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 industries 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 composites 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 materials. 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 fundamentally different from stiffened and sandwich constructions in that the load transfer mechanisms are different. Grids can be made by the widely available filament winding and pultrusion. We believe that both high performance 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 understanding, 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 outstanding 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 materials. 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, indicating 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 composites 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 analogous to the fiber volume fraction of a composite material. But fiber fraction 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 featured. 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 fraction 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