By: Tanveer Malik and Shivendra Parmar
Department of Textile Technology
Shri Vaishnav Institute of Technology and Science
Baroli (Indore-Sanwer Road), Distt: Indore (M.P) Pin-453331
When building bridges, airplanes and other structures engineering designers draw detailed plans on computers and use stress analysis to work out the effects of all the likely forces. In contrast, textile fabrics and textile products are still produced largely on the basis of experience, intuition and trial and error. Many textile fabrics have now penetrated into high performance areas from medical to civil engineering, from leisure to military and from space to undersea. These demanding areas require textiles to be engineered very carefully and precisely since failure could have fatal consequences. The 3-D fabrics are very challenging for these fields nowadays and will be a demanding technology in coming days.
3-D fabrics are technical textiles made on 3 planar geometry on contrary to 2-D fabrics which are weaved in 2 planes. In 2D fabrics, yarns are weaved perpendicularly, but in 3-D fabrics, yarns are not only weaved perpendicularly but also at an angle with each other depending upon the requirement.
Multi-warp weaving methods are used for weaving angle interlocked multi-layer 3D woven fabrics and can be constructed using specialized looms such as a reciprocative loom, and a conical take-up device.
Historically, applications of 3-D fabrics were restricted to aerospace development but nowadays these find applications commercially, particularly in marine structures and industrial components.
Scientists believe 3D woven preformed parts will be a key technology in achieving almost 100% atomization in respect of the manufacturing of complex shaped Composite parts.
This paper includes weaving methods, properties and applications of 3D fabrics in todays and coming era.
Three-dimensional woven, braided or stitched fibrous assemblies are textile architectures having fibers oriented so that both the in-plane and transverse tows are interlocked to form an integrated structure that has a unit cell with comparable dimensions in the all three orthogonal directions. This integrated architecture provides improved stiffness and strength in the transverse direction and impedes the separation of in-plane layers in comparison to traditional two-dimensional fabrics. Recent automated manufacturing techniques have substantially reduced costs and significantly improved the potential for large-scale production. Optimal orientations, fiber combinations and distributions of yarns have yet to be fully developed and perfected for 3D fabrics subjected to impact loading conditions.
The term �three-dimensional� is applied in the sense of having three axes in a system of coordinates. If no yarn system penetrating the depth is present, we are confronted with a simple textile flat (2-D) fabric. Simple flat fabrics have very good stiffness and strength in two directions i.e. in warp-way and weft-way, but they have problem in thickness direction. In thickness direction they have very low stiffness and strength.
2. 3-D Glass fabric can be applied in areas where high strength and/or weight reduction is needed and can act as an alternative to plywood, balsa, solid laminate, honeycombs, foams and more.
3-D WOVEN COMPOSITES
While the performance advantages of 3D composites are recognized, past applications have been restricted due to the high cost of producing the 3D reinforcement. Historically, applications that can afford the performance advantages have been restricted to aerospace development, typically including RTM (or other infusion). Recently, these materials have been finding increased usage in more commercial applications, particularly in marine structures and industrial components that are very cost sensitive. Due to the availability of heavy weight fabrics/reinforcements, and the subsequent reduction in lay-up labour, 3D fabrics can reduce the cost of finished composite structure.
The increasing interest and use of 3D textile composites is attributed to two factors: 1) improved performance due to controlled fiber distribution; and 2) lower cost through the use of automated textile processing equipment. Compared on a cost per square foot of finished composite structure, 3-D WEAVE reinforcements consistently outperform traditional 2D materials.
Application of 3-D woven composites
� In the marine company for building recreational boats.
� For manufacturing industrial pressure tanks.
� Alternative to a corrugated steel structure for the industrial/infrastructure market.
COMPARISON OF PROPERTIES OF 3-D WITH 2-D FABRICS
1. The absence of interlacing between warp and filling yarns allow the fabric to bend and internally shear rather easily, without buckling within the in-plane reinforcement which is not in case of 2-d fabrics.
2. The presence of Z-direction reinforcement in 3-d fabric is an obvious advantage, as dramatic improvement in composite transverse strength and impact damage tolerance is well documented. For example, tests of laminates made from these preforms have shown a 10�30% increase in short beam shear strength over 2D textile laminates.
3. These have shown improved compression after impact strength, reduced delamination area, and increased number of sub-perforation energy blows required to penetrate the panel.
4. Composites made from 3-d preforms exhibit high fiber content (% by weight). Although somewhat lower percentages can be expected, fiber content is still higher than in composites made from comparable 2D fabrics.
