The performance of any textile structure under most service conditions depends to a great extent on its bending behavior. The dependence of the drapability of an apparel fabric on its bending rigidity, in addition to its shear rigidity, is well known. The importance of the bending behavior is apparent in the folding of sleeping bags or the billowing of tents or for the apparels in the parts like knee, elbow, sleeves etc.The paper deals with detailed comparative study of present instruments available for measuring bending properties of fabrics. There are different instruments available working with different principles, hence direct comparisons can be done amongst them. Selection of type of instrument depends on principles required.
Bending, Bending Behavior, Hysteresis, Instron’s, KES
Comfort is an essential element of garments especially when garment comes in contact with skin. The body response decides the sweatability of garment. This aspect which is called as the skin sensorial aspect is expressed in terms of low stress mechanical properties. Bending is one of the important low stress mechanical properties which contributes the fabric comfort. Bending and recovery from bending is important for apparels in the parts like knee, elbow, sleeves etc.
The bending behavior of the material is expressed in terms of bending rigidity. Bending rigidity is a measure of ease with which the fabric bends. The fabrics bending rigidity basically depends on the constituent fibers and yarns from which the fabric is manufactured, the fabric construction and most importantly the nature of the chemical treatment given to the fabric. Inter-yarn and intra-yarn friction plays important role in deciding the bending behavior and the type of chemical treatment given to the material, mainly controls this frictional restraints.
Bending at low stress is more important because it has a direct relationship and greater association with fabric handle. The higher the rigidity, the lower the fabric handles value. The methods for measuring bending behavior are as described in below review.
Cloth bending recovery is measure of its ability to recover from gentle crushing; a fabric with poor bending recovery has a crumpled appearance. When crumpled gently, it can however be easily smoothed out as no permanent distortion in fiber has taken place. On the contrary in true creasing permanent distortion takes place and behavior is governed by elastic recovery of fibers.
Bending behaviour of fabric
- Cloth bending hysteresis tester
Figure 1 is a photograph of the apparatus which is shown diagrammatically in figure 2. The specimen AB is held at one end in a rotatable clamp C, which carries an index reading against a scale of degrees D, to the other end of the specimen is attached a long, light, pointer arm P which, under the action of gravity, provides the couple in the specimen. In order that the couple within the specimen should be as uniform as possible, the centre of gravity of the pointer arm should be as far from the specimen as possible. Provided the specimen is sufficiently short relative to the diameter of the scale and the distance from the specimen to the pointer centre of gravity, the curvature is proportional to the angle x and the couple to sin θ.
Figure 1: Cloth bending hysteresis tester Figure 2: Diagram of cloth bending hysteresis tester
In the present apparatus, the diameter of the scale is 15 cm; the operative specimen length l between the clamps is 0.5 cm, and the specimen width perpendicular to the plane of bending is l in., this being the standard width for stiffness testing. The edge B of the clamp is 0.25 cm from the centre of the scale so that, when the specimen is straight, its centre lies at the centre of the scale, and the pointer lies along a radius. When the specimen is bent this is no longer true but provided the angle x is less than 90, the curvature may be calculated with sufficient accuracy as x/l, which is equal to 2x cm-1 for the present apparatus if x is measured in radians, or 0.0349x cm-1 if x is measured in degrees. The angles actually read are β and θ; x is equal to β-θ.
The construction of suitable light, rigid pointers with provision for gripping the end of the specimen along a well-defined line presents some difficulty, but a reasonably satisfactory design has been evolved. The grip portion consist of a strip of 0.005 cm aluminium foil, 0.8 cm×2.8 cm folded along the centre line so as to form a channel 0.4 cm deep into which the of the specimen fits. The arm of the pointer consists of aluminium tube about 1 mm diameter and of weight 0.004 g/cm. one end of the tube is split so as to fit over the centre of the aluminium foil, to which it is attached with hard wax, the other end is flattened where it crosses the scale accurately. The extreme end of the tube may be weighted by winding it with wire if necessary. The specimens used are 0.5 in.×1in.; the mounting procedure is as follows. The aluminium foil grip is slightly opened and the specimen edge placed inside it. The aluminium foil is then pressed together by means of a Shirley creasing test loading device, modified so that pressure is applied to the grip only and not to the specimen or to wax joint. The other end of the specimen length of 0.5 cm between the grips, a jig, consisting of a flat plate which can be screwed to the back of the rotating clamp, is used to position the pointer. The clamp, with jig attached, is then rotated until the pointer is vertical, and the jig is removed. The clamp is next adjusted so that the pointer reads zero. The index on the clamp is movable, so that it may also be set to zero, thus eliminating the effect of any small initial distortion in the specimen or pointer.
