be required to handle and position the tacky pre-preg. Where the mold surface is doubly-curved, the prepreg needs to be further distorted, enabling it to fit the surface. Different types of material may be combined in the same lay-up as long as the materials are compatible. For instance, in sandwich structures, aluminium or Nomex honeycomb and adhesive films will normally be combined with carbonepoxy pre-preg to form the structure. Different fibers such as glass and carbon may be combined to form hybrid lay-ups, and different reinforcement arrangements such as unidirectional tape and woven fabric may be combined. 5.3.5 Automated Forming of Pre-Preg Stacks To reduce lay-up times and consequently labor costs, automated or semiautomated methods have recently been introduced to aircraft component production lines
122 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Instead of shaping and consolidating(laying up)each ply separately by hand,a flat stack can be assembled by manual or mechanical means.This flat stack can then be formed into the required shape using various methods;pressing,stamping, or diaphragm-forming.One version of the diaphragm-forming process is illustrated in Figure 5.4.A flat pre-preg stack is laid up and placed over a male- forming die.A diaphragm is fitted and sealed to the forming box.A vacuum is then applied to the box cavity.Because they are not extensible in the fiber direction,the plies must deform by shear to conform to the shape of the tool.It may be necessary to heat the flat pre-preg stack to a temperature above room temperature to assist forming.An infrared heating source is often used for this purpose. This process is most attractive for deep draws,and consequently the shear deformation required can be considerable.There are three main modes of deformation:intraply shear (a trellising action in which the fiber tows pivot at the crossover points),slippage between plies,and ply out-of-plane bending.The main problem is to avoid wrinkling of the plies caused by the development of compressive residual stresses.Computer simulation to assist in predicting the optimum conditions for forming is a recent development discussed later in this chapter. 5.3.6 Automated Lay-Up Lay-up of large components such as wing skins requires automation because. owing to the time required for hand lay-up,materials may be close to their out-life when the task is nearing completion. Fig.5.4 Schematic diagram of the diaphragm-forming process;below,carbon fiber-epoxy rib made using this process
122 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Instead of shaping and consolidating (laying up) each ply separately by hand, a flat stack can be assembled by manual or mechanical means. This flat stack can then be formed into the required shape using various methods; pressing, stamping, or diaphragm-forming. One version of the diaphragm-forming process is illustrated in Figure 5.4. A flat pre-preg stack is laid up and placed over a maleforming die. A diaphragm is fitted and sealed to the forming box. A vacuum is then applied to the box cavity. Because they are not extensible in the fiber direction, the plies must deform by shear to conform to the shape of the tool. It may be necessary to heat the fiat pre-preg stack to a temperature above room temperature to assist forming. An infrared heating source is often used for this purpose. This process is most attractive for deep draws, and consequently the shear deformation required can be considerable. There are three main modes of deformation: intraply shear (a trellising action in which the fiber tows pivot at the crossover points), slippage between plies, and ply out-of-plane bending. The main problem is to avoid wrinkling of the plies caused by the development of compressive residual stresses. Computer simulation to assist in predicting the optimum conditions for forming is a recent development discussed later in this chapter. 5.3.6 Automated Lay-Up Lay-up of large components such as wing skins requires automation because, owing to the time required for hand lay-up, materials may be close to their out-life when the task is nearing completion. I Fig. 5.4 Schematic diagram of the diaphragm-forming process; below, carbon fiber-epoxy rib made using this process