Cotton Fibre Maturity PDF

TRANSVERSE DIMENSIONS (FINENESS & MATURITY) 4.1 Fibre Maturity Cotton fibre maturity is measure of the secondary wall thickening of the fibres, which is established between the cessation of increase in fibre length and the bursting open of he fully developed boll. When the cotton fibres a...

TRANSVERSE DIMENSIONS (FINENESS & MATURITY) 4.1 Fibre Maturity Cotton fibre maturity is measure of the secondary wall thickening of the fibres, which is established between the cessation of increase in fibre length and the bursting open of he fully developed boll. When the cotton fibres are first formed, they start as thin tubules which, initially grow only in length. When their maximum length is reached, a secondary fibre wall then begins to build up on the inner surface of the thin primary wall. This process continues until shortly before the boll opens. After the opening the fibres dry and collapse to give the typical convoluted ribbon form of cotton. A ripened full mature cotton boll contains fibers of both mature and immature. A cotton fiber consists of a cuticle, a primary layer and secondary layers of cellulose surrounding the lumen or the central canal. In the case of mature fiber, the secondary wall thickening is very high. In the case of immature fibers, due to some physiological causes, the secondary wall thickening is practically absent, leaving a wide lumen throughout the fiber. Common cause of this is the slow rate of secondary growth, resulting in many of the fibres having developed only a thin wall by the time the boll opens. Cotton grown under favorable conditions largely consists of mature fibres with fairly thick walls, but always contains some poorly developed fibres. Figure Error! No text of specified style in document..1 Convolutions in Cotton Fibre  If growth conditions are not favorable possibly as a result of attack by disease or pests, of plant senility or occasionally because of the genetical nature of the variety, the onset of secondary wall thickening may first be delayed and then proceed at a reduced rate, and the ripened boll will contain a high proportion of poorly developed immature fibres. Harvesting period is one of the cause reasons of low maturity level. Generally,  First picking cotton - high maturity (85-90%)  Second first picking cotton - moderate maturity (75%)  Third first picking cotton - low maturity (60%) 4.1.1 Importance of maturity Because immature fibres have thinner walls, they are weaker and less stiff than the mature fibres affecting: 1. Spinnability 2. Nep Generation 3. Shade Variation 4. Yarn strength Spinnability There is an optimum degree of maturity for a cotton fibre, above which it tends to be too stiff and bristly for ease of processing and below which it tends to be too flabby and unresilient. It is not certain just where this optimum lies, though it is probably somewhere between θ=0.8 and θ=0.9. The spinner is much more concerned about the fibres those have little or no wall thickening. Nep Generation One of the main troubles caused by the presence of these thin walled immature fibers is nepping. Apart from maturity, causes like small bits or fragments of seed particles attached to the fiber also forms neps. Neps are created during processing starting at ginning stage. Further when rubbing of substances takes place, as in carding, minute knots of tangled fibers are caused and the immature fibers are more prone to this nepping effect. When fine cottons are being processed, the danger of nepping is even more acute, since even the mature fibers are likely to cause neps by faulty processing. In addition, the neps so formed are usually more prominent because of their size relative to the diameter of the yarn. Shade Variation Immaturity also affects the shade after dyeing. As the response of the primary wall to certain classes of dyestuffs is less intense, the thinner the secondary wall lighter will be shade. Hence fine cotton tends to be lighter in shade than coarse cotton. Apart from this the reflecting surfaces of the fibers of immaturity is with respect to the patches being shown or the weft bars seen in the fabric when yarn made of immature fibers or yarn spun from cotton of different maturity is used as warp and weft. So, summarizing the maturity, the following points are noted. Neps will show up as specks in the dyed cloth. Yarn Strength Immaturity affects strength of yarn since single fibre strength of cotton fibre is governed by its maturity level. 4.1.2 Laboratory Assessment A. Direct Methods Degree of Wall Thickening The extent of wall thickness development may be considered satisfactorily in terms of a ratio, the degree of wall thickening. This is defined as the ratio of cross sectional area of the wall to the area of the circle having the same perimeter as that of the fibres & is denoted by Greek letter θ. r Figure Error! No text of specified style in document..2 Cross-section of Cotton Fibre cros − sectional area of fibre wal 𝜃 = area of circle of same perimeter 𝐴 = 𝐴′ 𝐴 = 𝜋𝑟 2 𝐴 = 𝜋𝑟 2 4𝜋 𝐴 = × 2 4𝜋 𝜋𝑟 So,  4𝜋𝐴 𝜃 = Since, 𝑃 = 2𝜋𝑟 𝑃2 The ratio is unity if the fibre is completely solid & has a circular cross-section. The degree of wall thickening measures fibre maturity independently of the fibre perimeter expressing it on a numerical scale between zero & unity. The direct determination of the degree of wall thickening from measurement of the area & perimeter on fibre cross sections is not suitable for use as a practical everyday test, as it is slow & laborious errors arising cutting the sections. There are some indirect tests, which are much quicker to carry out. Caustic Soda Swelling Test The cotton fibers swell differently for mature and immature fibers due to variation of cellulose content when treated with 18% (Sp. Gravity 1.28) NaoH solution. Standard practice in U.K.: A projection microscope with traversing stage and magnification of 150x is used for the test. A thin tuft of fibers is drawn by means of tweezers and placed on glass slide and covered by a cover slip. A drop of 18% NaoH solution is added on the fibers. The slide is placed on stage of microscope. Five slides each of 100 fibres are to be tested. Frequently a maturity test is carried out on samples previously subjected to comb sorter test. Five tufts are collected at interval along the diagram. In the examination, the fibres are classified into three main groups as follows:  Normal Fibres (N)- After the swelling. appear rod-like with no continuous lumen, no well-defined convolutions.  Dead Fibres (D)- After swelling, the wall thickness is one fifth or less of the maximum ribbon width. Dead fibres vary from float ribbons, with no secondary flat fibre wall, to highly convoluted forms with a greater wall development.  Thin Walled(T)- They are not well defined. Some have a fairly rod-like form, but are not classified as normal because they have thin but continuous lumen. At the other end of the range in this group the fibres approach the dead class but their wall thickness is greater than one fifth of the ribbon- width.  Figure Error! No text of specified style in document..3 Cotton Fibre Maturity After identifying the fibres, Percentage normal and dead fibres are calculated. (𝑁 − 𝐷) Maturity ratio = + 0.7 200 Theoretical value of M will vary from 0.2 for all dead fibers to 1:2 for all mature fibers. M + 0.6H + 0.4I Maturity coefficient (Mc) = 100 Where, M - percentage of mature fibers. H - percentage half mature fibers I= percentage of immature fibers. B. Indirect methods In direct methods, have been evolved mainly to reduce time of testing and make it much simpler. Polarized Light Tests The fibres in the walls of the cotton fibre are anisotropic; they are doubly-refracting with two refractive indices n11 & n1. When plane polarized light passes through fibre it suffers retardation and assumes thickness of the fibre wall & the orientation of the fibrils to the fibre axis.  