Indian Journal of Fibre & Textile ResearchVol. 33, June 2008, pp. 146-150

Te~sile and tearing properties of PVC coated biaxial warp knitted fabrics underbiaxial loads

Yixi Luo', Hong Hu & Raul FangueirobCollege of Textiles, Donghua University, 2999 North Renmin Road, Songjiang 201620 Shanghai, China

Received 20 April 2007; revised received and accepted 30 October 2007

The tensile and tearing behaviour of biaxial warp-knitted fabrics made with polyester fibre and coated with PVC underbiaxial tensile loads has been studied. The typical tensile and tearing load-extension curves under biaxial tensile loads arepresented and discussed. The failure mechanism of samples, in particular, by evaluation of the influence of crack length anddirection under biaxial tensile loads is observed and analysed. The results show that the tensile properties of biaxial warpknitted fabrics coated with PVC depend on the yarn linear density, the densities of insertion and stitch yarns, the regularityof yarn tension during knitting and the quality of coated fabric. The pre-cracked tearing strength decreases with the increasein initial crack length and the maximum extension supported by a sample is increasing with the initial crack orientation.

Keywords: Biaxial warp knitting, Biaxial tensile load, Coated fabric, Polyester fibre, Tearing properties

IPC Code: Int. Cl.8 DO4C

Some studies have been undertaken on the tensileand tearing behaviour of soft composites reinforcedwith woven rabrics.3-6 However, only fewinvestigations have been made on the mechanicalperformance of soft composites reinforced withmultiaxial NCP7, above all with biaxial tensile means.The present study is therefore aimed at investigatingthe tensile and tearing behaviour of PVC coatedpolyester biaxial warp-knitted fabric under biaxialtensile loads.

1 IntroductionCoated fabrics have a very wide range of

applications. As soft composites, they can bestretched in multi-directions. The tensile and, tearingproperties of these fabrics are very important featuresrequired ,by their end-use applications, especially insome particular roofing membranes, airship, inflatableboats, rescue tents, etc.!

Woven fabric is the traditional reinforcement ofsoft composites. Biaxial and multi axial non-crimpfabrics (NCF) are a relatively new class of textilepreforms for soft composites. The most commonlyused types of NCF are biaxial, triaxial and quadraxialfabrics in which straight, uncrimped yams are alignedin the warp (0°), biis (30°< e <90°) and/or weft (90°)directions to provide multi-directional in-planeproperties. Besides higher production compared towoven structures, the biaxial and multiaxial warpknitted structures also provide better mechanicalproperties in tensile and tearing propagationresistance.l,Z

2 Materials and Methods2.1 Materials

Two types of biaxial warp knitted polyester fabricscoated with PVC matrix on both sides were selectedfor the study. The structural parameters of thematerials are given in Table 1.

'fable I-Structural parameters of materials

Parameter Material 1 Material 2

Polyester500*1000

Polyester500*500

99

440

PVC

0.37

18

610

PVC

0.56

'Present address: Department of Textile Engineering and DressDesigning, Shaoxing University, 312000 Shaoxing, Zhejiang,China.b-To.whom all the correspondence should be addressed.Pr~sent address: Department of Textile Engineering, University ofMinho, 4800 Guimaraes, Portugal. .E-mail: rfang@det.uminho.pt

Fibre

Yam liner density(warp*weft), dtex

DensityEnds/inchPicks/inch

2Mass, g/mCoating material

Thickness, mm

LUO et ai.: TENSILE & TEARING PROPERTIES OF PVC COATED BIAXIAL WARP KNITTETh FABRICS 147

2.2 MethodsThe dimensions of the samples used for the tensile

tests are shown in Fig. 1. The tensile tests were carriedout with TEXTEST multiaxial testing machine(Fig. 2), with a cross-head speed of 10 mmlmin. Thisequipment was developed by Centre for TextileScience and Technology, University of Minho,Portugal. Three tests were carried out for each material.

Pre-cracked tearing tests under biaxial tensile loadswere carried out with TEXTEST multiaxial testingmachine. The sample is cut to a small crack initiallyin the central part. Initial crack lengths (a) were 5,10and 1:$ mm and their orientation (a) were 0°, 15°,30°,45°, 60°, 75° and 90° (Fig. 3).

3 Results and Discussion3.1 Tensile Behaviour

The typical load-elongation curves of Materials 1and 2 under biaxial tensile loads are shown in Fig. 4.It is observed from Fig. 4 (Material 1) that the loads

Fig. 3-Samples for pre-cracked tearing test

Fig. 2- TEXTEST multi axial testing machine Fig. 4-Biaxial tensile load-elongation curves

148 INDIAN J. FIBRE TEXT. RES., JUNE 2008

in the warp direction are higher than those in the weftdirection. This is due to the fact that the influence ofstitch yarns in the warp direction is higher than that inthe weft direction. In fact, besides the insertion andstitch yarns, other factors are also influencing thetensile properties, such as the regularity of the yarntension during knitting, quality of coated fabric,structural uniform of the yarn density, etc. It is foundfrom Fig. 4 (Material 2) that the loads in the weftdirection are higher than those in the warp direction.As the stitch yarn is finer than the insertion yarn, thetensile properties of the coated biaxial fabric mainlydeptind on the insertion yarns. As shown in Table 1,the yarn linear density and the density of insertionyarns are different in the warp and weft directions.This leads to the difference of the curves along thesedirections. The higher linear density of the insertionyarns in the warp direction leads to a higher tensilestrength in this direction.

