Indian Journal of Fibre & Textile Research Vol. 30, September 2005, pp. 263-266

Effect of lay-up angle on mechanical properties of composites based on rib knit jute preforms

V R GiriDev " A Sivabalakrishnan & K Raghunathan Department of Textile Technology, A C College of Technology, Anna University, Chennai 600 025

and

A Karthik

Department of Textile & Apparel Technology & Management, North Carolina State University, Raleigh, NC 27695-8301, U S A

Received J July 2004; revised received and accepted 9 November 2004

The mechanical properties of knitted jute reinforced composites with different stacking sequences have been studied. Flat rib knitted preforms have been produced in manual flat bed knitting machine followed by composite laminate prepara­tion using simple hand Jay-up technique. It is observed that the mechanical properties are dependent upon the stacking se­quence. The i mprovement in course-wise mechanical properties of the laminates has been observed with [Oo/±450/00] and [0°/90°/90% °] Jay-up sequences compared to [0°)4 lay-up sequence which shows improvement in properties in wale-wise direction.

Keywords: Composite, Jute, Knitting, Mechanical properties, Textile preform

IPC Code: Int. CI.7 D04B7/00, D06M I7/06

1 Introduction Glass, carbon, boron and kevlar fibres are being

used as reinforcing fibres in fibre reinforced plastics which have been widely accepted as materials for structural and non-structural applications. Fibre rein­forced plastics have gained importance due to their high specific modulus, stiffness-to-weight ratio and strength-to-weight ratio compared to conventional materials. These materials are expensive and find their use in high class applications. Natural fibres, like ba­nana, cotton, coir, sisal and jute, have attracted the researchers for their application in consumer goods, low-cost housing and other civil structures. Jute ap­pears to be a potential candidate because of its com­mercial availability and inexpensiveness.

Researchers have characterized and discussed in detail the mechanical properties of woven jute rein­forced composite using thermoset and thermoplastic resins for its tensile strength and modulus, flexural strength and modulus, impact strength, in plane shear strength. interlaminar shear strength and hardness 1-5 .

"To whom all the correspondence should be addressed. Phone : +9 1 -44-22203562 /09444132585; E-mail : vrgiridev@yahoo.com

Knitting. as one of the preforms manufacturing techniques for composite, has caught the eyes of re­searchers for past few decades for its ability to con­form into complex contours because of its superior drapeability. Studies on knitted glass fabrics and other high performance fibres reveal that the knitted fabric reinforced composites possess superior impact prop­erties compared to their woven counterparts. Tensile properties of knitted reinforcements are found to be poor compared to that of woven fabric reinforced composites. Several attempts have been made to im­prove the properties of knitted composites by the in­troduction of inlay at first in course direction followed by in biaxial direction, and by the variation in knit architecture, stitch density, pre-stretching percentage on fabrics and tow size of bundles6-lo. Limited work has been reported so far on the mechanical behaviour of knitted fabric reinforced with different stacking sequences. It has been found that the mechanical properties of composites are sensitive to stacking se­quence, leading to variation of properties in wale-wise and course-" ,;ise directions I I .

In this work, an attempt has been made to study the mechanical behaviour of knitted jute reinforced com­posites with various stacking sequences [0°]4,

264 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2005

[00/±450/00] and [0°/90°190°/0°] . The laminates are characterized for their tensile, flexural and impact properties .

2 Materials and Methods

2. 1 Knitting of Jute

Jute yarn (272 tex and 1 87 twists/m), spun from a jute fibre of 29.5 cN/tex bundle tenacity, was knitted on a manual V-bed flat knitting machine with a gauge of 5 needles/inch. The tenacity and elongation-at­break of jute yarn were 1 1 .50 cN/tex and 1 .67% re­spectively. Care was taken to adjust the knitting feed tension so that there were few breakages while knit­ting of jute yarns. The wales/inch and courses/inch were 6 and 1 2 respectively.

2.2 Composite Fabrication

A simple hand lay-up method was used for the preparation of composites . The working surfaces were coated with wax and polyvinyl alcohol (PV A) for the easy removal of laminates from the moulds. The ma­trix material was prepared from commercially avail­able vinylester resin (cured at room temperature) pro­cured from Vaasivibala Resins Pvt Ltd. The resin, accelerator and catalyst were taken in a weight ratio of 1 :0.02 :0.02 respectively. The resin, catalyst and accelerator were thoroughly mixed together, followed by a vacuum treatment for 3-5 min to remove en­trapped air bubbles. Prior to commencing the com­posite fabrication, the knitted jute fabrics were dried in an oven at 1 00°C for 2.5 h to remove moisture in the fabric. Each layer of the fabric is then pre­impregnated with the matrix material and placed one over the other in the mould. A brush was used to push

any air cavity to sides, before applying pressure. The resin- impregnated fabrics were cured at room tem­perature for 24 h. Laminates of three different stack­ing configurations, i .e . [0°]4, [00/±450/00] and [0°/90°190°/0°], were prepared (Fig. O. The weight fraction of the jute fibre in the multi-layer laminates was 27 .27%.

2.3 Tensile Tests

Tensile tests were carried out according to ASTM D3039 standards on a Instron tester (Model 430 1 ). The size of the specimens was 250 x 25 .4 x 4 mm and the cross- head speed was kept at 2 mrnlmin.

2.4 Flexural Tests

Flexural tests were carried out according to ASTM D790 standards on a three-point bending load using Instron tester (Model 430 1 ). The size of specimens was 1 30 x 25.4 x 4 mm.