A microscope, with constant intensity source and polarizer and analyzer fitted. The test is usually made with a magnification of 100 x. A thin fringe of fibres is prepared and mounted on microscope slide protected by a cover-glass, with the fibres substantially parallel to each other. The slide is placed on traveling stage so that the fibres are parallel with the marked slow direction of the selenite plate. When the stage is advanced to bring the fibres into view beneath the objective they are seen to have different colours. Very immature fibres appear violet or indigo, immature fibres are blue, mature fibres are green & very mature fibres are yellow. Turning the slide through 900 causes, The very immature fibres to have their colour almost extinguished in the 450 position, changing to orange as the 900 position is reached, approached. The immature fibres change from blue to yellow in the 900 position. The green mature fibres tend to become more yellow in the 900 position. The yellow very mature fibres keeps the colour same on rotation through 900. The colour changes which appear on rotation are used as a check in classifying fibres subject to doubles. Under the conditions of observations used in this test, the colour of a given depends markedly upon the absolute thickness of the wall rather than on the degree of thickening. In a range of cottons of varied perimeter, it would be expected that those giving the same maturity count by polarized light method would not necessarily give the same estimates of degree of wall thickening as estimated by the swelling method. The intrinsically fine cottons of small perimeter would tend to have the same absolute thickness of as the intrinsically coarse cottons, but to be relatively more mature as judged by degree of wall thickening. The classification by the polarized light technique is subjective. There is no finite & sharp distinction between the fibre classes. The colours of each class change gradually one into another, because of the continuous form of distribution of wall thickness of different fibres in a sample. This leaves just as much scope for variation to occur in the judgment of differentials operatives as with the continuous change in the variation of the ratio of the wall thickness to ribbon width in the swelling test. Differential Dyeing method A 3-g sample is introduced into a boiling dye-bath consisting of Diphenyl Fast Red and Chlorantine Fast Green. After 15 minutes, 4% of (calculated on weight of fiber) of NaCl is added and, after a further 15 minutes, a further 4% of NaCl. When the sample has been in the bath for 45 minutes, it is taken out and rinsed three times in distilled water, after which it is centrifuged. The cotton is then rinsed in cold distilled water and carefully dried. The sample is now ground to powder in mill, thoroughly mixed, and pressed into the form of a pad. The pads are then compared visually with pads prepared from standard American cottons of known maturity as measured by the standard ASTM maturity count. Mature samples appear predominantly red and immature samples predominantly green. The test depends on two circumstances: (i) that, of the two dyes used, the red diffuses into, and also washes out of, the cellulose of the cell wall much more rapidly than the green; and (ii) that immature fibers have greater specific surface than the mature and so take up dye more rapidly. Thus, because of their greater specific surface, the immature fibers take up more green dye than the mature fibers do and, because of the slow diffusion rate of green dye, the difference between the two is not greatly affected by subsequent boiling wash. With rapidly diffusing red dye, on the other hand, a period of 45 minutes is long enough to cause both mature and immature to take up much the same amount of dye, but, in the 30-seconds boiling wash, the immature fibers lose much more of what they take up because of their greater specific surface. 4.2 Fibre Fineness and Measurement 4.2.1 Measures of Fibre fineness Diameter The quantity invariably used for defining the fineness or coarseness of a fibre was the diameter. For wool and synthetic fibres which are more preferably cylindrical, fibre diameter can be measured. For other fibres, however, which are of irregular cross sectional shape or which taper towards one or both ends, the term diameter has no useful meaning. Width Width frequently referred to as fibre diameter in early books was really the maximum width as viewed under the microscope. The convoluted fibre varies in apparent width over a wide range throughout the length of each convolution, and either the maximum or the minimum may be measured. Area of Cross Section For a given type of fibre, area of the cross section is proportional to the limiting weight per unit length at a section and if the fibre density is known, this can be easily calculated. Linear Density Linear density is the weight per unit length and provides the most useful general way of describing the fineness or coarseness of textile fibres. It is especially useful because it corresponds to the quantity used in describing the fineness or coarseness of yarn. With a rationalized system of units, the average number of fibres in the cross section of a yarn of any given size can be readily calculated. Perimeter The average fibre perimeter is referred as intrinsic fineness. Fibre perimeter is important mainly as a link between other dimensions. It is least affected by growth conditions.  Intrinsic fineness and are constant for that cotton variety. It is a characteristic of each botanically distinct species. Specific Surface It is defined as the surface area per unit volume or the surface area per unit weight of the fibre. Perimeter Specific Surface = Area of Cross Section This is very important because it governs the resistance of fibres to air flow and in turn relates to the maturity and fineness of the fibre. 4.2.2 Technical Significance of Fibre Fineness Before we go in more details of fibre fineness, let us understand some basic concepts of spinning and then we will try to relate how these properties have an influence on the same. Spinnability of a Cotton Fibre a) Finer the fibre, Finer the yarn can be spun: Some minimum number of fibres is required in the yarn cross section for satisfactory performance. This minimum number varies with staple length. For medium staple cotton about 80 fibres are required in cross section of a ring spun yarn. The finer limit of the yarn is reached earlier if the yarn is spun from coarser variety than if spun from finer variety. b) Quality of yarn (Strength and Evenness of yarn): Strength of the yarn depends on the inter fibre friction. Higher the inter fibre friction, more will be the strength. Inter fibre friction can be increased by increasing the overlap length or increasing number of fibres in the cross section of the yarn. Overlap length can be increased by using longer length fibres and number of fibre in the cross section can be increased by using finer fibres. c) Yarn cross section: Number of fibres in yarn cross section directly depends on fibre fineness. So finer the fibre, higher will be the fibres in yarn cross section, resulting into increased strength & uniformity of yarn. Twist in Yarn and productivity a) High strength to the yarn with same twist: Finer fibres produce stronger yarn for a given count with same level of twist as that of coarse fibres. b) Same strength with low twist: To spin a yarn of given count with required strength finer fibres require less twist, because less interfiber friction and thus less productivity. Ease of Spinning  ) Torsional Rigidity: As fineness varies and other things constant, the resistance to torsion changes increases more rapidly. Thus, the fineness plays an important part in determining the ease with which the fibres can be twisted together during yarn formation. a) Nep Generation: With increase in fibre fineness, flexibility goes on increasing, which will increase rolling tendency of fibres. So, nep formation becomes more frequent with fine fibres. The fineness affects several mechanical properties and therefore influences the behavior of the fibre during processing and properties of the resultant yarns and fabrics. Stiffness- Handle and Drape Fibre fineness is an important factor in determining the stiffness of a fabric or alternatively, its softness of handle and its draping quality. As fineness decreases and other things are equal, resistance to bending i.e. stiffness increases more rapidly. Luster (Reflection of light) The finer the fibres incorporated in a fabric, the greater is the number of individual reflecting surfaces per unit area of the fabric. Thus, fibre fineness has an influence on yarn and fabric luster. Fine fibres produce a soft sheen whereas coarse fibres give rise to a hard glitter. Absorption of liquids and vapor Since the rate at which dyes are absorbed into a fibre is dependent on how much surface is accessible to the dye liquor for a given volume of the fibre substance. It depends on the specific surface; it follows that the time required to exhaust a dye bath is shorter for fine fibres than for coarse fibres. 