APPLICATIONS OF 3-D FABRICS
With completely controlled and tailor able fiber orientations in the X, Y and Z directions, the ability to weave aramid, carbon, glass, polyethylene, steel fibers etc. and any hybrid combination, thickness up to one inch (2.54 cm), width up to 72 inches (183 cm) and the ability to make net shapes, an almost infinite number of 3-D materials are possible with a tremendously wide range of performance.
The floor was simulated with a finite element model for the wood panels and the 3-D sandwich panels. An iterative process was performed as a way to obtain the best material orientation. All parameters were kept constant except the material orientation, which was oblied to vary from 0� to 90� with a 5� gap. Best orientation was found to be 45�. Further lamination was necessary, because the total displacement was larger than the allowable. For the new 3-D sandwich design, the weight of the wood panels was reduced to 178 kg. from 289 kg.
3D GLASS FABRICS
3D glass fabrics are based onto the velvet weaving technology, whereby two deck-layers are connected by piles, which are integrally woven into the deck-layers. All fabrics are made from 100% E-Glass with a silane sizing. After impregnation, the fabrics expand by themselves to the specified width of 35-22 mm. Through this outstanding technology we are able to manufacture sandwich laminates in the simplest way.
� easy to use, no vacuum bagging necessary
� no drapeability problems
� no exact cut, no inclined edges
� simple stress points, you only press down the fabric to a solid laminate
� the space between the deck-layers can be streamed through by fluids for heating, cooling, monitoring
� fire resistive core material
Application of 3-D glass fabric
1. In the tank lining technology: - to have corrosion protection and leak detection With the 3-D glass fabric tank lining system, tank owners can transform their existing single-wall tanks into double-wall tanks with secondary containment and continuous leak detection, without reworking or replacing their single-wall storage tanks.
How does this technology work?
This tank conversion technology consists of a composite matrix of 100 percent solids epoxy and 3 mm thick three-dimensional (3-D) glass fabric that is bonded to the inside wall of an underground storage tank or, in the case of an above ground storage tank, the tank floor. The matrix is then top-coated with a layer of 100 percent solids epoxy specifically formulated to provide corrosion resistance against the cargoes stored. The fabric in the matrix, which is constructed from E-glass yarn into the 3-D glass fabric that is trademarked as PARABEAMTM worldwide, was developed in Europe in 1989.
The applications include:-
1. Textile applications
2. Reinforcement of composites
The various textile applications include:
a) Automotive application
� Seat coverings
b) Garment application
� Outer wear
� Inner wears
c) Medical application
� Artificial blood vessels
� Orthopedist fabrics
d) Architecture & Construction
� Membrane fabrics
� Canalization: tubes, fittings etc.
The various applications of reinforcements of composites are:
a) Automotive application
� Wheel rims
� Crash elements
� Instrument panels
� Seat shells
� Armour platings
b) Motorbike application
� Frame parts
� Mudguards, Fenders
c) Mechanical & Process Engineering
� Pressure vessels
d) Medical applications
� Artificial joints & Limbs
e) Architecture & Construction
� Conical pillars
� Light weight panels
f) Aerospace Industry
� Seat shells
ADVANTAGES OF 3-D FABRICS
1.) Although these materials are typically more expensive than 2D fabrics and mats, reduction of labor, higher performance and improved process efficiency result in overall cost savings in a variety of applications.
2.) The absence of interlacing between warp and filling yarns allow the fabric to bend and internally shear rather easily, without buckling within the in-plane reinforcement as in case of 2-d fabrics.
3.) The 3D preforms are seamless and directly shaped on the loom, with each part taking three minutes to weave. Moulding is said to have proven easier and more efficient using the new 3D preforms, since, because the textile is already shaped, there are no forces to affect fibre placing during the moulding process.
4.) Less conformable fabrics would require extensive cutting and darting to avoid wrinkles and/or buckling in the laminate. The conformability of 3-d preforms can result in reduced labor and faster cycle times, regardless of the composite fabrication process.
The ability to take complexity and labor out of manual composites fabrication processes through the innovative automation of engineered preforms is a key to more widespread use of composites. The trend is toward more control over fiber orientation and architecture while increasing productivity. This trend continues with the 3D weaving technology discussed in this paper. The 3-d weaving process and resulting preforms offer many advantages in both performance and economics. 3-d fabrics will continue to gain acceptance as more companies recognize the value these materials offer.
