《纺织复合材料》课程参考文献（3-D textile reinforcements in composite materials）05 Manufacture and design of composite grids

5 Manufacture and design of composite grids S.W.TSAI,K.S.LIU AND P.M.MANNE 5.1 Introduction Composite materials technology has emerged as the darling of many indus- tries over the past 30 years.This class of materials is light,corrosion and fatigue resistant and can be manufactured in a variety of methods.Most successes can be found in sporting goods and satellites where graphite com- WV IS:OE posites are the dominant materials.Here performance is the primary goal. Other notable achievements include components of aircraft,and many industrial applications where corrosion is critical. Composite materials have the potential to increase their market size significantly.As artificial fibers have all but replaced natural ones,we see composites as the structural materials of the future because they have unlimited supply and require less energy to process than metallic materi- als.There are many inhibitors to the growth of composites.They come from technological,economical and government regulatory sources.Maturing of any technology takes time,particularly if the technology involves public safety;however,innovation and favorable government regulation can hasten this process. Composite grids form the theme for this chapter.Grids are fundamen- tally different from stiffened and sandwich constructions in that the load transfer mechanisms are different.Grids can be made by the widely avail- able filament winding and pultrusion.We believe that both high perfor- mance and low cost can be achieved. Current manufacturing processes of composite materials and structures are based on weaving,braiding,pultrusion and/or lamination.They require expensive facilities,and costly manufacturing equipment and processes.As a result,processing costs are many times the material cost.We intend to show that the cost of manufacturing composite grids can be reduced to the level of materials cost.Such composite structures can then compete against most traditional materials. Grids are like the skeleton of a human body or the frame of old airplanes made of wood and cloth cover.The grid is the primary load-carrying 151

5.1 Introduction Composite materials technology has emerged as the darling of many indus￾tries over the past 30 years. This class of materials is light, corrosion and fatigue resistant and can be manufactured in a variety of methods. Most successes can be found in sporting goods and satellites where graphite com￾posites are the dominant materials. Here performance is the primary goal. Other notable achievements include components of aircraft, and many industrial applications where corrosion is critical. Composite materials have the potential to increase their market size significantly. As artificial fibers have all but replaced natural ones, we see composites as the structural materials of the future because they have unlimited supply and require less energy to process than metallic materi￾als. There are many inhibitors to the growth of composites. They come from technological, economical and government regulatory sources. Maturing of any technology takes time, particularly if the technology involves public safety; however, innovation and favorable government regulation can hasten this process. Composite grids form the theme for this chapter. Grids are fundamen￾tally different from stiffened and sandwich constructions in that the load transfer mechanisms are different. Grids can be made by the widely avail￾able filament winding and pultrusion. We believe that both high perfor￾mance and low cost can be achieved. Current manufacturing processes of composite materials and structures are based on weaving, braiding, pultrusion and/or lamination. They require expensive facilities, and costly manufacturing equipment and processes. As a result, processing costs are many times the material cost. We intend to show that the cost of manufacturing composite grids can be reduced to the level of materials cost. Such composite structures can then compete against most traditional materials. Grids are like the skeleton of a human body or the frame of old airplanes made of wood and cloth cover. The grid is the primary load-carrying 5 Manufacture and design of composite grids S.W. TSAI, K.S. LIU AND P.M. MANNE 151 RIC5 7/10/99 8:04 PM Page 151 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

