32 Testing for woven fabrics – Geometric and tensile

S. Sundaresan

Introduction

The reasons for testing fabrics are the same as those for testing fibres andyarns, viz. to check whether the product is up to the required specification or not.The performance of a fabric depends on its various characteristics andproperties, some of the major ones being their thickness, weight per unit area,thread density, pilling resistance, bursting strength, tensile strength, tearresistance etc. The measurement of all these require carefully designedinstruments and equipments.

Great care must be taken in making the measurements. We consider thesimple measurement of thickness of a fabric as an example. Fabric being acompressible material, the thickness would obviously be pressure dependent sothe pressure at which measurements have to be made has to be specified.

Woven fabric

Interlacement of warp and weft in a particular design manner. For making woven cloth loom is used. Woven fabric structure is also important.The structure of fabric depends on the end use application of fabric produced.

Testing of fabrics becomes essential. The reasons are

Check the specification.
Note the change in structure.
Note the change in physical, chemical, heat & weather etc. To investigate complaints.
Design the fabric for specific purpose.

Know the fabric properties in turn by the interaction of fibre, yarn or weave.

Different tester for different use

Tensile strength-ordinary dress material. Shirting, suiting etc. Wear & abrasion resistance-work’s man cloth

Tearing strength-Ribbon, tapes, bandage cloth, Bursting strength-Parachute cloth, filter cloth, non-woven, nets &Knitted. Structural properties

Cover factor

Yarn linear density Structure
Mass per unit area Crimp
Fabric thickness
Surface properties of fabric Face and back of the cloth
Warp and weft direction of cloth Use of counting glass
Find ends/inch Pick/inch
Warp count Weft count GSM
Thickness of fabric
Crease recovery ( warp and weft way)

Stiffness of fabric ( Flexural rigidity and bending modulus)

Geometric properties Cover factor Warp cover factor
Ends /inch ÷ √ warp count in N e K1
Weft cover factor
Picks/inch ÷ √ weft count in Ne K2

Cloth Cover factor

K1 + K2 – (K1K2)/28

Stability of fabric

Stiffness of fabric

This test method covers the measurement of stiffness properties of fabrics. Bending length is measured and flexural rigidity is calculated. Bending stiffness is a very important property which affects the appearance and comfort of a garment. It is defined as the bending momentrequired to produce a given curvature. It depends on the number of yarns being bent in the fabric and the bending rigidity of those yarns. A 6”x1” fabric strip is mounted on thehorizontal platform such that it overhangs like a cantilever and bends downwards.The mean values of the bending length in warp and weft directions are reported

Bending Modulus

The bending modulus is independent of the dimensions of the strip tested so that by analogy with solid materials it is a measure of ‘intrinsic stiffness’.

Where T = fabric thickness (mm)

G = Flexural Rigidity

Fabric thickness

A piece of the fabric is placed on the reference plate of the instrument ensuring that there are no creases in the fabric. While placing the fabric it should not be subjected to any stretching. The pressure foot is gradually brought down and after allowing it to reston the fabric for 30 seconds, the gauge reading is taken. The fabric thickness is read at 10different places on the sample and the mean of these readings is taken as the averagemeasured thickness of the sample.

GSM of Fabric

The round cutter is kept on the surface of the fabric and the weight ofthe round sample is measured with the help of the balance. On multiplying the weight by 100 we get the GSM of the fabric.

Thread density of fabric

In a woven fabric the warp yarns are sometimesreferred to as ‘ends’ and the threads of weft are referredto as ‘picks’. In the construction of any fabric the numberof ends and picks per unit length

Crease recovery of fabric

crease is caused by folding the fabric onitself and is identified by the sharp bend at the fold.If present where required it has utility, otherwise it is undesirable. Crease resistance is defined as the resistance of the fabric to bending deformation andcrease recovery is the ability of the fabric to recover from the imposed deformation. Ten fabric samples each 2”x1” or 4 cm x1.5 cm are cut in the warp / weft directions using the template.The specimens are folded end to end in half with their edges gripped in one line with the help of tweezers. Half of the test specimens in both cases are folded face to face and the other half back to back. The folded specimens are placed on the loading device and allowed to develop the crease under a load of two kg for one minute. The weight is removed and the sample is placed with the help of forceps in the clamp with half of the sample hanging vertically downwards. As the recovery takes place, the disc is rotated so that half the sample continues to hang vertically downwards. After a recovery time of one minute the crease recovery angle is read on the engraved scale.The specimen should be conditioned and tested in a standard testing atmosphere. The mean value of the crease recovery angle to the nearest degree is reported for the warp way and weft way test specimen.