4.2.3 Measurement of Fibre Fineness A. Micrometric Measures i. Width and Diameter: For all the fibres of cylindrical shape and if the between variation is small, so that only few number of observation is called for, the mean diameter is very satisfactory measure of fineness. This can be measured by use of microscope, with a micrometer eye piece, a camera, or a screen onto which magnified image can be projected for measurement. If the density of the fibres under examination is known, all other transverse dimensions and quantities can be readily calculated. In addition, the variability of the sample can be obtained. The same method can also be used for measuring the fineness of fibres of somewhat oval or flattened section. The width of a fibre of oval section can assume any value from a maximum across the major axis to the minimum across the minor axis, according to the orientation of the fibre with respect to the observer. Mean of all possible widths, which is virtually equal to the diameter of the cylinder of cross section, is taken as fineness.  Error due to swelling must also be avoided. Fibre pieces should therefore first be conditioned in a standard atmosphere and then mounted for measurement in a medium that does not change their moisture content on immersion. Liquid paraffin and cedar wood oil are suitable liquids. ii. Measurement of Fibre Sections Measurement made on transverse sections is capable of yielding the maximum amount of information about the transverse dimensions of fibres. They are however, laborious and time consuming, considerable skill is required for section cutting and its measurement. Therefore, unless it is carried out by experienced, skilled person, otherwise this method is prone to errors. The method is therefore, used only for special research purposes. The two main sources of error are 1. Oblique cuts in sectioning- It leads to systematic overestimates. 2. Swelling arising from the method of embedding and mounting employed. Dry sectioning by the plate method avoids the swelling difficulty, but it leads to oblique cuts and also to deformation of the section during cutting. B. Gravimetric Measure i. Cutting and Weighing Method: The cotton fibre is tapered at the ends, more at the ends away from the seed coat. The tapered extremity may have less secondary wall deposition and thus be more rapidly ruptured or broken during ginning and subsequent processing of yarn formation. For this and other reasons, the practice in Great Britain and many other countries is to measure Gravimetric Fineness at the centre portions of the fibres. The process involves straightening the parallel bundle of fibres over a piece of cork linoleum or similar material and slicing through its middle with a cutter consisting of two parallel razor blades, set the desired distance apart in a holder. The lengths cut should be as long as possible but not so long that an appreciable number of short fibres are rejected. For cotton this technique is commonly used, a 1 cm length is the most suitable for general use. W Gravimetric Fineness = NL Where, W= Weight of cut bundle of fibres N = Number of fibres in bundle L = Length of bundle fibre These direct methods of measuring fineness gravimetrically are useful for research purposes but they are too slow for use as practical measure for monitoring cotton quality. C. Air Flow Method  The measurement of fineness by gravimetric method is bit time consuming and is not well suited for routine testing. In order to obtain quick results to required accuracy some new testing instruments have been developed which offers the mill laboratories the opportunity of quickly assessing the qualities of raw material and of using the information in the selection of suitable varieties and also to check the deliveries. Principle: A sample of known weight is compressed in a cylinder to a known volume and subjected to an air current at a known pressure. The rate of air flow through this porous plug of fibres is measured. The flow meter is calibrated in terms of fineness. The rate of air flow through this porous plug of fibres depends on the specific surface. Specific Surface Area of the cylinder Let V be the volume of the cylinder V = 𝜋 (d/2)2 × L Where, d= Diameter of the cylinder L= Length of the cylinder. The surface area of the cylinder = 𝜋𝑑𝐿 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 Surface Area = Surface Area Volume = 𝜋𝑑𝐿 (𝜋𝑑2𝐿 )/4 = 4 d Relationship Between Air Flow and Fibre Fineness Figure Error! No text of specified style in document..4 Air Flow Principle  Let us consider two cylinders of similar dimensions, one filled with circular rods of large diameter(a) and the other with circular rods of small diameter (b) as shown in fig. The number and the diameter of the rods are so chosen that the total cross sectional area is equal in both the cases. If air is blown through two cylinders at the same pressure, it will be found that the rate of air flow through cylinder ‘b’ would be less than through ‘a’, even though the space through which air has to pass is the same in both the cylinders. The reason for this is that the air flowing through ‘b’ has more rod surface to flow past. This rod surface acts as a drag on the air and therefore, resistance increases. Hence a difference in the rate of air flow is a measure of the difference in the surface area of the large diameter and small diameter rods. This leads to consider a term called specific surface. Specific surface is defined as the ratio of surface area to the volume. Therefore, by measuring the rate of air flow under controlled conditions, the specific surface of the fibres can be determined and consequently the fibre diameter. Then by using a value for the density of the material the fibre weight per unit length can be obtained. If the fibres are assumed instead of rods so that for equal weights of the sample, the rate of air flow will be less for fine fibres than for coarse fibres. As has been shown, the fibre property on which the rate of air flow depends is the specific surface which can be expressed as the ratio of the perimeter to the cross-sectional area. If the perimeter remains constant, changes in the rate of air flow will reflect the changes in the cross section or linear density. The linear density depends upon thickness of wall i.e. maturity. Thus, low rates of flow caused by high specific surface will indicate low maturity. Micronaire Instrument Micronaire is the instrument based on the air flow principle, developed to estimate the fibre fineness in terms of micrograms per inch, the values so determined are known as micronaire values. Micronaire is an integrated value for both fineness and maturity when used for cotton fibres. Principle: The rate of airflow is inversely proportional to the surface area of the fibres. A sample of known weight is compressed in a cylinder to a known volume and subjected to an air current at a known pressure. The rate of air flow through porous plug of fibres, related to the fineness of the cotton fibres is measured.  Figure Error! No text of specified style in document..5 Micronaire Instrument Description: This is an American instrument originally designed to estimate the fibre fineness in terms of micrograms per inch for cotton by air flow methods. The values so determined became known as ‘Micronaire values’. When wool is tested the scale of the instrument is graduated in microns, the units used for fibre diameter. Initial Settings: A special master plug is pushed home into the fibre compression chamber. The instrument is then adjusted so that the float in the flowmeter tube rises to an upper limit, with air flowing through the plug, and to a lower limit, when the flow of air is restricted by placing the thumb over a hole in the top face of the plug. The adjustments are made by the float positioning knob and the calibration screw. When this initial adjustment is completed the master, plug is removed and the instrument is ready to test the samples. Testing Procedure for Cotton 1. 