152 3-D textile reinforcements in composite materials member.Skins or covers are there for another function.Optimally grids are formed by a network of ribs made of unidirectional composites.These ribs are many times stronger and lighter than metallic materials.The key is to exploit the unidirectional properties.While concrete and metallic grids have been made,their performance is limited because the ribs are isotropic.Only when ribs are unidirectional can the true potential of grids be realized.We will show how to capitalize on this principle and combine it with low-cost manufacturing. Grid structures are not new;they have been used in civil engineering for many years.The aeronautical industry used metallic grids as early as in World War II,for example in the British Vickers Wellington bomber.The grid was metallic and offered exceptional battle damage tolerance.This extra assurance made it the favorite among the flight crew.Nowadays,jet engine covers and some hulls of the International Space Station feature integral grids machined from aluminum plates.Based on our understand- ing,these applications do not constitute a very effective use of grids.On the other hand,Airbus A330 and A340 have composite grid reinforced skins in their horizontal and vertical tails.Presumably they are cost effective.They 1:09 are,however,hand-made.Our interest lies in developing new automatable 多2 2 manufacturing processes.It is hoped that with these processes,the out- standing performance of composite grids can be achieved at an affordable cost. 5.2 Grid description 豆 We wish to describe the geometric and material characteristics of grids and show why composite grids are unique. 8 5.2.1 Rib orientation Since grids have directionally dependent properties,we chose to adopt terms analogous to those commonly used for laminated composite materi- als.In Fig.5.1,grids are described based on the orientations of their ribs: square,angle and n/3 isogrids,respectively.In this figure all ribs are assumed to be in the same plane and to have the same height.But that restriction is not always followed:for example,ribs may run in different planes,like plies in a laminate.All grids shown here have identical rib intersections or joints.In particular the n/3 grid is isotropic and is often called an isogrid. 5.2.2 Rib construction There are at least two ways of making grids.The wrong way is to start with a slab of material and produce a grid by machining.As illustrated on the

member. Skins or covers are there for another function. Optimally grids are formed by a network of ribs made of unidirectional composites. These ribs are many times stronger and lighter than metallic materials. The key is to exploit the unidirectional properties.While concrete and metallic grids have been made, their performance is limited because the ribs are isotropic. Only when ribs are unidirectional can the true potential of grids be realized. We will show how to capitalize on this principle and combine it with low-cost manufacturing. Grid structures are not new; they have been used in civil engineering for many years. The aeronautical industry used metallic grids as early as in World War II, for example in the British Vickers Wellington bomber. The grid was metallic and offered exceptional battle damage tolerance. This extra assurance made it the favorite among the flight crew. Nowadays, jet engine covers and some hulls of the International Space Station feature integral grids machined from aluminum plates. Based on our understand￾ing, these applications do not constitute a very effective use of grids. On the other hand, Airbus A330 and A340 have composite grid reinforced skins in their horizontal and vertical tails. Presumably they are cost effective. They are, however, hand-made. Our interest lies in developing new automatable manufacturing processes. It is hoped that with these processes, the out￾standing performance of composite grids can be achieved at an affordable cost. 5.2 Grid description We wish to describe the geometric and material characteristics of grids and show why composite grids are unique. 5.2.1 Rib orientation Since grids have directionally dependent properties, we chose to adopt terms analogous to those commonly used for laminated composite materi￾als. In Fig. 5.1, grids are described based on the orientations of their ribs: square, angle and p/3 isogrids, respectively. In this figure all ribs are assumed to be in the same plane and to have the same height. But that restriction is not always followed: for example, ribs may run in different planes, like plies in a laminate. All grids shown here have identical rib intersections or joints. In particular the p/3 grid is isotropic and is often called an isogrid. 5.2.2 Rib construction There are at least two ways of making grids. The wrong way is to start with a slab of material and produce a grid by machining. As illustrated on the 152 3-D textile reinforcements in composite materials RIC5 7/10/99 8:04 PM Page 152 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

Manufacture and design of composite grids 153 8 0=0 0=±45 S 1 =60 SQUARE GRID ANGLE GRID TT/3 ISOGRID NW 0=0 n +Width b Length L Length L 5.1 Designation of grids by rib orientation,analogous to laminated composites. [1/31 LAM WV IS:OE:ZI II0Z “1S0” UNI" ISOGRID ISOGRID 5.2 Two ways of making grids.Left:the wrong way,by machining a quasi-isotropic laminate.Right:the correct way,by forming unidirectional ribs. let in F.qasisotropic laminate is taken as starting material a machined into an isogrid.We call the resulting grid an'iso'isogrid,indicat- ing that the starting material is isotropic.This class of grids is very costly and a very poor utilization of the material.The rib has the same stiffness as the starting material. The right way is to use directional materials such as composites. Instead of machining,unidirectional fibers are rearranged or regrouped to form unidirectional ribs as shown on the right of Fig.5.2.We call this class of grids'uni'isogrids.Here the superior stiffness of unidirectional com- posites is fully utilized.We will show later that the 'uni'isogrids are nearly three times stiffer than the 'iso'isogrids made from the same composite materials.This is indeed the right way.For the same reason,metallic grids are not effective.In fact,there is a close relation between composite laminates and composite grids.Grids can be viewed simply as a special case of laminates,and this will be used in deriving the stiffness and strength of grids

