Opportunities and Challenges for Textile Reinforced Composites · Opportunities and Challenges for...

Opportunities and Challenges for

Textile Reinforced Composites

Christopher M. Pastore

Philadelphia University

Philadelphia, Pennsylvania, USA

Textile Reinforced Composites

� Fiber reinforced composites whose repeating volume

element (RVE) is characterized by more than one

fiber orientation.

� Formed with hierarchical textile processes that manipulate individual fibers or yarn bundles to create

an integral structure.

� It is possible to join various sub-assemblies together to form even more complex structures.

Hierarchy of Textile Materials

Perceived Benefits

� Textiles are considered to have significant cost

savings compared to tape lay-up.� Individual layer of fabric is much thicker than tape.

� Fewer lay-up steps are necessary to create the final structure.

� Formed from dry fiber and infiltrated with resin in a secondary

operation.

� Handling and storage requirements of the material are reduced compared to prepreg.

� A single product is suitable for a variety of matrix materials,

reducing inventory and manufacturing costs.

2D and 3D Textiles

� Textiles are frequently classified as either 2D or 3D.

� Clearly all fabrics are 3D, but 2-D implies that the

fabric is fundamentally thin. � That is, the thickness of the fabric is formed by only 2 or 3 yarns in

the thickness direction.

� A 3-D fabric can have substantial thickness, limited

only by the machine, not some fundamental physical phenomenon.

Types of Textiles

� Direct-formed fabrics are those made directly from

fibers.

� Woven, knitted, and braided fabrics are made from

manipulation of yarns.

� These four classes represent the vast majority of

fabrics used in composite materials. � woven fabrics are formed by inter-lacing yarns,

� knitted by inter-looping yarns,

� braided by inter-twining yarns, and

� direct formed fabrics by inter-locking fibers.

Direct Formed Fabrics

� Created directly from fibers without a yarn processing

step involved.

� No interlacing, intertwining, or interlooping of fibers

within the structure.

� These fabrics are called nonwovens in much of the

literature, despite the obvious inadequacy of this

term.

Direct Formed Fabrics

� Generally there are 2 steps� First a web is constructed of fibers. This sets the distribution of in-plane fiber orientation.

� Next the web is densified. This typically involves through thickness

entanglement or bonding.

Web formation

� Opening process: mechanically separates the fibers.� Deposit fiber mass onto a belt, creating a continuous roll of low-density material � width of roughly 1-meter and a thickness 10-20 cm called a picker lap.

� The fibers have a virtually uniform, random orientation in the plane,

with substantial out of plane orientation.

� To thin the picker lap, it may be passed through a card.

� Individual fibers are mostly oriented in the direction of material flow through the machine. � This orientation allows the fibers to pack closer than previously resulting in a

thickness reduction, increased density, and a preferred distribution of fiber

orientations in the machine direction.

� The resulting material is called a carded web.

Densification of web

� The carded web may be used as input to the needle

punch, or it may be cross-lapped first. � The cross-lapper places carded web transverse to the machine

direction allowing the preferred fiber orientation to be in the cross

direction.

� Needle punch creates mechanical interlocking

through barbed needles

� Bonding can be done to chemically adhere the fibers� Adhesive application

� Thermal bonding (sacrificial low melt fibers are pre-included in the

web)

XYZ Orthogonal Nonwoven

Knitted Fabrics

� There are two basic types of knitting - weft knitting

and warp knitting.

� They are distinguished by the direction in which the

loops are formed. � Weft knitting, the most common type of knitting in the apparel industry, forms loops when yarns are moving in the weft direction,

or perpendicular to the direction of fabric formation.

� Warp knitting differs from weft knitting in that multiple yarns are

interlooped simultaneously. A set of yarns are supplied from a creel or warp beam and interlooped in the cross (course) direction.

Jersey Knits

� The simplest weft knit structure is the jersey.

� Inherently bulky due to curvature

of the yarn.

� The “natural” thickness of a jersey knit fabric is roughly three times

the thickness of the yarns, resulting in maximum yarn

packing factors of 20-25%, and

thus Vf around 15%.

� High extensibility (up to 100%

strain to failure) which allows

complex shape formation capabilities.

Rib Knits

� In a rib knit structure it is possible to incorporate large yarns in the weft to create a weft inserted rib knit.

� In such a way a “unidirectional” preform can be constructed. However

it is difficult to achieve fiber volume fractions greater than 30% in these structures due to the inherent bulkiness of the fabric.