Having set the pointer and clamp index to read zero, the test is commenced; the procedure to be described is quite arbitrary, but convenient in operation. A stopwatch is hung at the side of the instrument and is left running. At zero time the clamp is moved 100 anticlockwise, gently but quickly. At 15 sec, a mental note is made of the pointer reading, and the clamp moved a further 100 anticlockwise. This procedure is repeated every 15 sec until the angle x is about 900.
Figure 3 shows a typical bending-hysteresis curve for a woven fabric, the parameters are defined in table 1. the curvature amplitude used in all the tests on the fabrics was 3 cm-1 in reading off the parameters, the curvature at A was standardized at 0.2 cm-1, and that at I, B [1,2].
Figure 3 Typical bending-hysteresis curve for woven fabric
Table 1: Bending hysteresis curve parameters
- Attachment to Instron
The frictional loss in a fabric during a bending cycle is directly related to the force required to roll the fabric between parallel constraints, and a complicated simultaneous measurement of couple and curvature and a subsequent integration may be reduced to a simple measurement of force. This principle is used in the apparatus shown in figures 4 which is designed as an attachment to the Instron tensile tester. The upper assembly A is suspended from the load cell’ of the Instron, and consists of a connecting rod bearing an aluminum triangular block 1 in. thick. The Instron crosshead carries the lower assembly B which consists essentially of a truncated aluminum framework; the sloping arms are a little more than 1 in. wide and are .sufficiently thick ( ~ in.) to resist bending under normal operating loads. The arms are hinged at the bottom to facilitate the insertion of test specimens; during the test, they are rigidly attached to the stop blocks carried by the horizontal beam. The sloping arms and the faces of the triangular block make angles of 14.5 to the vertical; this choice of angle provides a geometrically convenient apparatus and simplifies the manipulation of the experimental results. The test specimens consist of two identical 15-cm loops of fabric that are attached to the block and to the arms at points C and D, respectively, so that the bending axes of the loops are horizontal. When the Instron crosshead is driven up and down, the relative vertical position of the block and arms changes, as does the normal separation between them. Accordingly, the fabric loops are rolled along between parallel surfaces of varying separation, and the load cell at any instant experiences a force, the magnitude of which is affected by both the stiffness and the bending hysteresis of the fabric appropriate for the prevailing curvature conditions in the specimens; and by suitable manipulation of the experimental results, these effects may be separated [3,4].
Figure 4: Bending apparatus-diagrammatic
- KES (FB2):-
The tester is used for pure bending tests of thin films materials such as fabrics, leather etc. A fixed chuck holds one edge of the sample, while moving chuck holds the other. The moving chuck follows a fixed orbit turning its head at an angle, so that a uniform curvature is maintained on the sample to find the relationship between the curvatures and bending moment. Clamp interval is 1 cm. and rate of bending is 0.5/cm.sec. Maximum curvature is +/- 2.5/cm. The bending parameters, their definitions and units are given in the Table 2. The typical bending moment-curvature curve is depicted in Fig. (5). Bending rigidity (B) and bending hysteresis (2HB) are calculated within the curvature of 0.5 and 1.5 and -1.5 and 1.5 respectively .
Table 2. Kawabata’s Bending and Compression Parameters
Figure (5) KES bending measurement: Typical bending moment-curvature curve for fabric in forward and backward direction.