2.5 Impact Tests

Impact tests were carried out according to ASTM D256 standards using the un-notched specimens. The size of specimens was 65 x 1 2 .7 x 4 mm.

3 Results and Discussion

3.1 Tensile Tests

The major factors which affect the tensile strength of a knitted composite specimen are the design archi­tecture, volume fraction of fibres or number of fibres bridging the fracture plane, apart from knitting pa­rameters l ike loop length, wales per inch and courses per i nch. Moreover, the anisotropy of the knitted samples i s already known, leading to superior tensile property in wale direction compared to that in course direction.

II II II • • • • • II

II • II (a) (b) (c)

GIRIDEV et (II. : EFFECT OF LAY-UP ANGLE ON MECHANICAL PROPERTIES OF COMPOSITES 265

Figs 2 and 3 give average tensile properties (tensile modulus and tensile strength) of multi-layered knitted jute fabric composites subjected to a uniaxial tensile loading. It can be seen from the figures that the ani­sotropic nature of the knitted composites to a particu­lar direction of loading is known.

The tensile modulus and tensile strength of the specimens subjected to uniaxial loading are dependent upon the yarn orientation along the applied load. From the figures, it can seen that the tensile strength of [0°]4 composites is superior to all other lay-ups in wale-wise direction, whereas in course-wise direction the sequence [0°/90°/90% °] is better than the other two lay-ups. Moreover, it should be noted that [0°/90°/90% °] specimens show more or less similar properties in both directions .

The fracture mode of [00/±450/00] is different from the other lay-ups. The [0°]4 and [0°/90°/90% °] speci­mens fracture normal to the vertical axis of the speci­men when loaded in wale or course direction. The failure in case of [00/±450/00] is unpredictable, leading to different failure modes.

Recent study on glass-epoxy system offers an ex­planation for this kind of failure by matrix digestion method12• For a particular case of a = 45° (angle of inclined surface to the applied stress), the fracture of monolayer laminate occurs along the inclined surface since the shear strength of the composites is higher than its tensile strength which implies that the failure of single layer composite subjected to the uniaxial tensile load is mainly caused by the normal (tensile) stress component on the inclined surface. On the other

� "-8 '" " ;; "8 ::E � .� " t-

3

2.5

2

1 . 5

0.5

° [0°] [0° 1 ±4So 1 0°]

Lay-up Sequence Fig 2-Tensi le modulus of multi-layer knitted jute laminate as a function of lay-up sequence

hand, the experiment shows that the fracture of [00/±450/00] laminate does not occur along a similar inclined surface. Therefore, the resulting normal (ten­sile) stress component must be smaller than the shear stress component, leading to different fracture modes .

3.2 Flexural Tests

The flexural modulus and flexural strength values of multi-layered knitted jute laminates are given in Figs 4 and 5. The flexural strength of weft knitted

50 45 40

-;;;- 35 "-6 30 ..c 0;, � 25 </) � 20 c � 1 5

1 0

: LEl.-

W] [0° 1 ±45° 1 0°] Lay-up sequence

Fig 3-Tensi le strength of multi-layer kni tted jute laminate as a function of lay-up sequence

W] [0° 1 ±45° 1 0°] [0°/90°/90°/0°] Lay-up Sequence

Fig 4-Flexural modulus of multi-layer kni tted jute laminate as a function of lay-up sequence

266 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 2005

100

90

<ll 80 .... 6 70

� 60 c:: � 50 '" � 40 I<

30 " Ii: 20

10

0

Lay-up Sequence

Fig 5-Flexural strength of multi-layer knitted jute laminate as a function of lay-up sequence

preform is determined by the strength of fibre bundles bridging the fracture planes. The fibre bundles bridg­ing the course fracture planes are less than those bridging the wale fracture planes, leading to superior wale-wise flexural property to that of course-wise property. With the change in lay-up sequence from

[0°]4 to [Oo/±4So/00 ] and [0°/90°/90% °], the im­provement in course-wise property is observed due to the contribution of more fibre bundles in course-wise direction.

3.3 Impact Tests

The impact tests involve the relatively high contact forces acting on a small area over a period of short duration with both elastic and plastic deformation, leading to fracture. While metals absorb energy in elastic-plastic deformation, the composites generally absorb energy through fracture mechanisms, such as delamination, shear cracking and fibre breakage; however, some portion of energy may be absorbed through the elastic-plastic deformation of the fibre and the matrix. The impact values of multi-layer laminates are given in Fig 6. It is observed that the impact properties improve with the change in lay-up sequencing, leading to improvement of course-wise impact strength compared to wale-wise impact strength . Similar kind of observation has been re­ported in the study of knitted glass-epoxy matrix sys­tem for a four-ply sequencing8. This is attributed to the multi-directional nature of knitted loops and in­termeshing of loops with different plies on application of load while preparing laminates, leading to superior impact resistance.

300

250

= 0. 200

'! c � 1 50 ell ... u os Po. 1 00 .5 50

O · roo]

le wa�

I o Course

[0° / ±45° / 0°]

Lay-up Sequence

Fig 6-lmpact strength of multi-layer knitted jute laminate as a function of lay-up sequence

4 Conclusions Mechanical properties, like tensile, flexural and

impact strengths, are found to vary with the stacking

sequence; [0°]4 laminates exhibit superior properties in wale- wise direction whereas [Oo/±4So/00] and [0°/90°/90% °] exhibit superior properties in course­wise direction. The fracture mode of [00/±4So/00] multi-layer laminate is found to be different compared to the other laminates when subjected to a uniaxial tensile load.

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