50 grains (3.24gm) of the sample weighed on a precision balance is taken with accuracy of +/- 0.1 grain (6 milligram). 2. The sample taken should be well opened and free from the foreign matter. 3. The sample is introduced into the compression chamber. 4. The sample is compressed in a chamber of 1 inch diameter and 1 inch length by compression plunger. 5. Then the air is allowed to enter inside the chamber and the reading is taken with the top level of float. 6. Then the fibre sample is removed from the compression. 7. Reintroduce the specimen by reversing its direction and the same procedure is repeated. 8. Two samples should be tested from each test lot and the average value is calculated to the nearest of 0.1 micronaire unit. 4.3 Micronaire Value and Nep Micronaire value is a combined index of fineness and maturity. Therefore, cottons of lower micronaire value than average for the growth are usually lower in maturity. If micronaire value falls below average, the lower is maturity and greater is the chance of nepping and bad dyeing. 4.4 Use of Micronaire Value 1. A spinner can buy cotton based on graded staple and micronaire value. 2. The estimate of micronaire value avoids nepping and trouble in dyeing if proper care is taken. D. Vibroscope This is an indirect method of estimating the mass per unit length of a fibre or yarn and is based on the theory of vibrating strings. This method is not suitable for measurements in cotton because of the within specimen variability. It is used extensively in work on man made fibres. Principle: When a violinist tunes his instrument he adjusts the tension in each string until the pitch of the note produced by bowing or plucking the string is correct. In other words, the string is vibrating at the right frequency, the violin has four strings tuned to E, A, D and G. The count of strings varies too, the finest being the E string and the coarsest the G. Let m = Mass per unit length of a perfectly flexible string l = length f = Natural fundamental frequency of vibration (c/s) T = Tension in the string Then Figure Error! No text of specified style in document..8 Vibroscope Principle Where a = Correction factor involving the elastic modulus of the material. For negligible a, m can be found for a specimen of fixed length l either by finding frequency of vibration f , for given tension T, or by varying T until a given neutral frequency f is obtained. If a string of length ‘l’ is clamped at one end, led over a knife- edge support, loaded by a weight W, and is induced to vibrate at its natural fundamental frequency f then, Construction and working ( BTRA Vibroscope) Figure Error! No text of specified style in document..8 Vibroscope Instrument The weighted specimen is clamped to the vibrator at A and passed over a knife edge K. The clamp and knife edge are connected to a 150v source so that the specimen is electrically charged. Transverse vibrations of the specimen will therefore induce a charge in a brass screw s, situated midway between the clamp and the knife edge and spaced 1 mm from the specimen. The screw thus acts as a transducer, if the signal from it is amplified suitably and fed back to the vibrator an oscillatory loop is formed, thus causing the specimen to vibrate at its resonant frequency. The voltage across the vibrator can then be fed into the frequency measuring circuit and the frequency of the oscillation indicated on the meter. Hence weight (Wg)T is known,λ wavelength is (2l) and f has been accurately measured, the mass per unit length can be calculated. It would be possible to calibrate the meter directly in denier. The range is generally from 0.4 to 450 denier. For the same several weights are to be used. The meter scaled from 0 to 100 and calibration chart is prepared for each weight.  TENSILE PROPERTIES OF FIBRES AND YARNS 5.1 Introduction The tensile properties of textile fibres (fibre strength and elongation) are very important from the point of view of their behavior during processing and the properties of the final product. 5.2 Terms related to Tensile Properties 5.2.1 Load Load is the force applied either by dead weight or by any other means to a specimen in the direction of its axis. The load causes tension to be developed into the yarn. The load is generally expressed in terms of gm weight (gravitational unit of force) in case of a dead weight or in terms of Newton or Centinewton (cN) in case other types of load applications. 5.2.2 Breaking load The load at which the specimen breaks is called as breaking load. 5.2.3 Stress To compare the tensile properties of different types of fibres independently of the direct effect of their dimensions in place of load ‘stress’ is used. Stress is defined as the load or force acting per unit cross section area of the material. Load /( Force) 𝑆𝑡𝑟𝑒𝑠𝑠 = Area of Cross − section The units of stress may be g/cm2 or Newton/m2 (pascal) or dynes/cm2 5.2.4 Mass Stress The cross sections of many fibres and fibre structures are irregular in shape and difficult to measure. To simplify the matter a dimension related to corss-section is used. The linear density is such a dimension. The linear density may be expressed in denier or tex count and the ‘mass stress’ then becomes the ratio of the force applied to the linear density (mass per unit length). 𝐹𝑜𝑟𝑐𝑒 𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝑀𝑎𝑠𝑠 𝑆𝑡𝑟𝑒𝑠𝑠 = 𝐿𝑖𝑛𝑒𝑎𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 The unit of mass stress therefore become grams weight per denier or grams weight per tex. Here again abbreviations are used (g/denier or g/tex). 5.2.5 Tenacity or the Specific Strength  The tenacity of a material is defined as the mass stress at break. The units are same as those of mass stress i.e. g/tex, g/den. An alternative term for tenacity is specific strength. 5.2.6 Breaking Length The breaking length is the length of the specimen which will just break under its own weight when hug vertically. Naturally we do not build tall towers in order to measure this breaking length but calculate it from the results of tests or short lengths. The expression of strength in terms of breaking length is useful for comparing the strength of different fibre structures e.g. for comparing single fibre strength with the yarn strength. The unit of breaking length is kilometers. The other unit for breaking length is RKM. RKM stands for “Reiss Kilometre” in German and “Resistance Kilometrique” in French. RKM means kilometers of yarn for break. 5.2.7 Strain When a load is applied to a linear specimen such as fibre or yarn the specimen stretches or elongates. The amount of this elongation will vary with original length of the specimen. The ‘strain’ is the term used the relate the elongation with initial length. 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 𝑆𝑡𝑟𝑎𝑖𝑛 = 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 5.2.8 Extension Extension is the strain expressed as a percentage 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛 = 𝑥 100 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ The extension is sometimes referred as ‘strain percent’. 5.2.9 Breaking extension The ‘breaking extension’ is the extension of the specimens at the breaking point. 5.2.10 Stress Strain Curve An extremely important curve is produced when the load on a specimen is plotted against the elongation. This curve describes the behavior of a specimen from zero load and elongation upto breaking point.  From a close study of this curve very important information can be obtained such as initial young’s modulus, work of rupture, yield point etc. Stress strain curve of viscose derived from a load extension curve of a 300 denier yarn in approximate standard atmosphere. Figure Error! No text of specified style in document..6Stress-Strain Curve The textile fibres differ from metals in that metals are crystalline materials while the fibres are visco- elastic. The load elongation curve of fibres is therefore somewhat different. The load elongation curve of purely elastic materials like metals would be a straight-line while the load elongation of most of the fibres show a straight line in the initial part upto small stresses. However, as the stress is increased further, the stress strain curve bends sharply and large extensions are produced by small stresses. A sort of plastic flow of material occurs. Since different fibre materials have different molecular structures their stress strain curves will be different. 