DRAPE

The term used to describe the way a fabric hangs under its own weight determines how good a garment looks in use.It differs from fabric to fabric and depends on end use.A particular value cannot be classified as either good or bad. The multi direction curvature formed is dependent on shear property and bending stiffness.

Fabric Strength Test

Tensile strength Tear strength

Bursting strength

Fabric Strength

Breaking strength is a measure of the resistance of the fabric to a tensile load or stress in either the warp or weft direction. Tearing strength is a measure of the resistance to tearing of either the warp or weft series of yarns.Bursting strength is a measure of composite strength of both the warp and weft yarns simultaneously and indicate the extent to which a fabric can withstand a bursting type of force with the pressure being applied perpendicular to the surface of the fabric

BREAKING STRENGTH

Mainly used for woven fabric because the tests are unidirectional and woven fabrics haveunidirectional yarns. Factors that produce a strong fabric include fiber content, yarn size and type, weave, and yarns per inch.Breaking strength is expressed in pounds and the elongation is expressed as a percent and in both cases a number of warp tests and weft test are performed.

IMPORTANCE OF BREAKING STRENGTH TEST

Breaking strength is important in various end uses such as automotive safety belts, parachute harness and pants.Breaking strength can also be used to test the effects of destructive forces upon a woven fabric including sunlight, abrasion, laundering etc.

TEARING STRENGTH

Tearing strength is the force required to continue a tear or rip already stated in a fabric.This test is used mainly in woven fabrics because the test is unidirectional and woven fabrics have unidirectional yarns. A number of tear strength test is carried out and the average is taken to determine the tear strength .Tearing strength is expressed either in pounds or grams.

Tensile Strength

The tensile strength of a fabric, that is its strength when a load is applied in either the warp or weft direction of the fabric. The warp direction of woven fabrics is often stronger than the weft simply because there may be more threads in that direction.Also, the warp yarns probably have more twist than the weft to give them the greater strength they need during weaving.

Objectives of Tensile Testing

To check the fabric’s conformity to specifications.To note the effects of changes in structural details and chemical processes.To obtain some indication of probable performance in use.To investigate causes of failure and customer complaints.To study the interaction of fibres, yarn and fabric properties.

Testing method

Three tests may be used.

Ravelled strip method Cut strip method
Grab method
Ravelled strip method

Tension Test on a strip of fabric

Specified specimen width is secured by ravelling away yarn Specimen size:
Width of 2 inches and 8 inches length
To cut the samples of 12” x 2.5”, the extra length apart from the above specimen is used for gripping in jaws.
The threads form both the edges are removed until the width is reduced to 2” exactly.
The rate of traverse for the bottom clamp 12”/min

Cut strip method

Tension test on a strip of fabric, the specimen width is secured by cutting the fabric.
Specimen size: Same as ravelled strip method Width of 2 inches and 8 inches’ length
To cut the samples of exact width and no ravelling of the sample is necessary.
The rate of traverse for the bottom clamp 12”/min.
This method is used for coated and heavily sized fabric, where ravelling of thread is difficult.

Grab Test method

Tension test on a fabric, in which only a part of the width of the specimen is gripped in the clamps.
Ex: If the specimen width is 6” and the width of the jaw is 2”, the specimen is gripped centrally in the clamps.

Specimen size:

Test Sample size 4” x 6” are cut from the master sample.
The 6” length is parallel to the yarn to be tested and is dependent on the gauge length.
In setting the testing instrument the clamp must be set 3” apart. The rate of traverse for the bottom clamp 12”/min.

Tear Strength

A fabric tears when it is snagged by a sharp object and the immediate small puncher is converted into long rip by what may be a very small extra effort.
It is probably the most common type of strength failure of fabrics in use.
The tearing force is related to the energy loss by the following equation: Energy loss = tearing force X distance

BALLISTIC STRENGTH TESTER

Consist of a stand to which is fixed a semicircular (quadrant) scale
Scale calibrated in inch.lbs to indicate the energy required to break the specimen Weighed pendulum is fulcrum at centre
Grip is provided on the bob of the pendulum and another at base of the instrument at

RHS

Catch is provided at the top , RHS of the scale to hold the pendulum
Pendulum is raised to catch on the right hand of quadrant and kept in position Pointer is placed near to anticipated breaking position
Lea to be tested is secured to base grip and grip of pendulum bob in proper way without any twist
Catch is then released & pendulum swings to other side pulling the sample & pointer and rises to a height & ruptures the yarn
Position of the pointer reads the energy spent in rupturing the sample in inch.lbs Experiment is repeated for 16 samples & mean is calculated.