Technocrats using 3-d performs could efficiently and accurately design totally new materials, novel manufacturing processes and new fabric structures to accelerate the fabric development process and foster an innovation.
First of all we would like to express profound gratitude to the management of the institute, Administrative officer Shri. R.C.Parmar, Principal Dr. Ing.V.P.Singh, Advisor Prof. Dr. H.V.S.Murthy and Head of the department Prof. Dr.Prabhakar Bhat for giving encouragement and guidance to work on 3-D FABRICS � An Overview.
� D. Mungslov and A. Roudaruiviirri, "Automated 3-D Braiding Ma�chine and Method," (To 3TEX, Inc.). US Patent No. 6.434.0% Bl. Aug. 27, 2002. 2, A. Bogdanovich and D. Mungalov, "Innuvaiive 3-D Braiding
� Mohamed, M. H., Bogdanovich, A. E., Dickinson, L.C., Singletary, J. N., and Lienhart, R. B., "A New Generation of 3D Woven Fabric Preforms and Composites," SAMPE Journal, 2001, Vol. 37, No. 3, May/June, pp. 8-17.
� Dickinson, L.C. and M. H. Mohamed, "Recent Advances in 3D Weaving for Textile Preforming", Proceeding of the ASME Aerospace Division, Book No. H01214, pp. 3-8, (2000).
� Brandt,J. Drechsler, K., Mohamed, M., and Gu, P., " Manufacture and Performance of Carbon/Epoxy 3-D Woven Composites", Proc. 37th Int. SAMPE Symposium, Anaheim, CA,(1992).
� USP 5,085,252, "Method of Forming Variable Cross-sectional Shaped Three-Dimensional Fabrics," Mansour H. Mohamed and Zhonghuai Zhang, February 4, 1992.
About the author:
I am Shivendra Parmar completed my M.Tech in Textile Technology at Textile institute of technology and Science,Bhiwani CDLU University during 2002 .I did my B.E in Textile Technology at Shri Vaishnav Institute of Technology and Science, Indore and passed out in First Class during the year 2002.So far published 4 articles in National and International Journals and presented one paper in national conference. Currently working as lecturer & placement coordinator in the department of Textile Technology, Shri Vaishnav Institute of Technology and Science, Indore. Also contribute in TEXTILE ASSOCIATION OF INDIA,M.P UNIT as a Jt. Secertary. EMAIL: email@example.com
I am Tanveer Malik completed my M.Tech in Textile Manufacturing at Veermata Jijabai Technological Institute (VJTI), Mumbai University during dec 2001 .I did my B.E in Textile Technology at Shri Vaishnav Institute of Technology and Science, Indore and passed out in First Class during the year 2000.So far published 3 articles in National and International Journals. Currently working as lecturer in the department of Textile Technology, Shri Vaishnav Institute of Technology and Science, Indore. Email: firstname.lastname@example.org
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This limitation restricts the use of simple 2-D fabrics in the field of space engineering, automotive engineering and sports goods. One way to get added strength in thickness direction is by the use of fibre reinforced composites. Composites using long fibres for reinforcement purposes have long been popular in aviation and space engineering as well as for sports goods. But according to market projections, the annual increase for the raw materials used in such products, i.e. carbon fibres and polyaramide fibres are in the range of 15 to 20% only. The reasons for the comparatively limited proliferation of high performance composites (HPC) are found to be:-
� High cost for raw material, and
� High manufacturing cost.
Above mentioned limitations led the researcher to think in the direction of new concept, which led the discovery of 3-D fabrics.
HISTORY OF 3-D FABRICS
The history of three-dimensional textiles reaches back to the 19th century. Already in 1898 it was recognized that the interlaminar shearing properties of rubber drive belts could be improved by adding reinforcements made of multi-layer fabrics, thus effectively eliminating the lateral displacement of layers. Subsequently, multi-layer fabrics which feature additional reinforcement by yarns arranged vertically to the fabric layers have been applied in a wide range of sectors:-
� Conveyor belting
� Dry felts for papermaking
� Interlinings for shirt collars
At the end of the sixties, the aerospace industry began to demand fibre- based composite structures which could withstand multi-directional stress at extreme thermal conditions. In France and later on in the United States and Japan, carbon fibre composites were developed whose yarn systems were arranged multi-dimensionally.
