Directional Stiffness of Fabrics and Fabric Piles

Transcription

1 Indian Journal of Textile Research Vol. II, June 1986, Pp Directional Stiffness of s and Piles A R KAlYANARAMAN The South India Textile Research Association, Coimbatore India Receiz'ed 22 July 1985; rel'ised and accepted 29 November 1985 The directional stiffness of a single layer of fabrics and of a pile of fabrics was measured with an electronic stiffness meter designed earlier by the author. The actual stiltnessdeviated from that calculated by using Peirce's formula. The investigation is preliminary and relates to some woven and knitted fabrics. Keywords: Directional stiltness,, pile. Knitted fabric. Woven 1 Introduction The bending behaviour of a fabric is an important parameter which determines the drapability. handle and aesthetic appeal of a fabric. A fabric represents a two-phase structure of air and fibre wherein each fibre is arranged parallel in a staggered manner with others of equal or unequal length and strength and is wound helically and twisted into a cylinder of varying hardness called yarn. These rod-like structures are placed parallel to each other in a plane and are interlaced with similar structures orthogonally so as to form a woven fabric or three-dimensionally looped to generate a knitted fabric. Such a geometry introduces enormous permutations and combinations of the fibre orientation and the fabric may have highly varying physical properties depending upon the geometry and construction. Therefore the fabric properties have to be individually assessed by practical methods and stiffness measurement is one such important parameter of concern to fabric and garment manufacturers. fabric cutters and end users. With this objective, we have studied this property ip detail for the woven and knitted fabrics. The measurements were made on an electronic stiffness meter designed by the authorl. The instrumentl is based on the principle of cantilever bending. where a fabric is allowed to advance on a smooth platform and bends on its own weight. The process cuts the light falling on an optical sensor from a standard light source. The decrease in light intensity is a direct measure of the fabric bending. Thus the instrument effectively translates the overhanging length of the fabric into a measurement of its stiffness. Also, the measurement correlates very well with that obtained with the Shirley stiffness meter as reported earlier2 In the present case,the instrument is further used for studying the stiffness of fabrics of different constructions and the directional bending of fabrics and fabric piles. This paper gives a detailed estimate of the bending behaviour offabrics when they are cut at angles to the warp or weft direction. The values are aiso compared with those obtained by using Peirce's formula3. The bending behaviours of a pile made by samples cut at angles and placed one over the other and those of a few knitted samples were studied. 2 Experimental Procedure Five different fabrics. four of cotton and one of polyester-viscose, were chosen. The (abrics were made up of different types of warp count. weft count. cover and weight per square yard. Two knitted fabrics were also used in the investigations. The physical properties of the fabrics used are given in Table I. strips of 25 mm x 200 mm were cut and were allowed to bend at a straight edge as described earlier1.2 When the bending fabric completely hides the electronic sensing element. the sensor output current comes down and the length corresponding to the minimum current is noted. 2.1 The fabric strip was moved forward and backward and the strip length corresponding to the minimum current was noted. The strip was reversed upside down and the experiment was repeated. Observations were made again with live such strips. each reading being the average of 20 different observations. 2.2 The fabric strips were carefully placed one over the other for a complete overlap and the bending lengths under the conditions, described above were noted. 2.3 strips of the same dimensions were cut such that they were at 15. ~Oo. 45, etc. to the warp direction and their bending behaviour was measured as described. Also. they were placed one over the othe' and the stiffness was measured.

2 INDIAN J. TEXT. RES:, VOL. 11, JUNE 1986 The same experiments were repeated with knitted fabrics also. In the case of knitted fabrics, the above directions were with respect to the course direction. Also, in the case of knitted fabric, they were fully relaxed and the specimens used for the measurement had a tendency to curl (only at the top) and this was avoided by ironing the fabric with a hot iron and the readings were taken. As mentioned above in 2.1, all observations were made with fabrics reversed upside down also. 3 Results and Discussion The properties of the fabrics analyzed are given in Table I. The stiffness values at different directions are given in Table 2. All the fabrics show a typical hump at 15 and a hump in between 60 and 75 with both the limits included. As far as the strips are concerned, the supporting members are the yarns in the warp and weft directions and these directions are the directions of the stiffness vectors. The resultant vector should act along the diagonal of the parallelogram (in the case of woven fabrics) and this direction depends upon the warp and weft forces involved. Thus the directions corresponding to the hump represent the directions of the resultant stiffness vectors or the direction in which the warp and weft yarns offer maximum support. The nature of stiffness with directions for woven fabrics is shown in Fig. 1. The stiffness values were also calculated in accordance with Peirce's formula3 and are given in Table 3. Fig. 2 shows that variation of stiffness with direction for a woven fabric and Fig. 3, for knitted inch 63.6 per 95.3 Dhoti (cotton fabric,plain weave) 2 Shirting-I (polyester-viscoseblended, plain weave) 3 Cotton shirting 4 Cotton suiting 5 Cotton sheeting Table I-Physical Properties of s Used Wovens Picks Warp inch per yarn count Ne Weft Ne Ends cover Knitted s I Parameter RibInterlock fabric Wovenfabric Rib164.7 Ix fabric 25 39Knitted I fabric Stiffness e2 -~---- direction 2 2 Inclination ble 2-Stiffness at Different Directions of s thickness mm wt per unit area g/m