left in Fig. 5.2, a quasi-isotropic laminate is taken as starting material and machined into an isogrid. We call the resulting grid an ‘iso’ isogrid, indicat￾ing that the starting material is isotropic. This class of grids is very costly and a very poor utilization of the material. The rib has the same stiffness as the starting material. The right way is to use directional materials such as composites. Instead of machining, unidirectional fibers are rearranged or regrouped to form unidirectional ribs as shown on the right of Fig. 5.2. We call this class of grids ‘uni’ isogrids. Here the superior stiffness of unidirectional com￾posites is fully utilized. We will show later that the ‘uni’ isogrids are nearly three times stiffer than the ‘iso’ isogrids made from the same composite materials. This is indeed the right way. For the same reason, metallic grids are not effective. In fact, there is a close relation between composite laminates and composite grids. Grids can be viewed simply as a special case of laminates, and this will be used in deriving the stiffness and strength of grids. Manufacture and design of composite grids 153 5.1 Designation of grids by rib orientation, analogous to laminated composites. 5.2 Two ways of making grids. Left: the wrong way, by machining a quasi-isotropic laminate. Right: the correct way, by forming unidirectional ribs. RIC5 7/10/99 8:04 PM Page 153 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

154 3-D textile reinforcements in composite materials SuuO f 23 percent f 39 percent 5.3 Rib volume fractions of sparse and dense grids. 5.2.3 Rib geometric parameters The principal geometric parameters of grids are the length L,width b and WV IS:OE height h of the ribs.A useful dimensionless measure is the rib area fraction f within a unit cell.This fraction is related to the length and width of the ribs and their orientations in the grid.Two values of fare shown in Fig.5.3: for a sparse grid on the left and for a dense grid on the right.A dense grid can also be called a waffle plate,characterized by the fact that its ribs would not buckle. The value of fis the same as the rib volume fraction as long as the grid pattern remains constant along the grid height.The rib fraction is analo- gous to the fiber volume fraction of a composite material.But fiber frac- tion in composite plies is not a common design variable because such a fraction is often predetermined by material suppliers.For grids,however, rib fraction is an important design variable and must be deliberately selected for a given design.We recommend f-values in the range shown in Fig.5.3. Rib height h is also a critical design parameter,in determining flexural rigidity in particular.A low height-to-width ratio or h/b is a shallow grid;a high ratio,a tall grid.We assume in the present work that this ratio is higher than 1.Euler buckling of ribs occurs only in the lateral direction.It is then governed by the length-to-width ratio,L/b.Such a failure mode must be compared with failure by compressive strength.Whichever is lower will be the controlling failure mode. The relation defining the area fraction f of a grid is a function of the grid configuration.In Fig.5.4,we show the definition of f for iso-and square grids.A visual presentation of an isogrid compared with square grids is fea- tured.All grids have the same rib width.The smaller square grid on the left has the same area fraction f,whereas the larger square grid on the right has

5.2.3 Rib geometric parameters The principal geometric parameters of grids are the length L, width b and height h of the ribs. A useful dimensionless measure is the rib area fraction f within a unit cell. This fraction is related to the length and width of the ribs and their orientations in the grid. Two values of f are shown in Fig. 5.3: for a sparse grid on the left and for a dense grid on the right. A dense grid can also be called a waffle plate, characterized by the fact that its ribs would not buckle. The value of f is the same as the rib volume fraction as long as the grid pattern remains constant along the grid height. The rib fraction is analo￾gous to the fiber volume fraction of a composite material. But fiber frac￾tion in composite plies is not a common design variable because such a fraction is often predetermined by material suppliers. For grids, however, rib fraction is an important design variable and must be deliberately selected for a given design. We recommend f-values in the range shown in Fig. 5.3. Rib height h is also a critical design parameter, in determining flexural rigidity in particular. A low height-to-width ratio or h/b is a shallow grid; a high ratio, a tall grid.We assume in the present work that this ratio is higher than 1. Euler buckling of ribs occurs only in the lateral direction. It is then governed by the length-to-width ratio, L/b. Such a failure mode must be compared with failure by compressive strength. Whichever is lower will be the controlling failure mode. The relation defining the area fraction f of a grid is a function of the grid configuration. In Fig. 5.4, we show the definition of f for iso- and square grids. A visual presentation of an isogrid compared with square grids is fea￾tured. All grids have the same rib width. The smaller square grid on the left has the same area fraction f, whereas the larger square grid on the right has 154 3-D textile reinforcements in composite materials 5.3 Rib volume fractions of sparse and dense grids. RIC5 7/10/99 8:04 PM Page 154 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