NEED FOR 3D FABRICS:
Traditional 2D weaving has been around for thousands of years. 2D weaving is a relatively high-speed economical process. However, woven fabrics have an inherent crimp or waviness in the interlaced yarns, and this is undesirable for maximum composite properties. Most of the marine and wind blade industry currently use glass non-crimp stitch-bonded or knitted fabrics, more properly known as multi-axial-warp-knits. These materials are cost-competitive. However, they do not conform well to complex forms and often have significant fiber distortion in the final composite. A fully automated 3D weaving process with simultaneous multiple filling insertions, has been developed at the NC State University College of Textiles . This process is inherently 3D from the onset, and does not involve the building up of layers one layer at a time. Rather, a single unit of thick fabric is formed during each weaving cycle. The essence of the innovation/patent centers around this simultaneous multiple insertion from one or both sides of the fabric.
In the picture below, it is layout of Para beam 3D fabric. The distance between each facing can be varied to suit the application. When both facing are coated, an airtight space is obtained which can be inflated by either air or foam. Goodyear already produced an inflatable aeroplane using rubberized air mat in the mid nineteen- fifties. It is example of the high- performance applications of 3D fabric.
Inflation of 3D fabric can be done by a variety of media such as air, water or foam. Another group of students looked at the 3D fabrics. This material is used in lightweight structures since long time. Pneumatic use of 3D fabrics is mostly used as a plane structure.
Three- dimensional woven fabrics are usually produced on a multiwarp loom. In a conventional 2D loom, harness alternately lift and lower the warp yarns to form the interlacing pattern. In a multiwarp loom, separate harness lift different groups of warp yarns to different heights. So the some are formed into the layers while other weave the layers together to form net shape performs.
The heart of the 3D-weaving process is the dual-directional shedding operation. Through this operation the multiple layer warp can be displaced to form alternately multiple column wise and row-wise sheds.
Subsequent picking of wefts in the corresponding sheds of the two directions results in the complete interlacing of the multiple layer warp (Z) with the two mutually perpendicular sets of wefts (X and Y).
MANUFACTURING OF 3-D FABRICS
Manufacturing method involves special peddles which are designed to sort the warps into three sections that form the mainframe and flanges of the 3D woven preforms. The preforms are woven into a plane construction and then unfolded to form a near-net-shape construction.
Since reinforcements play a major role in dominating the mechanical properties of composites, the continuity and integrity of the architecture of fiber preforms becomes a main concern in 3-d composites. Textile reinforcements have received widespread use in composites based on their flexibility to accommodate various reinforcing requirements. From a textile constructional perspective, there are four reinforcing constructions, including chopped fibers, filament yams, simple fabrics, and 3-d fabrics. There are four basic textile techniques-weaving, knitting, braiding, and stitching-that are capable of fabricating 3D textile reinforcements .
These 3D woven preforms with various architectures can be fabricated using different weaving methods. Multi-warp weaving methods are used for weaving orthogonal and/or angle interlocked multi-layer woven fabrics. On the other hand, preforms with cylindrical profiles can be constructed using specialized looms developed by different methods, such as reciprocative loom and conical take-up device.
The warp/weft knitting technique has been widely used to produce non-crimped fabrics (NCFS) in which tows all lay flat, straight, and fully extended and are subsequently knitted by fine filaments to fix them in place. NCFS can be produced with single-layer or multi-layer constructions where each layer has a specific fiber direction. Stitching is a fairly convenient and cheap method for fabricating 3D textile preforms, which simply bind the fabrics, forming a 3D construction (thick plate, T-shaped, and so on) by chain or interlock stitching of a thread structure.
Since a 3D construction, especially for complex shapes, is difficult to fabricate at a reasonable production rate, very few automatic manufacturing systems available have been developed commercially. Two of the most successful automatic manufacturing systems for 3D textiles are the multi-axial warp knit (MWK) by Liba.
Basic Weaving Concept
The basic automatic weaving motion includes three major mechanisms, known as shedding, picking, and beating-- up, to interlace the warps and wefts forming the woven fabrics. In addition, two assisting operations, let-off and take-up, are included for weaving fabrics continuously.
Since the 3D fabrics are difficult to achieve on a weaving loom, warps must be placed in a flattened form.
Above figure shows the schematics of plane and unfolded constructions of 3-D woven fabrics. To form the mainframe and flanges of the fabrics, multi-eye heddles are used to arrange the warps into three sections, where one set of the warps goes back and forth between the adjacent sections as a joint of mainframe and flanges, shown in Figure.