5.2.11 Initial Young’s modulus Material in this portion, behaves like an elastic material and obeys Hooke’s law. The significance of this portion is that when the load is removed the material recovers its original length or very nearly so. The tangent of the angle between the initial part of the curve and the horizontal (x axis) is the ratio stress/strain. This ratio is defined as initial Young’s modulus and it describes the initial resistance to extension of the material. If the stress unit is grams/denier, the initial Young’s modulus will also have the same unit as the strain has no units. 5.2.12 Yield Point The yield point can be defined in terms of yield strain or stress. Alternative terms for yield point are limit of proportionality and elastic limit. This yield region is located by the yield point; which is determined geometrically. The point at which, the tangent to the curve is parallel to the line joining the origin and the breaking point is taken as the yield point. 5.2.13 Work of rupture This is a measure of toughness of the material (see Fig. below).  Figure Error! No text of specified style in document..7 work of rupture and work factor The work of rupture and work factor: (a) Work of rupture = are a OAB (b) Work factor > ½ (c) Work factor < ½ Work of rupture is the energy or work required to break the material. The area under the load elongation curve represents the work done in stretching the specimen to breaking point and therefore the units of the work of rupture will be the units of work e.g. gram – centimeters. Figure Error! No text of specified style in document..8 Work of rupture If we consider a fibre under load F, increasing its length by an amount dl, we have 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 = 𝐹𝑜𝑟𝑐𝑒 × 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 = 𝐹 × 𝑑𝑙 Hence the total work done in breaking the fibre (work of rupture) will be  Break 𝑊𝑜𝑟𝑘 𝑜𝑓 𝑟𝑢𝑝𝑡𝑢𝑟𝑒 = ∫ 𝐹 × 𝑑𝑙 O = Area under the load elongation curve 5.2.14 Work factor If the fibre had obeyed Hook’s law the load elongation curve would be a straight line and the work of rupture would be given by: 1 𝑊𝑜𝑟𝑘 𝑜𝑓 𝑅𝑢𝑝𝑡𝑢𝑟𝑒 = (𝐵𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝐿𝑜𝑎𝑑 × 𝐵𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛) 2 The actual curve differs from this ideal curve. It is therefore, convenient to define a quantity ‘the work factor’ to show the difference from the ideal state. 𝑊𝑜𝑟𝑘 𝑜𝑓 𝑅𝑢𝑝𝑡𝑢𝑟𝑒 𝑊𝑜𝑟𝑘 𝐹𝑎𝑐𝑡𝑜𝑟 = 𝐵𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝐿𝑜𝑎𝑑 × 𝐵𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝐸𝑥𝑡𝑒𝑛𝑠𝑖𝑜𝑛 Figure Error! No text of specified style in document..9 Work factor In the ideal state the work factor will be 0.5. If the load elongation curve lies mainly above the straight line, the work factor is more than 0.5 and it below the straight line, work factor is less than 0.5. 5.2.15 Elastic recovery Elastic recovery may be defined as that property of a body by which it tends to recover to its original size and shape after deformation. The power of recovery from a given extension may be expressed by the term ‘elastic recovery’ value. Elastic extension 𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 = Total extension For example, see the following example.  Figure Error! No text of specified style in document..10 Elastic recovery If the original specimen length was AB and this has been stretched to a length AD. The total extension is BD. After removal of the load the length may become AC. The length ‘CD’ is thus the elastic extension. Therefore, 𝐶𝐷 𝐸𝑙𝑎𝑠𝑡𝑖𝑐 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 = 𝐵𝐷 Perfectly elastic materials have an elastic recovery of 1.0. while materials without any power of recovery (plastic or viscose material) will have a recovery of 0. The fibres will have a value of elastic recovery between 0 to 1. The elastic recovery can also be expressed as percentage. 5.3 Modes of Loading Specimen 5.3.1 Constant rate of elongation (CRE) This is the most widely used method for fibre testing. The principle of working of the instruments based on this method are as given in figure below: Figure Error! No text of specified style in document..11 CRE Consider a specimen gripped between two jaws, jaw J1 at the top which is fixed and the jaw J2 at the bottom which is movable, downwards. With the help of a screw rotating at a uniform speed, the jaw can be moved downwards at a uniform downwards speed. The downward movement of the bottom jaw makes the specimen to elongate at a uniform speed. This principal is therefore called as ‘constant rate of elongation’. The tension developed in the specimen can be recorded by a suitable sensor placed at the top Jaw J1. The elongation is continued until the specimen breaks. ‘Instron’ is an instrument working on the CRE principle. Instruments of this type are very versatile in load ranges, traverse rates, testing sequence and auxiliary facilities. 5.3.2 Constant rate of loading (CRL) Opposite to the constant rate of extension in the testing principle of constant rate of loading, the load is varied uniformly with time. The operating principle of CRL instruments is as shown in Fig. below: Figure Error! No text of specified style in document..12 CRL The specimen is gripped in a fixed top jaw J1 and in bottom jaw J2 which is movable. A force F, initially zero but increasing at a constant rate, is applied to the specimen until it eventually breaks. The movement of the bottom jaw is recorded against the increase in the force F and thus the load elongation curve can be plotted. The load is the cause in this case and the elongation is the effect. 5.3.3 Constant rate of traverse (CRT) Normally in instruments working on CRE principle the top jaw is fixed and only bottom jaw moves with a constant rate. However, in certain instruments the top jaw is not fixed but needs a certain amount of movement in order to operate the load measuring mechanism. In such case the top jaw also moves and the bottom jaw also moves. Though the bottom jaw traverse downwards at a constant rate due to the movement of top jaw the rate of elongation is not constant. Hence such mechanisms are referred to as “Constant rate of Traverse” mechanism. At first glance it might appear that it is immaterial which method is used, but a closer study will reveal some important differences. 5.4 Factors affecting the Tensile Properties 5.4.1 Test Specimen Length The length of the specimen plays an important role in tensile testing. Due to inherent variability in the cross-sectional area normally as the test specimen length is increased the strength of textile materials (fibre, yarn and fabrics) recorded will go on decreasing up to a certain point, as shown in the Fig. below:  Strength Specimen Length Figure Error! No text of specified style in document..13 Effect of Specimen Length It is therefore necessary that whenever we want to compare two different fibres for strength the length tested in both cases should be same. The testing length therefore becomes an important specification in the process of standardization of tensile testing methods. 5.4.2 Rate of loading / elongation and the time to break Because the textile fibres exhibit visco-elastic properties the elongation depends upon the time for which the load is applied. If a constant load is applied to fibre, it will after its instantaneous extension, continue to extend for a considerable time and if the load is great enough it will eventually break. The load to break the specimen therefore will depend on the speed of the test and a rapid test will require a greater load to break the specimen than a slow one. Thus, the results of the test are affected by the time allowed for loading or elongation. Thus, the rate of elongation or rate of loading and consequently the time to break a specimen will affect the results of the tests. A higher rate of loading or elongation as the case may be or lesser time of break will exhibit higher strength and vice-versa. The time to break or the rate of loading or elongation thus become other important specification while standardizing the test methods. 