3 KAlYANARAMAN: DIRECTIONAL STIFFNESS OF FABRICS AND FABRIC PilES fabric (interlock). The calculated values are juxtaposed so. as to compare the observed and calculated one~. Some important trends of bending.-, are' discernible from Table 2. The two distinct directions, namely 15 and an angle in between 60 and 75, where stiffness is higher than in other directions, seem to be a general feature for the woven fabrics. However, for fabric 2, the hump at has flattend out and for fabric 5 both have flattened, Apart from the difference in the basic fibre of which they are made, fabric 5 and fabric 2 have some geometrically common features. Although the individual counts and ends and picks per inch are totally different for the fabrics, the following similarities are seen. For fabric 2, the ends and picks are almost the same but the warp and weft counts are slightly different. For fibric 5, the ends and picks are approximately the same and the warp and weft counts are almost equal. This perhaps may be the reason for the flat response of stiffness with direction. However, this particular observation is of interest and further work is under way. In regard to knitted samples, the rib fabrics show a gradual increase in stiffness and a hump at 67S (Table 2), implying a stiff direction, whereas interlock fabrics show a fall in stiffness at 15Q and thereafter a gradual rise (Fig. 3). This property is under further study with ~ 2.8 ~ 2 6 IL "= I INCLINATION TO WARP DIRECTtON,deg Fig. I-Directional stiffness of woven fabric (cotton shirting) fabrics knitted with varying course lengths and counts. Peirce3 has described a formula to predict the' stiffness in any direction of a fabric knowing, the stiffness values along the warp and weft directions. According to him, if C I and C2 are the stiffness values 3"5 ILl E 3'0 1Il- u Z It i= 2-8 2'4 2 6 Fig. 2-0irectional E 2 2 u 2 0 ILl ~ 1-8 IL tii o 15 Fig. 3 -Oirectional e -' MEASURED x - CALCULATED INCLINATION TO WARP DIRECTION,deg stiffness of a woven tabric {cotton shirting),with calculated values X-MEASURED 0- CALCULATED INCLINATION TO COURSE DIRECTION,deg stiffness of a knitted fabric (interlock) with calculated values 0 Table 3-Measured and Calculated Stiffnessat DifferentDirections of s InclinationMeasured S Calculated Interlock Rib I Measured Woven Calculated fabric Measured Knitted Calculated fabric Measured Calculated Measured Calculated or course deg. 79

4 INDIAN J. TEXT. RES., VOL. 11, JUNE 1986 along the warp and weft directions, the stiffness value C along the direction ex to the warp is given by the formula: C = Cl(cos2a + K2 sin2a)- 2/3 where K =(Cl/C2)3/4. The C values have been calculated and are given in Table 2 and Figs 2 and 3. One finds that the agreement is not good and it appears that Peirce's formula does not seem to exactly evaluate the directional stiffness of fabrics. When fabric strips are cut along a particular direction and placed one over the other, each angular pile exhibits a particular bending parabola of its own (Table 4 and Fig. 4).The 0, 15 and 30' piles show the same type of bending behaviour and 15" direction shows the lowest stiffness of the directions. The other four seem to follow a different stiffer parabola. However, the characteristic bending behaviour of the 4 ~'3r~~i E E-60 - ~ F-7S - G-90 -_ A-O x I lock C-30 - A D-4S - 0 _A ~ 8 _. 11"--'-. F pile is directional and needs further study. With knitted fabrics, the behaviour is different from that of woven material and also it appears to show a structuredependent directional stiffness property. Now, if one forms a pile with one strip from each at angles of 0, 15,30, etc. and gradu!llly measures the stiffness, the bending behaviour appears as in Figs 5 and 6. The values are given in Table 5. In the case of a woven fabric (Fig. 5) the stiffness comes down when the 4th layer (corresponding to the layer representing 45 ) l 4 FABRIC-l \f) \f) w3 lj.. ~ I. f= I \f), layer j I o FABRIC-2 ~ I.-- 2 L_: ~.., Inclination to warp direction deg. Inter-I \ FABRIC LAYER NUMBER Fig. 5-Bending behaviour of multi-directional overlapping piles of woven fabrics Table 5-StitTness Values of s [ Strips Cut at Specified Angles to the Warp or Course Direction when Placed One Over the Other] Stiffness Knitted fabric Woven fabric Rib NUMBER OF OVERLAPS Fig. 4-Bending behaviour of uni-directional overlapping piles (Blended shirting fabric) Table 4-StitTness _~--_._--_._._ Values of s [ Strips Cut at Angles to Warp Direction and Placed One Over the Other] Number of ' 75 90' overlaps I /~

5 4 5 0-RIB KALYANARAMAN: DIRECTIONAL STIFFNESS OF FABRICS AND FABRIC PILES x -INTERLOCK the fabric pile bends. However, in the case of knitted structures, althoj,lghthe fabric bends, the stiffness of the pile increases with the addition of each layer (Fig. 6). This observation warrants further study. The bending behaviour of piles may perhaps find applications to create certain fancy effects. 4 Conclusions. W z [::2 I Fibrics have a directional stiffness; and for plain wo"v~n fabl:ics the stiffness seems to be high around 15 and 75. Peirce's formula to calculate stiftnessin a particular direction does not seem to hold good at least for the fabrics investigated. Piles cut at an angle to warp or weft direction do not have the same bending behaviour as the piles made along warp or weft. Knitted piles have a markedly different bending behaviour. o FABRIC LAYER NUMBER 7 Acknowledgement The author is thankful to Shri A. Sivaramakrishnan for calculations and data collection and to Shri T.V. Ratnam, Director, SITRA, for his keen interest and encouragement in this work. Fig. 6-Bending behaviour of multi-directional overlapping piles of knitted fabrics is added and the trend does not however stay there and the stiffness seems to increase after the 5th fabric. The rate of increase in stiffness decreases with each pile and References I Kalyanaraman A R & Sivaramakrishnan A. Text Res J. 53 (1983) Kalyanaraman A R ~ Sivaramakrishnan A, Text ResJ. 54 (1984) Peirce F T, J Text Inst, 21 (1930) T