Manufacture and design of composite grids 155 SQUARE GRID ISOGRID SQUARE GRID 3 b b 3 EQUAL f→EQUAL L/b→ fsq=L/ 2/3 =f1s0 fo=2语 2 fiso fsq=L/b=3 5.4 Definition of area fraction f of iso-and square grids.Slenderness ratio L/b is related to Euler buckling of ribs. poo the same slenderness ratio L/b as the actual isogrid.For the smaller square 1: grid,the length L is reduced by3:for the larger square grid,the area frac- tion f is reduced by v3. While there is a one-to-one relation between fraction f and L/b,each serves its own purpose in the design of composite grids.Area fraction fcan be treated as a material property that governs both in-plane and flexural stiffnesses in a consistent manner.Slenderness ratio,L/b,is useful in its direct relation to Euler buckling of the ribs.We prefer the use of area fraction f because it reflects the weight and amount of material used in a grid. Another geometric parameter of grids is their height or height-to-width ratio,h/b.Grids have characteristics similar to those of solid and sandwich panels.The ribs of a grid should be as tall as possible,i.e.having a high height-to-width ratio.Like plates,flexural rigidity increases with the cube of the height.Short or shallow ribs are not effective.For sandwich panels, flexural rigidity depends on both the height of the core and the laminated face sheets.If a grid has one or two face sheets,its flexural rigidity is like that of a sandwich panel.The rigidity factors are more numerous than for a grid without facing. 5.3 Manufacturing processes Composite grids have been explored in the former Soviet republic,South Africa,Germany as well as in the USA for over 20 years.In the USA,James Koury of the USAF Phillips Laboratory(now retired),Larry Rehfield of Georgia Institute of Technology (now with the University of California

the same slenderness ratio L/b as the actual isogrid. For the smaller square grid, the length L is reduced by ; for the larger square grid, the area frac￾tion f is reduced by . While there is a one-to-one relation between fraction f and L/b, each serves its own purpose in the design of composite grids. Area fraction f can be treated as a material property that governs both in-plane and flexural stiffnesses in a consistent manner. Slenderness ratio, L/b, is useful in its direct relation to Euler buckling of the ribs. We prefer the use of area fraction f because it reflects the weight and amount of material used in a grid. Another geometric parameter of grids is their height or height-to-width ratio, h/b. Grids have characteristics similar to those of solid and sandwich panels. The ribs of a grid should be as tall as possible, i.e. having a high height-to-width ratio. Like plates, flexural rigidity increases with the cube of the height. Short or shallow ribs are not effective. For sandwich panels, flexural rigidity depends on both the height of the core and the laminated face sheets. If a grid has one or two face sheets, its flexural rigidity is like that of a sandwich panel. The rigidity factors are more numerous than for a grid without facing. 5.3 Manufacturing processes Composite grids have been explored in the former Soviet republic, South Africa, Germany as well as in the USA for over 20 years. In the USA, James Koury of the USAF Phillips Laboratory (now retired), Larry Rehfield of Georgia Institute of Technology (now with the University of California, 3 3 Manufacture and design of composite grids 155 5.4 Definition of area fraction f of iso- and square grids. Slenderness ratio L/b is related to Euler buckling of ribs. RIC5 7/10/99 8:04 PM Page 155 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

