3D Fabrics An Overview

INTRODUCTION:
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.

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:-

1. Conveyor belting
2. Straps
3. Carpets
4. Dry felts for papermaking
5. 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 [1]. 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.

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.

The applications include:
1. Textile applications
2. Reinforcement of composites

The various textile applications include:

a) Automotive application
 Seat coverings
 Carpets
 Airbags

b) Garment application
 Hats
 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
 Bumpers
 Crash elements
 Instrument panels
 Seat shells
 Armour platings

b) Motorbike application
 Helmets
 Monocoques
 Frame parts
 Mudguards, Fenders

c) Mechanical & Process Engineering
 Pressure vessels
 Fittings

d) Medical applications
Artificial joints & Limbs
Prothesis

e) Architecture & Construction
Conical pillars
Light weight panels

f) Aerospace Industry
Radomes
 Seat shells
 Pipes

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.