Bursting Strength

Bursting strength is a method of measuring strength in which the material is stressed in all the directions at the same time and is therefore more suitable for materials such as knitted fabrics, lace or non-woven.
Fabrics used in parachute, filters, sacks and nets are simultaneously stressed in all the directions during service. In service, a fabric is more likely to fail by bursting than by a straight tensile fracture;
Example: The stress present at elbows and knees of clothing
Importance of Bursting Strength
Tensile test is unidirectional and thus suitable for woven fabrics where definite warp and weft direction strength is measured.
In case of knitted or nonwoven fabrics, where no definite alignment of yarns/fibres is there, multidirectional force is required.
Some fabrics (even woven also) stressed in all direction during use (parachute, filters, sacks, nets etc.)
These types of fabrics more likely to fail by bursting in service than it is to break by a straight tensile fracture.

SAMPLING:

Cut circular fabric specimen so that the specimen is ½” greater in diameter than the outside diameter of the clamp ring
The specimen should be free from fabric faults. Avoid selvedges Avoid same end in the different specimens to get reliable results.

TEST PROCEDURE

The material to be tested is taken and clamped by a ring over a thin flexible rubber diaphragm.
The material is clamped over a circular hole in the upper face of the reservoir.
The pressure i.e. the liquid is increased by values (or) screw driver piston. Due to increase i.e. pressure, the diaphragm bulges, taking with it the specimen.
At some point the fabric bursts and the pressure at that point is indicated by the pressure gauge.
Average strength, CV% are calculated.

FABRIC ABRASION RESISTANCE

Abrasion resistance is the ability of a fabric to resist surface wear caused by flat rubbing contact with another material. Abrasion is the rubbing away of the component fibres and yarns of the fabric.
Abrasion is of three types :
Plain or flat abrasion – A flat area of material is abraded.
Edge abrasion – Kind of abrasion which occurs at collars and folds.
Calculate the weight loss %, which is the measure of abrasion resistance (if the weight loss is less, the fabrics is highly resistant to abrasion).

PILLING

Pilling is a problem in staple fibre fabrics.
Pills are small knots or balls of large number of small fibres. Accumulated on the surface of the fabric.
Entangled due to the mild frictional action during processing/wearing.

Factors responsible for pilling

Fibre characteristics-morphological, chemical, structure, length, strength, extensibility.
Yarn characteristics-blend, count, twist, ply, hairiness, irregularity. Fabric Characteristics- weave, ends and picks
Frictional/ abrasive force-linear/ rotational rubbing forces during wear.

Fiber characteristics

Morphological: Fibers with smooth surface, cylindrical shape & cross section have greater tendency to pilling.
Chemical: Pilling depends on the chemical composition of fiber. Synthetic fibers are more to pilling than natural fiber.
Fiber Structure, Fineness, Length, Strength, Orientation of the chain molecules, short staple length

Yarn characteristics

Plying: Less pilling than Single Yarn
Regularity: Uneven Thick and thin yarn have great Tendency to pill.
Hairiness: Greater the Hairiness, More pilling.
Short fibers: Higher shorter fibers – Greater Pilling.

Fabric Characteristics

Weave: Open structure, long float length which increases pilling-Twill , satin weaves.
Knitting: Higher the interlacement
Ends & Picks: Higher the EPI &PPI

METHOD OF TEST

A specimen (125 mm x 125 mm) is cut from fabric (2 for warp 2 for weft). Stitched face-to-face and turned inside out.
Placed round a rubber tube 6 inch long, 1.25in. outside dia and 1/8 in. thick. Cut ends of the fabric are covered by cellophane tape.
Four such tubes are placed in a box (9 in. x 9 in. x 9 in.) lined with cork 1.8 inch. in thick.
The box is rotated at 60 rpm for 5 hours.
For garments which are normally subjected to repeat washing as well as to wear, washing may be done prior to sample preparation.

Visual Assessment of Pilling

After the test is over, the extent of pilling is assessed visually by comparison with the arbitrary standards 1,2,3,4 and 5.

5 – No change (very weak formation of pills) 4 – Slight pilling (Weak formations of pills)

3 – Moderate pilling (Moderate formations of pills) 2 – Severe pilling (Obvious formations of pills)

1 – Very severe pilling (Severe formation of pills)

CRIMP

When warp and weft yarn interlace in fabric they follow a wavy or corrugated path.Crimp% – Measure of waviness in Yarn.Crimp % – Excess of length of the yarn axis over the cloth length.

Crimp and its influence on fabric properties
Resistance to abrasion Shrinkage
Fabric behavior during Tensile Testing Faults in fabric
Fabric design Fabric costing

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