156 3-D textile reinforcements in composite materials Davis)and McDonnell Douglas Astronautics Company [1]have been pioneering the use of aluminum and later composite grids principally for the fairing and the interstage cone of missiles.W.Brandt Goldsworthy of Rolling Hills,California,pioneered not only pultruded but also filament wound grids.He first proposed this for the Beechcraft Star Ship in the 1970s. Recently,Burt Rutan of Scaled Composites in Mojave,California,built the fuselage of a corporate jet out of composite grids.The USAF continues to explore composite grids with new applications.The McDonnell Douglas Handbook [1]has been updated with the use of composite materials by Chen and Tsai [2]and by Huybrechts [3].The modeling used in this work draws heavily from these earlier publications.The software developed by these authors is instrumental in the analysis and figures used throughout the current effort. It has been recognized by many people that filament winding would be an optimal method for manufacturing grids if the composite tows could be guided by some soft tooling.Grids are assembled by carving out slots or grooves in a rubber tool. 5.3.1 Assembly methods 2502 We believe that new approaches can improve performance and,at the same time,lower cost.A variation in the grid assembly is the configuration of the rib intersection or joint.Three possible joints are shown in Fig. 5.5. ne SLOTTED JOINT STACKED JOINT TRIG JOINT (in carpentry) (a bird cage) bonded or interlaced 5.5 Three types of joints in a grid.The slotted joint is not recom- mended.Stacked and TRIG joints can be produced more easily and have better properties

Davis) and McDonnell Douglas Astronautics Company [1] have been pioneering the use of aluminum and later composite grids principally for the fairing and the interstage cone of missiles. W. Brandt Goldsworthy of Rolling Hills, California, pioneered not only pultruded but also filament wound grids. He first proposed this for the Beechcraft Star Ship in the 1970s. Recently, Burt Rutan of Scaled Composites in Mojave, California, built the fuselage of a corporate jet out of composite grids. The USAF continues to explore composite grids with new applications. The McDonnell Douglas Handbook [1] has been updated with the use of composite materials by Chen and Tsai [2] and by Huybrechts [3]. The modeling used in this work draws heavily from these earlier publications. The software developed by these authors is instrumental in the analysis and figures used throughout the current effort. It has been recognized by many people that filament winding would be an optimal method for manufacturing grids if the composite tows could be guided by some soft tooling. Grids are assembled by carving out slots or grooves in a rubber tool. 5.3.1 Assembly methods We believe that new approaches can improve performance and, at the same time, lower cost. A variation in the grid assembly is the configuration of the rib intersection or joint. Three possible joints are shown in Fig. 5.5. 156 3-D textile reinforcements in composite materials 5.5 Three types of joints in a grid. The slotted joint is not recom￾mended. Stacked and TRIG joints can be produced more easily and have better properties. RIC5 7/10/99 8:04 PM Page 156 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

Manufacture and design of composite grids 157 Slotted joint grids The traditional slotted joint grids are shown on the left in Fig.5.5 and are most frequently used in carpentry.Slots are cut into ribs and assembled. The disadvantages of this design include: cost of machining slots, difficult assembly of many ribs having multiple slots, low rib strength introduced by machined slots and notches. low grid stiffness and strength from imperfect fit at slotted joints. We understand that Composite Optics Incorporated of San Diego,Califor- nia,used [/3 laminates as the rib in order to increase the rib strength.The use of laminates for ribs,however,degrades the grid stiffness by a factor of 3 from unidirectional ribs.It is therefore our opinion that slotted joint grids should remain as a popular technique for carpenters and cabinet makers. poo Stacked joint grids 1S: We believe that the stacked joint grids shown in the middle in Fig.5.5 can be as effective as slotted joint grids and can be simpler to manufacture.An Tontun for tries To build stacked rid,longitudinal and hoop or r example of stacked joint grid is the bird cage,which has been in existence members are stacked.Members run on separate planes,similarly to the plies in laminate.There are at least two variations.The longitudinal members (longis)are pultruded,filament wound or made in a female mold by blow molding.The cross members (circs)can be skins applied by filament winding to form a circular or conical grid or shell. The longitudinal tubes may be fan-shaped,for example,and serve the same purpose as a sandwich core between the inner and outer filament wound skins.Although winding can also have a helical pattern if an increase in shear rigidity is desired,such a process increases the cost of manufac- turing over pure hoop winding.The longitudinal and cross members may be fully or partially populated,i.e.the longis do not have to be placed adja- cent to one another.The hoop wound plies can be continuous or discon- tinuous like bands or rings.An example of a ring reinforced cylinder is shown in Fig.5.6. Other examples of a stacked grid include cross-members made by molding or vacuum infiltration.A multi-hole bar or ring through which lon- gitudinal rods or tubes are threaded and bonded forms a bird cage-like structure.There are many possible configurations for different applications. Stacked grids,however,are currently limited to orthogrids.Isogrids,for example,are difficult to make because ribs in three levels must be stacked and joined