- BES-FY SYSTEM:-
The fundamental structure of BES-FY is illustrated in figure 6 which is a quasi three point bending device combining a fixed pin with two U-shaped pins. A yarn or a fabric is clamped and hung on the two jaws and is bent between the fixed pin and the two U-shaped pins. If the yarn/fabric has no extension; then the cross-section of the yarn/fabric does not change in the whole bending; the bending between the fixed pin and the U-shaped pins follows Timoshenko’s elastic theory; and there exists only a small deformation in the bending of the yarn/fabric in the initial period. The U-shaped pins with the sample are of circular cross section. For modeling conveniently, however, it was assumed that one is circular in cross-section for shifting-point bending (fabric slides over the pin), and the other is semicircular for fixed-point bending (no sliding). The contacting part of the fixed pin is at the center point of the sample length between the U-shaped pins. The maximum deflection, x, of the sample is increased by the pulling up of the U-shaped pins. The assumed arc length, l(x), of the center line of the sample between the U-shaped pins and the reaction force, F(x), on the fixed pin are both changed with the pulling shift, i.e., the length and the reaction force are functions of maximum deflection. Therefore, by designating the center line vertical to the sample in the horizontal linear status as the X-axis and the horizontal linear sample as the Y-axis to be the Cartesian coordinates of the structure .
Figure 6: The fundamental structure of BES-FY
- TH-7 Tester:-
A new device, TH-7, is described on which the bending rigidity of circular samples, and also of square and rectangle ones, can be measured. The device TH-7 was developed by means of innovation of device TH-5 on which only rectangle samples sized 2.5 cm_5 cm could be measured. The device enables the measuring of non-textile materials, for example paper, foils and membranes; however, it was constructed mainly for measuring fabrics. It has three ranges of measured bending force. The range of measuring force of bending is from 40 to 4000 mN. The output from the device is the value of bending force Fm [mN]. This value can be measured for various sample widths, with 50mm being the maximum and the minimum being unlimited. The suggested length of the sample is 50 mm; however, textiles of 25mm minimum length can be measured, too. Materials whose thickness does not exceed 1.5mm can be bent. The distance between the clamping and the sensor jaws is 14 mm. The scheme and photography of bending the fabric on device TH-7 is given in Figure 7 .
- CANTILEVER METHOD:-
Cantilever method was used for the determination of direct indicators of fabrics bending ability (stiffness and bending modulus). The stripe of fabric, to be tested, in dimensions of 16×3 cm was placed on horizontal supporting platform and fixed in place by a weight. When the toggle switch was cut in the mechanism, it smoothly and uniformly lowered the movable side shelves of the platform, thus imparting flexural deformation to the test stripe flexed under the action of its own mass. When the side shelves were completely lowered, the flexure indicator was displaced upwards by screw showing on scale the flexure (f) of both free ends of the stripe. The size of relative deflection is not allowed to be greater than 0.65 and the size of average deflection not less than 1 cm. if at 16 cm long test stripe these conditions are not satisfied, the length of the stripe is reduced for 1 cm and relative as well as average deflection are determined again. The procedure of reducing the length of the test stripe is repeated as long as it is needed to obtain the values of relative deflection and average deflection which will satisfy the mentioned limited conditions .
Figure 8: Apparatus for determination of fabric stiffness and bending modulus (Cantilever method)
- BUCKLING METHOD:-
Two aluminium clamps were used to hold thesample sheet in place during tests. All tests were performed using a Hounsfield Universal Testingmachine. The lower clamp is fixed on the base of theHounsfield machine, while the upper clamp isconnected to a load cell which can record the appliedforce during experiments. In this study, constant displacement rates were applied to the upper clampwhich would move downwards to the requireddisplacement, and the reaction force of the upperclamp from the sample sheet during bending versusthe displacement of the upper clamp was recorded .
Figure 9: Schematic of bending test
The satisfactory method for measuring bending properties depends on the material used, weave and other working of the fabric manufactured. All above methods are good according to their principles and gives the proper results as compared with each other.Cloth bending hysteresis tester is a time consuming method but gives accuracy in result and it also determines the factors like flexural rigidity and bending modulus. Apparatus used as attachment to instron reduces the frictional loss during bending cycles. KES-(FB2) and BES-FY systems are automatic and hence give results in less time. TH-7 tester also measures non-textile materials as it was constructed mainly for measuring fabrics. Cantilever method is useful to determine fabric stiffness and also the bending modulus. Buckling method is useful for determine buckling as well as bending.
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