5.4.3 Previous history of the specimen Due to the visco-elastic nature of textile fibres, it is observed if the fibre is loaded beyond its yield point after removal of load the elongation is only partly recovered. Certain elongation is permanently set. Secondly, degradation due to certain chemical treatment or mildew fungus also affect the tensile properties of fibres. It is therefore desirable to know about the previous history of fibre before interpreting the results of tensile tests. 5.4.4 The Form of the Test Specimen (single or bundle) The fibres can be tested for the purpose of tensile strength either in single or in bundle form. Due to the variability in many aspects such as diameter, strength and elongation between the individual fibres the fibre bundle strength is not a simple multiple of individual fibre tenacity and the number of fibres in the bundle. The actual bundle strength shown by any instrument is much less than this simple multiple value. The difference between this value and the actual bundle strength goes on increasing with increasing variability of the fibre properties. The measurement of single fibre strength is the fundamental aspect and useful for fibre engineering such a test is very difficult and time consuming. Secondly, the textile structures such as yarns and fabrics are made from group of fibres and therefore it is of practical interest to know how the fibres will behave in groups or bundles rather than, individually. Therefore, though for research purposes the single fibre strength is tested for practical purposes the fibre bundle strength is tested. These tests are fast, less complicated and the results of these tests are more correlated with the performance of fibre structures such as yarns and fabrics. 5.5 Fibre Strength 5.5.1 Single fibre strength Testing Strain Gauge Transducer Principle In Strain Gauge Transducer Principle, a beam is mounted as a cantilever, as shown in Figure. The upper jaw J1 is attached to the free end of the beam and the lower jaw J2 has a controlled vertical movement through a screw mechanism. By moving J2 downwards a tensile force is developed in the specimen which causes the free end of the beam to be deflected. The effects of the deflection are used to measure the magnitude of the load on the specimen. Figure When the beam bend, the length of the upper face AB increases and the length of the lower face CD decreases. These changes in length being proportional to the applied load. Between the two outer faces there is a neutral plane NL whose length remains unchanged. When a piece of resistance wire is stretched, its electrical resistance increases; conversely, if it, contracts its resistance decreases. Further, the change, in the resistance value is proportional to the change in length. Consider a resistance wire R firmly bonded by cement to the face AB of the beam so that elongation of AB will produce an elongation of R. Thus, a load applied to the end BC causes change in the length of the resistance wire, and the change in the value of the resistance is proportional to the magnitude of the load. It is now necessary to convert this change in resistance into a visual recording of the load. This conversion is usually achieved by means of a Wheatstone bridge. If four resistances are employed in the loading system, two on the upper and two on the lower surface, they are connected in the form of a Wheatstone bridge-as in Figure. Wheatstone bridge Figure.  With the beam undeflected, no voltage will be developed across CD when a voltage is applied across AB. The bridge is said to be 'balanced'. When a load is applied to the beam the deflection causes changes in the values of the resistances and a voltage is produced across CD, its value being proportional to the load. This voltage output is fed to suitable electronic circuits which finally drive a pen recording instrument. Advantages of a strain gauge instrument 1. Free from inertia errors and friction. 2. The deflection of the end of the beam is very small, therefore the tester virtually tests under 'constant rate of extension' conditions. 3. Versatility in the types of testing possible. Disadvantages of a strain gauge instrument 1. The services of expert technicians required for maintenance and repair. 2. Constant checks required to counter 'drift' in electronic circuits. 3. High initial cost. Universal tensile testing instrument A general view of the table model is seen in Figure.  In order to accommodate a wide variety of specimens, several interchangeable load cells containing the strain gauges are used. In this way fibre, yarn and fabric strength testing is possible. The load cell is located centrally in the fixed crosshead. The upper jaw is suspended from the cell through a universal coupling. The lower jaw is mounted on the traversing cross head which is driven upwards and downwards by screwed rods on each side. On the bottom panel of the crosshead assembly the controls which govern the movement of the crosshead can be seen. Preparation of Sample Sample is collected from population by suitable technique. Paper window is prepared first and single fibre is sticked to this window as shown in Figure, The load cell output is fed by cable to the control cabinet which houses the various electronic circuits to give the result in the form of numerical values as well as graphical form. 5.5.2 Bundle Strength of Cotton Single fibre tensile tests have enabled much useful information to be found both in relation structure and in comparing different types of fibres. Nevertheless, they are of little or no practical importance when applied to assessing raw cotton quality in everyday mill practice. Though the results of bundle fibre tests will be less accurate due to errors in testing including slippage at grips, the variations in the degree of fibre parallelization and tension, the tests are faster and save time. Secondly, the yarns are made of a bundle of fibres and hence when the yarn ruptures a bundle is ruptured and therefore the results of fibre bundle strength tests are more closely correlated with the yarn strength. Because of these facts the fibre bundle strength tests have become more popular. The three main types of fibre bundle testers that are most widely used are (i) Stelometer (ii) Press ley Tester and (iii) Clemson Flat bundle test. Stelometer The pendulum lever principle of loading is used in this cotton fibre bundle strength testing instrument. Its name is coined from ST-rength and EL-ongation, the two fibre properties measured. Measurement of elongation is possible since the sample may be tested with the Pressley jaws separated by ⅛ in., as opposed to the more usual zero-gauge length used in flat bundle testing. A random sample of cotton fibres is prepared, short fibres being removed by combing so that all the fibres in the test specimen extend all the way through the jaws. One of the Pressley jaws, J 1 is mounted in the adjustable jaw holder carried by the beam. The other jaw, J 2 is mounted at the top end of the pendulum. The beam carries the pendulum which is pivoted at O. The centre of gravity of the beam is to the right of its axis of rotation, A, and therefore when a retaining catch is taken off, the beam rotates in a clockwise direction. The centre of gravity of the pendulum coincides with the axis of rotation, A; thus, the heavy mass of the pendulum bob has minimum movement and inertia effects are largely eliminated. Consider the conditions obtaining when the beam is inclined at an angle θ to the vertical. Assuming for the moment that the specimen has not stretched; the pendulum will also be inclined at an angle θ to the vertical. In Figure 5.9 let GOQ represent the pendulum and let F be the force in the fibre bundle. The design of the instrument ensures that the force  Figure Error! No text of specified style in document..14 Stelometer F will nominally act at right angles to GOQ. The clockwise moment of F about the pendulum pivot O will be: 𝐹 × 𝑂𝑄 This moment is balanced by the anticlockwise moment of W, the weight of the pendulum, about 0, 𝑊 × 𝑂𝐺 sin 𝜃 Hence, 𝐹 × 𝑂𝑄 = 𝑊 × 𝑂𝐺 sin 𝜃 And 𝐹 = 𝑘 sin 𝜃 (since W, OG, and OQ are constant) Hence, 𝐹 ∝ sin 𝜃 Thus, the load on the fibre bundle is directly proportional to the sine of the angle through which the pendulum has moved. Therefore, if the rotation of the beam can be so controlled that sin 𝜃 varies at a constant rate, then the rate of loading will be constant.  