3.4 Failure Modes

Figure 7 shows the failure modes of materials. Forbiaxial tests, the initial crack progressively takes anelliptic shape, but without any wrinkle at the notchtips. In the ideal case of a perfectly balanced fabric,the crack propagates perpendicular to the locally moststretched yarn direction, depending on the crackorientation. The fabric studied is not perfectlybalanced. Stiffness in warp and weft directionsindeed is not equal. Moreover, the tensile strength of

3.2 Influence of Initial Crack LengthThe tearing load-extension curves for different

initial crack lengths, oriented at 450 are giv.en in Fig.5for samples cut parallel to the warp and weftdirections. Table 2 shows the results of differentinitial crack lengths obtained in the tearing tests. Themaximum force supported by the material decreaseswith the increase in initial crack length. This is due tothe fact that the number of cut yams increases withthe increase in initial crack length. The tearingstrength of Material 1 is lower than that of Material 2as the linear density and the density of the insertionyams of Material 2 is higher.

3.3 Influence of Crack OrientationThe crack orientation influence is shown in Fig. 6.

Table 3 shows tqe results of different initial crackorientations obtained in the tearing tests. For a givencrack length, it is observed that the maximumelongation and load supported by a material isincreasing with the crack angle.

0 5 10 1S 2) 25

Elongation (%)Fig. 5- Tearing load-elongation curves for different initial cracklengths

Table 2- Tearing test results 9f initial crack lengths

Initial crackorientation

deg

Initial cracklengthrnm

Material Material 2Elongation, % Load,N Elongation, % Load, N

Warp Weft Warp Weft

454545

51015

21.9820.0419.05

762.44681.47647.31

604.25536.51500.25

24.3120.2818.80

1268.021068.671058.12

1544.261339.151255.20

LUO et ai.: TENSILE & TEARING PROPERTIES OF PVC COATED BIAXIAL WARP KNITTED FABRICS 149

g)

Fig. 6-Crack orientation influence

(d) (e) (f)

Fig. 7-Faill.tre modes of Material! [(a) 30° , (b) 45°, and (c) 60°] and Materia12 [(d) 45°, (e) 60°, and (f) 75°]

Table 3- Tearing test results of initial crack orientationsI

Initial crackorientation

deg

Initial cracklengthmm

INDIAN J. FIBRE TEXT. RES., JUNE 2008150

Industrial Importance: The strength of the materialdecreases with the increase in initial crack length;high quality coated fabrics are required in order toavoid initial cracks and long initial crack lengthsshould be repaired on time.

Material 1 is higher in warp direction, and the tensilestrength of Material 2 is higher in weft direction. Thecrack propagation often occurs in weave direction(warp or weft). For Material 1, the crack propagationoften occurs in warp direction at first, and then in weftdirection. This is due to the fact that the tensilestrength of Material 1 is less in weft direction. Theyams in weft direction break at first. For Material 2,the crack propagation often occurs in weft directionfirstly and then in warp direction. This is due to thefact that the tensile strength of Material 2 is hi.gher inwef\ direction. The yams in warp direction break atfirst~followed by the yams in weft direction.

AcknowledgementThe authors are thankful to the European

Commission for sponsoring this project (Asia LinkProject 2004/82158).

4 Conclusions4.1 The biaxial tensile properties of PET fibre

biaxial warp knitted fabric coated with PVC dependon the count and density of the insertion and stitchyams, regularity of the yam tension during knittingand quality of coated fabric.

4.2 The pre-cracked tearing strength decreases withthe increase in initial crack length and the maximumelongation supported by a sample is increasing withthe initial crack orientation.

References1 Jiao W H & Chen N L, J Donghua University, 30 (2004) 91.2 Ning Y & Chen N L, Technical Text Products, 8 (2005) 19.3 Bigaud D, Szostkiewicz C & Hamelin P, Compos Struct, 62

(2003) 129.4 SzostkiewiczC & Hamelin P, Mater Struct, 206 (1988) 118.5 Wei D, Wang C, Xiang Y & Shen S Zh, Spatial Struct, 8

(2002) 37.6 Witkowska B & Frydrych I, Fibers Text Eastern Eur, 46

(2004) 42. -

7 Xu Y, Mechanical Properties of the Directionally OrientedWarp Knitted Flexible Composites, Master Thesis, DonghuaUniversity, 2006.