WEAVING MECHANISMS OF 3D WOVEN FABRICS
Shedding, Picking, and Beating-up Mechanisms
A harness with single-eye peddles performs the shedding motion in a conventional weaving loom, but a complicated weaving pattern needs more harnesses to control the shedding. As mentioned above, warps are sorted into three sections to form the plane form of the 3-d woven construction. We use special peddles, called multi-eye peddles, to sort the warps and perform the shedding motion. As implied by the name, a multi-eye peddle is made of wire with several heddle eyes on it. Multi-eye peddles allow warps be sorted into three sections in a single harness. A number of sheds are formed within a single shedding motion, depending on the arrangement of peddles and number of eyes. The use of multi-eye peddles reduces the number of harnesses and simplifies the shedding motion, thus increasing the potential of automatic manufacturing.
The shedding motion for single-layer fabrics requires four harnesses, but six harnesses are needed for the complicated tri-layer constructions. Three and nine picks are used to perform the picking motion for the singleand tri-layer fabrics, respectively.
The weaving cycle for forming the 3-d fabric is as continued:
First, harnesses 1, 3, 4, 5, and 6 go upward, whereas 2 goes downward, spreading the warp threads into several layers, thereby forming nine clear sheds and nine picks passing through the sheds from left to right. Second, the reed pushes the newly inserted picks into the fell. Third, the harness switches to the reverse direction, forming the new cloth and clear sheds during the backward movement of the reed and then the picks passing through the sheds from right to left. Note that each of the four fabrics has a set of warps that goes back and forth between the adjacent sections, forming an interlacing structure as a joint of the web and flanges.
Let-off and Take-up Mechanisms
The warps are fed in an equal amount for conventional weaving looms in which a warp beam is used to perform the let-off motion. However, warp-shedding lengths are different since the multi-eye heddles are used, so the let-off warps must be controlled separately to fit the differential shedding motion. Therefore a negative let-off motion is performed using a creel loaded with bobbins. The differential feeding length between the warp yams gives rise to extra friction, and therefore hairiness may occur. In order to reduce this friction, the warps are passed through the tensioner and weight with ceramic eyes individually between the creel and weaving loom. The thickness of the central portion of the flattened fabrics is different from the two side portions. Therefore the cloth roller cannot be used to take up the flattened fabrics. The fabric is clipped and pulled by a pair of rollers set in front of the loom as a take-up device.
The 3-d woven fabrics are successfully fabricated by modifying the conventional weaving mechanisms. Harnesses with multi-eye heddles are used to arrange the warps into three sections in plane form for weaving convenience. Mainframe and flanges are interlaced by a set of warps moving to and fro as a joint. Wefts pass through the clear warp sheds separated by the multi-eye heddles to form the 3-d woven fabrics in plane form. A creel is used for the let-off motion, while a pair of rollers replaces the cloth roller.
CLASSIFICATION OF 3-D FABRICS
A large number of processes and machines for manufacturing 3-D fabrics have been developed, often with a view to very different objectives in terms of application. Considering that many of these processes have there origin in conventional textile machinery, it would appear sensible to apply a classification system which is based on conventional fabric forming technologies. The 3-D fabrics are classified in following groups:-
3-D WOVEN FABRICS
3-D fabrics with multiple warps:-
Such fabrics are also referred as �multi-layer fabrics�. They are the most important group among 3-D fabrics in terms of actually consumed volume in the future. Since they are based on weaving technology dating far back, multi-layer fabrics may also be regarded as the entry point into 3-D fabrics.
3-D fabrics with multiple wefts:-
The basic construction of this fabric is similar to that of multi-layer fabrics. Three yarn system which are precisely arranged rectangularly result in a compact fibre package featuring a cross-section with often widely varying rectangular shapes. Development work for this type of fabric was mainly carried out in the sixties and seventies in Europe (UK, FRANCE) as well as in Japan and United States.
Woven 3-D shapes:-
3-D shaped fabrics are not based on compact yarn packages but are flat fabrics forced into curved shapes. Therefore, we are looking at two-dimensional fabrics which have been transformed into a three-dimensional shape to suit the component geometry.
Circular braids are manufactured in compact form (for ropes, ship�s lines) as well as in tubular form (cable insulation). Tubes, ribbon, and ropes are braided using an ancient manufacturing technology. The braiding bobbins are in motion on two circular paths enabling counter-rotation around a centre point (braiding point), where the yarns are brought together to form the braid. Interlacing of the yarns is achieved by alternating transfers of braiding bobbins from the inner to the outer circular path.
Typical applications of circular braids are sports goods (tennis rackets, skis, bicycle frames and wheel rims), medical engineering (human implants), general engineering (torsion rollers in paper machinery) and lightweight construction elements (robot technology, aerospace engineering).