Figure Error! No text of specified style in document..15 Pendulum lever principle with CRL Beam rotation is controlled by a special dash pot device. A rod secured to the beam is connected to the piston rod of the dashpot, thereby applying a restraining force to the beam as it swings around the axis A. The geometry of the system is so designed that the angular velocity of the beam is such that sin θ varies at an approximately constant rate and therefore the rate of loading is approximately constant. Dashpot adjustment is made so that the rate of loading normally used is I kg/sec. The breaking load is indicated by the pointer P1 which moves over the large scale graduated from 2 to 7 kg. The percentage of elongation is indicated on a small scale by the pointer P 2 which is suspended from the arm of pointer P1. After the bundle, has been broken, the fibres are collected and weighed. From the indicated breaking load at t in. and the fibre weight in milligrams the tensile strength of the material is calculated. Breaking load in kilograms x 1 · 5 x 10 Tenacity in grams per tex = Sample weight in milligrams The sample length is 1.5. cm when the l in. gauge length is used, hence the 1.5 in the formula; the tenacity calculation at zero-gauge length will use 1.18, i.e. the sample length is then 1.18 cm. 5.6 Yarn Strength Yarn strength is measured in the form of single yarn or in the form of bundle i.e. a lea. Conventional instruments work on pendulum lever principle while modern yarn testing instruments work on strain gauge transducer principle, which has been discussed in pt. 5.5.1. 5.6.1 Pendulum Lever Principle  Fig.5.11 Pendulum lever principle with CRT Consider a specimen clamped in the upper jaw J1 and the lower jaw J2. J1 is attached to a steel tape which runs over a small pulley of radius r. J2 is given a constant rate of traverse in a downward direction, perhaps by a screw mechanism. The small pulley is pulled round and in doing so swings the pendulum P from the vertical. Let the mass of the pendulum be M and its centre of gravity at a distance R from the pivot of the small pulley. Assuming the specimen to be an inextensible and an absence of any dynamic effects. Let us consider the conditions obtaining at any chosen instant when the angle through which the pendulum has moved is θ radians. Taking moments about the pivot of the small pulley- 𝐹𝑟 = 𝑀𝑔𝑥 = 𝑀𝑔𝑅 𝑠𝑖𝑛𝜃 The values of Mg R and r are constant. Therefore, F α 𝑠𝑖𝑛𝜃 Since, F is the force acting in the tape; it is also equal to the tension in the specimen. Thus, the tension in the specimen is proportional to the sine of the angle through which the pendulum has been swung. There are essentially two ways in which the yarns are characterized as to their strength behavior. These are Single thread test. The single thread strength tester, working on the pendulum lever principle, is shown in figure.  Fig.5.12 Single thread strength tester This is a motor driven pendulum type strength tester. It is made up of a weight arm pivoted in ball bearings and having a quadrant at the top. The arm is connected to an upper clamp by a chain, which runs over the quadrant and fixed to it. The pulling force acting on the specimen is transferred to the pendulum through the clamping arrangement, which in turn displaces the weight in proportion to its own magnitude, and this can be read on the quadrant scale. This is made in two ranges one for lower and other for higher range. The lower scale is used when there is small weight on the pendulum and the higher scale is used when the pendulum carries an additional weight. The lower jaw is carried by a rack, which is connected by a screw rod connected to a driving mechanism consisting of a clutch, and declutch arrangement. A rod, which carries the lower jaw, can be changed to different length so that different length specimens can be tested in the tester. The specimen shall break within 20+/- 3 seconds. The rate of traverse of the bottom jaw is 300+/15 mm/min (12”/min). Testing: The pendulum is arrested with a catch and also the movement of the upper clamp is arrested. The material to be tested is taken and clamped between the upper clamp and the lower clamp. The extra material is cut off exactly at the clamp position and then the catches are taken out. When the machine is started, the lower jaw traverses downward imposing the tension on the specimen and thereby pulling the upper clamp and in its turn will make the pendulum to move over the quadrant scale. When the specimen ruptures, the pendulum arm is retained in the position by a set of pawl working over the serrated portion of the quadrant. The position of pendulum arm gives the breaking load of the specimen. Comparatively 50 tests are done for single yarns and 25 for plied yarns, every time bringing the pendulum arm to zero position and arresting the movement of the upper jaw. Then the tenacity is calculated using the formula- 𝑀𝑒𝑎𝑛 𝑏𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝑙𝑜𝑎𝑑 𝑖𝑛 𝑔𝑟𝑎𝑚𝑠 𝑇𝑒𝑛𝑎𝑐𝑖𝑡𝑦(𝑔𝑚𝑠/𝑇𝑒𝑥) = 𝑌𝑎𝑟𝑛 𝑐𝑜𝑢𝑛𝑡 𝑖𝑛 𝑇𝑒𝑥 Apart from the breaking load, the elongation is also measured by noting down the relative position of the upper clamp. The elongation scale directly reads the difference in the movement of both the lower and upper jaws. This can also be calculated in terms of the original length of specimen as follows- 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 𝑠𝑐𝑎𝑙𝑒 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 % = 𝑋100 𝐺𝑎𝑢𝑔𝑒 𝑙𝑒𝑛𝑔𝑡ℎ Several tests with reference to breaking load and elongation are carried out. Personal error is there in mounting the sample. To avoid this, continuous lengths are taken and strength values are obtained by applying equal pressure to both the clamps when fixing the specimen to avoid jaw break. The material taken for testing is finally collected and count can be assessed. Since small lengths are being tested, the range in the strength value will be very high and thereby increasing the C.V%. In the standard test procedure, the minimum number of 20 inch test specimens for single spun yarn is indicated as 50. Thus the actual length of material is relatively short. This must be recorded as a disadvantage of the tester. Lea Strength Test: In this test, a hank or skein of 120 yards is broken on a pendulum lever tester from which the lea strength of the hank is obtained. This test does not give a measure of the absolute value of the yarn strength, but yields a relative value that is quite useful for quality control purpose. The strength of the yarn in a lea test is determined largely by the thinnest places in the yarn and the frictional forces between the threads and the hooks on the tester. The strength of the yarn is expressed in terms of the product of lea strength and count, which is called the “Count Strength Product” or “Break Factor”.The count strength product is used for comparing yarns of slightly different counts. A hank of yarn, with its starting and finishing ends knotted together, is placed over the hooks of the lea tester, as shown in figure. As the lower hook descends, a load is imposed on the loops of yarn constituting the hank. At some point one of the threads breaks, yet the hank remains intact and capable of sustaining further loading. The hank suffers a succession of thread breakage and ultimately becomes unable to sustain any further increase in load. When this happens, the pendulum stops moving and the maximum load is indicated on the dial. This is called the lea strength. Lea may be 120 yards with a circumference of 1.5 yards to get 80 rounds or 100 meters to get 100 rounds. Lea strength is not the true representative of the strength of the individual ends put together but it is something different because here only the fraction of the ends break and the rest of the hank slipped. The end point of the test is determined by the rate at which the lea unravels or slips as the threads break. At the first instance a few threads break and when the rate of unraveling of threads equals to