Slotted joint grids The traditional slotted joint grids are shown on the left in Fig. 5.5 and are most frequently used in carpentry. Slots are cut into ribs and assembled. The disadvantages of this design include: • cost of machining slots, • difficult assembly of many ribs having multiple slots, • low rib strength introduced by machined slots and notches, • low grid stiffness and strength from imperfect fit at slotted joints. We understand that Composite Optics Incorporated of San Diego, Califor￾nia, used [p/3] laminates as the rib in order to increase the rib strength. The use of laminates for ribs, however, degrades the grid stiffness by a factor of 3 from unidirectional ribs. It is therefore our opinion that slotted joint grids should remain as a popular technique for carpenters and cabinet makers. Stacked joint grids We believe that the stacked joint grids shown in the middle in Fig. 5.5 can be as effective as slotted joint grids and can be simpler to manufacture. An example of stacked joint grid is the bird cage, which has been in existence for centuries. To build a stacked grid, longitudinal and hoop or cross members are stacked. Members run on separate planes, similarly to the plies in laminate. There are at least two variations. The longitudinal members (longis) are pultruded, filament wound or made in a female mold by blow molding. The cross members (circs) can be skins applied by filament winding to form a circular or conical grid or shell. The longitudinal tubes may be fan-shaped, for example, and serve the same purpose as a sandwich core between the inner and outer filament wound skins.Although winding can also have a helical pattern if an increase in shear rigidity is desired, such a process increases the cost of manufac￾turing over pure hoop winding. The longitudinal and cross members may be fully or partially populated, i.e. the longis do not have to be placed adja￾cent to one another. The hoop wound plies can be continuous or discon￾tinuous like bands or rings. An example of a ring reinforced cylinder is shown in Fig. 5.6. Other examples of a stacked grid include cross-members made by molding or vacuum infiltration. A multi-hole bar or ring through which lon￾gitudinal rods or tubes are threaded and bonded forms a bird cage-like structure. There are many possible configurations for different applications. Stacked grids, however, are currently limited to orthogrids. Isogrids, for example, are difficult to make because ribs in three levels must be stacked and joined. Manufacture and design of composite grids 157 RIC5 7/10/99 8:04 PM Page 157 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

158 3-D textile reinforcements in composite materials Pultruded longi's Round. Fan-shaped 5650602红 Inner cylinder +-+ 299互 Retaining circ's k-Spacing→ 5.6 A stacked grid with round or fan-shaped longitudinal members sandwiched between inner and outer windings. (sL6-LS-IL)Kus -Helical pattern Interlacing filling the gap Tooling positioned 1/ 5.7 A filament wound cylinder made by the TRIG process.Left: tooling from contoured tubes.Right:wound interlacing fills the V-shaped grooves for grid strength. Interlaced joint grids For the interlaced grid,the thin wall tubes,again,are the starting compo- nents.The filament wound tubes with all-hoop plies provide maximum stiff- ness for the final grid.The tubes are sliced to a contour that fits a mandrel. They are then positioned as tooling on the mandrel.This is shown on the left in Fig.5.7.The V-shaped gaps between tooling are filled with interlac- ing tows,as shown on the right.The interlacing tows carry sufficient resin to bond the tooling and interlacing together to form a solid,continuous rib. The interlacing gives superior strength to the grid.The tooling becomes part of the finished grid and provides high stiffness to the grid.Although tooling contributes to the grid stiffness,it terminates at the rib joints.The disconti- nuity is small relative to the length of the rib.The effect on the grid stiff- ness is small