Conformable Rib Knit

Warp Knits

� In the WIWK, the load bearing yarns are locked into

the structure through the knitting process

Braiding

� Biaxial braided fabrics may incorporate a longitudinal

yarn creating a triaxial braid.

� The braided fabric is characterized mainly by the

braid angle, θ, (10° - 80°).

� Braids are tubular and frequently compared with

filament winding. They have been shown to be cost

competitive.

� The braided fabric is flexible before formation, and

thus the fabric can conform to various shapes. The

braided fabric may be formed around a mandrel, and

rather complex shapes can be formed.

Braiding

� Braids are formed by a circular “maypole” pattern of

yarn carrier motions

Types of 2D Braids

3D Braiding Machine

Woven Fabrics

� Generally characterized by two sets of perpendicular

yarns systems

� One set is raised and lowered to make “sheds” (these

are warp yarns)

� The other set is passed through these sheds,

perpendicular to the warp yarns (these are fill, or pick

or weft yarns)

Elements of a loom

Woven Fabrics

� The structure of the woven fabric is the pattern of

interlacing between the warp and weft yarns

� Yarns can “float”, or not interlace for some distance

within a woven fabric

Basic weave structures

Crimp in Weaves

� The crimp is defined as one less than the ratio of the

yarn's actual length to the length of fabric it traverses.

� Crimp levels influence fiber volume fraction,

thickness of fabric, and mechanical performance of fabric.

� High crimp leads to� Reduced tensile and compressive properties

� Increased shear modulus in the dry fabric and the resulting

composite

� Fewer regions for localized delamination between individual yarns.

Applications of Weaves

� Weaves can be formed into composites with fiber

volume fractions as high as 65%.

� High harness count satins – 8 and 12 –serve the role

previously held by 0/90 tape lay-ups.

� There is a significant cost benefit to using the fabrics

in that much fewer layers need be applied because

the woven fabric is usually many times thicker than the tape (depending on the yarns used in the fabric).

3D Weaves

Layer-to-layer Through thickness

XYZ

Doubly Stiffened Woven Panel

Variations in Weave Design

� If large yarns are used in the warp direction and small

yarns are infrequently spaced in the weft direction,

the resulting fabric resembles a unidirectional

material.

� Weaves can be formed with gradients in a single or

double axis by changing yarn size across the width or

length

� Complex shapes can be achieved through “floating”

and cutting yarns to reduce total number of yarns in

some section of the part

Gradations through yarn size

Shape through floats

Issues with shaping woven fabrics

� Tailoring the cross-section of a woven fabric will

generally result in � a change in weave angle,

� a change in the distribution of longitudinal, weaver, and fill, and

� a change in fiber volume fraction in consequence to the change in

thickness.

� Some fiber volume fraction effects can be controlled

by tooling. The tailoring occurs in a discrete manner,

using individual yarns, whereas most tooling will be approximately continuous.

Example of single taper weave

� Consider a tapered panel where gradation in

thickness is achieved by changing yarn size/count

across the width

Design of tapered woven panel

� Pick count is constant,

warps and wefts per

dent are modified to

taper

� Z yarn path changes

to accommodate the

weave.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Dent

Number

Pick Columns per inch Picks per column

Warp per dent

40%

42%

44%

46%

48%

50%

52%

54%

56%

58%

60%

0.000 0.500 1.000 1.500 2.000 2.500

Distance from Thin Edge (in)

FiberVolumeFraction

Calculated

Target

Variation in Fiber Volume Fraction

� This variation in

yarn packing results

in variations in Vffor the resulting

composite.

Variation in weave angle

� The weave angle will

also change throughout

the width of the part due

to varying warp yarn

count and part thickness.

25 °

30 °

35 °

40 °

45 °

50 °

55 °

0.0 0.5 1.0 1.5 2.0 2.5

Distance from Thin Edge (in)

WeaveAngle

Calculated

Target

Yarn Distributions

� The distribution of warp,

weft, and Z yarn will also

vary throughout the part.

15%

20%

25%

30%

35%

40%

45%

50%

55%

60%

0.0 0.5 1.0 1.5 2.0 2.5Distance from Thin Edge (in)

YarnDistribution % Z % Warp % Fill

Variations in Modulus

� All mechanical properties will vary throughout the part

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0 2.5

Distance from Thin Edge (in)

TensileModulus(Msi)

E11 E22 E33