the rate of movements of hooks, the top hook will stop and maximum load would have been reached and this is taken as the lea strength. In the normal test, the presence of the frictional forces between the threads and the thread and hooks, the angle of inclination of threads with the hook restrict the unraveling of the hank. Hence, the indicated strength is a combination of complex composite specimen strength plus a measure of the frictional properties of yarn in the hank form and yarn/hook friction. Fig.5.13 Lea strength tester It is a motor driven, pendulum type strength tester. It consists of an upper jaw and a lower jaw. The lower jaw can be engaged with a screw mechanism, which is driven at a constant speed by a motor. Thus a constant rate of traverse of 12 inches per minute is given to the bottom jaw. The top jaw is connected to a pendulum arm by means of a steel tape. A heavy bob is attached to the pendulum arm and the arm moves over a serrated quadrant. A pawl is attached to the pendulum arm to control the movement of the arm and helps the arm to stop when the lea breaks. There is a dial, calibrated in pounds, over which a pointer moves through the geared movement of the pendulum arm. The pointer indicates the lea strength in pounds on the dial. Samples of lea are prepared from the ring bobbins or cones using wrap reel. The length of the lea is 120 yards. The bobbins are reeled under the same tension and with a small traverse to separate the layers. When the set length is wound, the reel automatically stops and the lea is transferred from the wrap reel to the lea tester. The lea is mounted over the jaws and when in doing so, care should be taken to avoid the formation of any twist in the lea and the grouping of threads on the jaws. Then the bottom jaw is engaged with the screw mechanism and the motor is switched on. Because of the pull on the upper jaw the pendulum arm is pulled which moves the pointer over the dial. At one point, one or two strands break and then many will slip and at the stage, there will not be any further movement to the pendulum. The pendulum will be prevented in falling back suddenly by the pawl which engages with teeth over the serrated quadrant. At that place, the pointer also stops moving and indicates the maximum load on the dial. This load is called the strength of the lea. The lower jaw is then brought up and the lea is removed from the jaws. Drawbacks of the Lea Tester: Lot of personal errors are liable to occur while preparing and mounting the specimen on the hooks of the lea tester. Further, during the wrapping of the test material, there must be uniform tension throughout. Otherwise unequal tension will introduce errors to the results because tighter threads will break soon, resulting in low breaking load or strength. The frictional forces present in the lea test reduce the sensitivity of the test to the detection of weak places in the yarn. A yarn which is generally weak will, of course, produce a low lea strength but the presence of abnormally weak place or weak places in one or two threads will always escape detection, especially damages during winding is not shown in lea tester. Therefore, it can be said that the lea tester is not sensitive to weakest points in the lea. Some of the leas testing machines have elongation scale also. Considering the form of material for testing and the way in which it is tested, during strength testing, the elongation has no meaning and the elongation scale therefore does not serve any purpose. Since, the elongation here is not a representative of the elongation of any individual thread. It will not indicate multiple strength of the single thread but it is only the capacity of individual threads to withstand stress and strain of the several processes such as winding, warping etc. Therefore, the lea strength has no bearing on the performance of material during preparatory or weaving.  The lea tester belongs to the class of constant rate of traverse machines, in particular to the pendulum lever principle. Therefore, it suffers from the effects of inertia and hence the final rate of loading is not constant. The test results are affected by the change in the length of specimen elongation and time to break the specimen etc. The lea testing does not serve any purpose from the point of further processing. The Lea Tester is still popular because of the following Merits, The material taken from strength testing can be directly used to determine the count. Therefore, simultaneously both count and strength values can be determined. Though the lea tester is not sensitive for weakest point in the yarn, it is sensitive enough to detect changes in the yarn quality, for example addition of waste, changes in settings, changes in weighing system etc. Lea tester does not contain any complicated part or mechanism. It is a strong and robust machine and hence it cannot be spoiled by defective handling and gives a good service. The lea tester is there with the spinning mills for the past so many years and many are adapted to this form of testing and hence to convince them about the defects of this test is rather difficult. They will not be prepared to change this method because the testing is simple and easy and without much practice, testing can be done. Many samples can be tested in a short duration of time to have a comparative idea of the material spun in the spinning room. This information is useful for both producer and user with reference to the change in character on spinning and therefore lea test comes as handy to all the spinners. There is a readymade table, which is prepared based on practical experience on several cottons for count strength product, which is called CSP values. Count Strength Product The count strength product (CSP) is a measure used for cotton yarns and is the product of the yarn count (English Count, Ne) and the lea (Hank) strength in pounds. It is based on measuring the strength of 80 turn hank made on 1.5 yard wrap reel to give a total length of 120 yards; the strength being measured in pounds force (lbs). This value enables a comparison to be made among the yarns of a similar but not necessarily of identical count. While assessing the character of cotton yarn, the product of count and lea strength is a very useful measure of the merit of yarn from the strength point of view. This is called as break factor or CSP. The count strength product can be calculated using the formula- 80𝑋 𝑁𝑋𝑆 𝐶𝑜𝑢𝑛𝑡 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 = 𝑊 Where, S= Average breaking load in lbs.  N= Average count in an indirect system (Ne). W= Number of wraps in the skein. For skeins of 80 wraps, the formula becomes, 𝐶𝑜𝑢𝑛𝑡 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑃𝑟𝑜𝑑𝑢𝑐𝑡 = 𝑁𝑋𝑆 Since, 𝐻𝑎𝑛𝑘𝑠 𝐶𝑜𝑢𝑛𝑡 = 𝑃𝑜𝑢𝑛𝑑𝑠 so, 𝐻𝑎𝑛𝑘𝑠 CSP = 𝑋 𝑃𝑜𝑢𝑛𝑑𝑠 = 𝐻𝑎𝑛𝑘𝑠 𝑃𝑜𝑢𝑛𝑑𝑠 Thus, the CSP is actually the number of hanks, the yarn can support from the stand point of strength. For example, if a 30s yarn has a CSP of 1800 hanks, it means a skein of 30s yarn could support the weight of 1800 hanks of the same yarn. Actually, CSP is the breaking length of the yarn with the units of hanks. Comparison of Lea and Single Thread test Results The lea strength is the most widely used measure of yarn quality in cotton industry. It has certain advantages over single thread strength. As a number of threads simultaneously participate in lea test, errors due to faulty sampling are much reduced. The effect of yarn irregularity is more clearly seen in the results of lea strength tests than the results of single thread strength tests. The Assessment of Yarn Quality on the basis of Lea Strength has the following Drawbacks: In winding or weaving, yarn is used as single strand and not in the form of skein except in the sizing and dyeing operations. The rupture of a single thread at a weak place affects the results of the whole skein. The skein test does not give an indication of the extensibility and elastic properties of a yarn which are the important characteristics required during weaving. This method has the following Advantages over Single Thread Strength Test: This test requires less time to complete. It involves less sampling error. Count of the yarn can be determined easily. It is used for comparing the quality of yarns spun from different cottons.