Interlaced joint grids For the interlaced grid, the thin wall tubes, again, are the starting compo￾nents.The filament wound tubes with all-hoop plies provide maximum stiff￾ness for the final grid. The tubes are sliced to a contour that fits a mandrel. They are then positioned as tooling on the mandrel. This is shown on the left in Fig. 5.7. The V-shaped gaps between tooling are filled with interlac￾ing tows, as shown on the right. The interlacing tows carry sufficient resin to bond the tooling and interlacing together to form a solid, continuous rib. The interlacing gives superior strength to the grid.The tooling becomes part of the finished grid and provides high stiffness to the grid. Although tooling contributes to the grid stiffness, it terminates at the rib joints. The disconti￾nuity is small relative to the length of the rib. The effect on the grid stiff￾ness is small. 158 3-D textile reinforcements in composite materials 5.6 A stacked grid with round or fan-shaped longitudinal members sandwiched between inner and outer windings. 5.7 A filament wound cylinder made by the TRIG process. Left: tooling from contoured tubes. Right: wound interlacing fills the V-shaped grooves for grid strength. RIC5 7/10/99 8:04 PM Page 158 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

Manufacture and design of composite grids 159 Significant cost savings can be obtained when the interlacing is filament wound with one helical winding angle.Having a V-shaped groove along a helical pattern would allow high-speed winding.That would further reduce the cost of assembly. 5.3.2 Features of grids We have discovered theoretically that grids are more efficient if the ribs are tall and thin.This process,identified as the tooling reinforced interlaced grid (TRIG),yields a high geometric definition for the ribs and also high grid stiffness.Several current interlacing and fiber placement processes use rubber or foam as guide and tooling.These processes do not produce the high definition and stiffness that the TRIG process does. The advantages of composite grids are derived from the availability of mass-producible rods and tubes,and from the final assembly by filament winding.This winding process is one of the most advanced and widely avail- able processes.Curing is done at room or elevated temperature.Debulk- ing,bagging and autoclaving are not required.With this process the cost of making a grid can be close to the cost of materials,not many times the cost. Assembly by adhesive bonding in the case of some stacked grids can also be cost effective. 争 Although the stiffness of composite grids is nearly equal to that of lam- inates,the strength is many times higher.This is because unidirectional ribs do not fail by microcracking or delamination,but by loss of strength or buckling.Where foamed tubes are used,the grids will have superior damping and acoustic properties that cannot be matched by metallic struc- tures.Composite grids are also more resilient.There is no permanent defor- mation upon unloading.Thus composite grids do not dent or crumple like sheet metals. While the advantages of composite grids are high strength and low cost, there are also disadvantages.As of now,grids can only be made in simple geometric shapes.Such a limitation is often imposed by filament winding. Circular and conical shells are the easiest.Spherical shells can be done using the TRIG process.But doubly curved or concave surfaces are not suitable for grids.Bolting is not recommended without local reinforcement. Finally we recommend that grids be designed to carry all the loads.Skins are present for functional reasons only:in sandwich panels the skins carry the load. 5.4 Mechanical properties of grids We wish to describe the stiffness and strength of grids and compare them with comparable properties of laminates

Significant cost savings can be obtained when the interlacing is filament wound with one helical winding angle. Having a V-shaped groove along a helical pattern would allow high-speed winding. That would further reduce the cost of assembly. 5.3.2 Features of grids We have discovered theoretically that grids are more efficient if the ribs are tall and thin.This process, identified as the tooling reinforced interlaced grid (TRIG), yields a high geometric definition for the ribs and also high grid stiffness. Several current interlacing and fiber placement processes use rubber or foam as guide and tooling. These processes do not produce the high definition and stiffness that the TRIG process does. The advantages of composite grids are derived from the availability of mass-producible rods and tubes, and from the final assembly by filament winding.This winding process is one of the most advanced and widely avail￾able processes. Curing is done at room or elevated temperature. Debulk￾ing, bagging and autoclaving are not required. With this process the cost of making a grid can be close to the cost of materials, not many times the cost. Assembly by adhesive bonding in the case of some stacked grids can also be cost effective. Although the stiffness of composite grids is nearly equal to that of lam￾inates, the strength is many times higher. This is because unidirectional ribs do not fail by microcracking or delamination, but by loss of strength or buckling. Where foamed tubes are used, the grids will have superior damping and acoustic properties that cannot be matched by metallic struc￾tures. Composite grids are also more resilient.There is no permanent defor￾mation upon unloading. Thus composite grids do not dent or crumple like sheet metals. While the advantages of composite grids are high strength and low cost, there are also disadvantages. As of now, grids can only be made in simple geometric shapes. Such a limitation is often imposed by filament winding. Circular and conical shells are the easiest. Spherical shells can be done using the TRIG process. But doubly curved or concave surfaces are not suitable for grids. Bolting is not recommended without local reinforcement. Finally we recommend that grids be designed to carry all the loads. Skins are present for functional reasons only: in sandwich panels the skins carry the load. 5.4 Mechanical properties of grids We wish to describe the stiffness and strength of grids and compare them with comparable properties of laminates. Manufacture and design of composite grids 159 RIC5 7/10/99 8:04 PM Page 159 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9