Compact 3-D braids:-
These braids are mostly yarn structures without cavities. The compact 3-D braids enable yarns to be guided through the centre of the braid. This technology was first developed in the USA and in FRANCE in the early seventies, in order to limit to a minimum delamination tendencies in materials.
3. 3-D KNITTED GOODS:-
Multi-axial warp knitted goods:-
In multi-axial warp knitted goods, we find up to 7 layers of stretched layers of fibres held together by a very thin polyester yarn. Only this interconnecting yarn forms the loops and run vertically to the plane of knitting.
Warp knitted 3-D shapes:-
In this type of knitting, needles are controlled individually and temporarily put out of function during the manufacturing process. A special attachment avoids the build-up of tension in the ends not actively used in the knitting process.
What is Spacer Fabric?
Spacer textiles are 3-D fabrics that comprises of two outer textile substrates that are joined together and kept apart by an insert of spacer yarns, mostly monofilaments. This creates a ventilated layer of air, allowing heat and moisture to escape. To enhance specific stiffness of textile composites (i.e. stiffness referring to weight), top/bottom layers of woven, uni or multiaxially oriented high performance fibres are separated by spacer structures like foams, honeycomb or fibres/textile spacers.
Spacer fabrics are produced in the knitting industry on warp knitting machines that work on the principle of rib knitting technology with two needle rows. Plain as well as colour and design and surface texture effects can be produced on the face of the fabric knitted by the cylinder needles with up to 15mm between the two fabric faces.
The construction of the ground fabric, chosen according to the purpose of the final product, is achieved by combining lapping and threading in such a way as to obtain the desired strength. The yarn material used to join the plain ground fabric at the front side and the plain ground fabric at the back side with a defined "space" is mostly a stable, pressure-tolerant material. Such fabrics are usually made of polyamide or polyester monofilament, but glass threads can also be used. All other yarn materials can be used as necessary.
Spacer fabric is a lightweight fabric comprising two outer textile substrates joined together and also separated by a ventilated inner layer of spacer yarns, to allow heat and moisture to escape. Various properties can be added to the fabrics, including anti-microbial, anti-mildew, anti-static, flame-retardant, absorptive, water-repellent and abrasion-resistant attributes.
On weaving the fabric keeps its shape and, depending on the construction, is also stretchable, flexible and soft. Garments made out of this versatile fabric are also comfortable to wear. The 3 mm air-filled gap between the two layers of fabric is said to create a comfortable microclimate around the body. The dimensionally stable and the perforated construction on the upper side of the fabric also offer a number of exciting design possibilities. The mesh construction provides the garment with additional functional characteristics, by increasing its breathability and ensuring that it is crease resistant.
3-D WOVEN SANDWICH FABRIC
In the year a new kind of sandwich structures basing on 3-D woven fibre preforms has been developed. The main feature of this material is the integral connection of the woven skins by z-directional pile-fibre systems. This concept allows an easy manufacturing of highly damage tolerant light-weight structural panels.
The 3-D fabric consists of two layers, connected by orthogonal threads, resulting in a textile sandwich preform. The production of 3-D fabric is based on velvet weaving technology. Velvet is produced by cutting the connecting threads between the two fabric layers of a 3-D fabric.
The Integral Composite Sandwich structure consists of 3-D fabric impregnated with polymer resin. The result is a sandwich structure where top and bottom core are completely integrated with core. Hence, these structures combine the well known advantages of a sandwich construction with some unique characteristics not found in most other sandwich structures:
� High damage tolerance.
� Extremely high delamination resistance.
� Able to contain functional foams.
� High and well-distributed energy absorption.
APPLICATION OF 3-D WOVEN SANDWICH FABRIC
1. Subfloor of Helicopter
The sub floor of a helicopter is responsible for the energy absorption being necessary to avoid injury of the passengers after a crash from low flight height. This situation occurs after an engine failure in ground-near flight-operation when autorotation is not possible. The sandwich-based floor is necessary because fuel and various electrical equipment have to be placed in the sub floor. Picture shows the concepts for energy absorbing structures based on 3-D panels. To enable a good energy absorption a special triggering device was developed leading to a cutting of the pile fibres.
2. Bus floor
The traditional wood plate bus floor was intended to be substituted by a new one formed by 3-D sandwich panels. The objective was to obtain a considerable weight reduction and to improve passive security thanks to the high specific strength of composite materials.