160 3-D textile reinforcements in composite materials 5.4.1 Stiffness of quasi-isotropic laminates It is useful to compare the stiffness of laminates and equivalent grids.The simplest comparison is that between isotropic laminates and isogrids.Lam- inates become quasi-isotropic with equally spaced ply orientations of [/3], [/4],[/5]and so on.Similarly isotropy of the grid is assured when the three ribs are spaced 60 apart.There are closed-form solutions of the plane stress stiffness components [4]. The quasi-isotropic invariants are linear combinations or the ply stiffness components shown below: 3 1 u=g0.+0m)+40,+0s U=3 1 3 10 [5.1 3 u=80a+0m)+40w+22s The quasi-isotropic Young modulus,Poisson ratio and shear modulus of the laminates are functions of the invariants: diu [5.2] where D=U2-U2. On the other hand,when the degree of anisotropy of a composite ply increases to the upper limit,the only dominant stiffness component is the longitudinal Young modulus E.The matrix-related components become vanishingly small.Then the invariants above approach: 4-君u-g6-g 3 [5.3 The resulting engineering constants of this limiting quasi-isotropic laminate are: =G-古D=g,- [5.4 The mathematical results in the last equation may be explained physi- cally by viewing a laminate having three independent plies of equal thick- ness.The effective stiffness is equal to of the unidirectional stiffness because each ply occupies of the total laminate thickness.Having the same stiffness in 60 intervals,the laminate becomes isotropic.This can be shown by averaging the transformed stiffness components

5.4.1 Stiffness of quasi-isotropic laminates It is useful to compare the stiffness of laminates and equivalent grids. The simplest comparison is that between isotropic laminates and isogrids. Lam￾inates become quasi-isotropic with equally spaced ply orientations of [p/3], [p/4], [p/5] and so on. Similarly isotropy of the grid is assured when the three ribs are spaced 60° apart.There are closed-form solutions of the plane stress stiffness components [4]. The quasi-isotropic invariants are linear combinations or the ply stiffness components shown below: [5.1] The quasi-isotropic Young modulus, Poisson ratio and shear modulus of the laminates are functions of the invariants: [5.2] where D = U1 2 - U4 2 . On the other hand, when the degree of anisotropy of a composite ply increases to the upper limit, the only dominant stiffness component is the longitudinal Young modulus Ex. The matrix-related components become vanishingly small. Then the invariants above approach: [5.3] The resulting engineering constants of this limiting quasi-isotropic laminate are: [5.4] The mathematical results in the last equation may be explained physi￾cally by viewing a laminate having three independent plies of equal thick￾ness. The effective stiffness is equal to 1 –3 of the unidirectional stiffness because each ply occupies 1 –3 of the total laminate thickness. Having the same stiffness in 60° intervals, the laminate becomes isotropic. This can be shown by averaging the transformed stiffness components. n iso iso iso [] [] [] === = 1 3 1 8 1 8 1 3 2 , ,, G ED E E E xx x U EU EU E 145 xxx 3 8 1 8 1 8 === , , E D U U U G U iso iso iso [] [] [] == = 1 4 1 , , n U QQ Q Q 5 xx yy xy SS 3 8 1 4 1 2 = + ( ) + + U QQ Q Q 4 xx yy xy SS 1 8 3 4 1 2 = + ( ) + - U QQ Q Q 1 xx yy xy SS 3 8 1 4 1 2 = + ( ) + + 160 3-D textile reinforcements in composite materials RIC5 7/10/99 8:04 PM Page 160 Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://woodhead.metapress.com Hong Kong Polytechnic University (714-57-975) Saturday, January 22, 2011 12:30:51 AM IP Address: 158.132.122.9