Topic7x Kawabata FAST

Introduction to the Principles of Textile Testing - MA Wilding

Topic7x-Kawabata-FAST.doc 1

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The handle of a fabric (its qualities generally associated with "touch and feel") is clearly determined largely by its various mechanical properties. However, whilst these individual properties can be (and frequently are) assessed, and inferences perhaps drawn as to the handle of a particular piece of material, this is not the same thing as assessing the handle per se – ie in its own right. Traditionally, that has always been the subjective territory of "expert" panels, who arrive at their judgement by means of actually handling the cloth. For many reasons – not least economic – it would be preferable to have a laboratory-based system available that could perform this task objectively. In order to achieve this though, two important and connected questions had to be addressed: 1) Can the various objectively-determined properties be used to provide a formal and reliable link to more subjective notions of fabric handle? 2) Might a conveniently small number of the many fabric properties be combined in such a way as to provide an overall assessment of a particular fabric’s suitability for a given end-use? A positive answer to the first question would bridge the gap between laboratory testing and traditional assessment methods, such as the use of panels of expert “judges”. Ideally, it would also enable a universally-recognised system to be developed for describing a fabric’s handle; one that would allow rapid communication of easily-interpreted data worldwide. The second question deals with identifying what are the optimum properties a fabric should have for a particular end-product. For instance, we might wish to determine the best fabric (and fabric construction) for men’s winter suiting, or for women’s summer outerwear. Which of the many mechanical properties are important, and what combination works best, are obvious concerns for manufacturers. Over recent decades these questions prompted the development of a number of integrated systems for the objective assessment of handle. Two of these – the ‘Kawabata Evaluation System for Fabrics’ (KES-F), and the ‘Fabric Assurance by Simple Testing’ (FAST) system are now universally-acknowledged as industry standards. These were originally intended for the assessment of apparel fabrics, but they have also been successfully applied in textile applications more widely.



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The development of an integrated system for the objective assessment of fabric handle was pioneered by the Japanese academic, Professor Sueo Kawabata. Although fabric handle had traditionally been assessed by experts, which is a wholly-subjective procedure, he recognised that the stimuli causing the psychological response of handle must ultimately be determined by the physical & mechanical properties of the fabric. It was therefore logical to assume that a more objective approach should be possible. In order to progress, however, it was first necessary to get universal agreement as to how the most important handle qualities should be described subjectively. Allied to this was agreement on how each of these attributes should contribute to a fabric’s overall “rating” in terms of its handle. First, Kawabata defined eight descriptive terms (or 'descriptors') to be associated with the various subjective aspects of a fabric's handlea. These were essentially terms such as "stiffness", "smoothness", "fullness" and "softness"; they were of course Japanese, and many of them had subtle nuances which it is hard to translate directly into English. Taken together, the eight descriptors were considered to determine a quality which Kawabata called the ‘Primary Hand’. The terms adopted, along with their approximate English translations are shown in Table 1.

Table 1. Kawabata’s eight primary hand definitions

Japanese term Approx. English translation

Koshi Stiffness

Numeri Smoothness

Fukurami Fullness & softness

Shari Crispness

Hari “Anti-drape” stiffness

Kishimi "Scroopy" feeling

Shinayakasa Flexibility with soft feeling

Sofutosa Soft touch

Next, Kawabata introduced several distinct categories of apparel-fabric end-use. He associated with each a relatively small subset of the primary hand descriptors, which he regarded as being of particular importance for that category. Hence, the precise combination of primary hand descriptors relevant to the handle of any given fabric depends on the end-use category it is in. For example: Men’s winter suits – Koshi; Numeri; Fukurami Men’s summer suits – Koshi; Fukurami; Shari; Hari Women’s thin dress fabrics – Koshi; Hari; Shari; Fukurami; Kishimi; Shinayakasa

a which is sometimes referred to as its 'hand'



The values these individual properties should (ideally) have also differs from one category to another; which leads us to the question of how the various primary hand descriptors can be quantified. The procedure adopted was to rate each on a 10-point scale, where 10 represents a high value of the particular property and 1 represents its opposite. So, if a fabric is judged to display a very high Koshi feel, it might be assigned a value around 8, 9 or10 for this primary hand property. In contrast, a very limp fabric could have a Koshi rating in the range 1-3. This meant that a given fabric could be ascribed a set of numbers – perhaps four or five – that taken together conveyed information about its overall handle qualities. These numbers were called 'Primary Hand Values' (PHV). However, Kawabata realised that it would be very convenient if this kind of information could be encapsulated in just one single value. To this end, he developed a further concept, which he termed the 'Total Hand Value' (THV) of a fabric. This entailed much fundamental work in understanding the underlying fabric mechanics involved in the various handle characteristics, and the production of a 'translation equation' for each fabric category which could be used to manipulate the individual PHVs to give an overall THV. It is possible to make an assessment of the handle of an individual piece of fabric in complete isolation – ie without reference to any other fabric. However, this is not usually very satisfactory: it would be far better to have a reproducible "ideal" fabric, of known handle properties, against which to compare. This is especially important if the assessment is to be made generally meaningful, and the results are to be communicated within or between companies, say. In order to accomplish this, Kawabata, in collaboration with an organisation known as the Hand Evaluation & Standardisation Committee (HESC), produced books of standard fabric samples. There was a reference sample for each of the primary hands, and also standard samples corresponding to the ideal total hand, for each of the following five categories:

1. Men’s winter/autumn suiting 2. Men’s summer suiting for a tropical climate 3. Ladies’ thin dress fabrics 4. Men’s dress shirt fabrics 5. Knitted fabrics for undershirts

It will be realised that up to this point the assessment of handle was still completely subjective – ie the rating assigned using Kawabata's scheme to a particular piece of cloth still depended on someone's impression from actually handling it. To address this difficulty, he derived a further set of translation equations linking the fundamental fabric mechanical properties to the various primary hand values. Alongside this, he invented a set of four test instruments designed to measure the appropriate



properties. This formed the integrated system that subsequently became known as the KES-F system. The instruments themselves (or their corresponding properties) were given short-hand codes:

FB1 Tensile & Shear FB2 Bending FB3 Compression FB4 Surface friction & variation

The great advantage of this development was that it enabled any operator to measure PHVs and THVs reproducibly, hence turning what was a purely subjective exercise into an objective one. The KES-F system actually facilitates the measurement of 17 different mechanical parameters at the low levels of force typical of a fabric in normal use. In fact, the fabric weight is also determined, thus raising the total number of parameters to 18. These are grouped, and given short-hand codes, as follows:

Tensile EMT Extension at max. load (500 gfcm-1, = 4.9 Ncm-1) LT Linearity of load-elongation curve WT Tensile energy RT Tensile resilience Shear G Shear rigidity 2HG Hysteresis of shear force at 0.5o shear angle 2HG5 Hysteresis of shear force at 5o shear angle Bending B Bending rigidity 2HB Hysteresis of bending moment Lateral compression To Fabric thickness at 0.5 gfcm-2 (= 4.9 mNcm-2) Tm Fabric thickness at 50 gfcm-2 (= 0.49 Ncm-2) LC Linearity of compression-thickness curve WC Compressional energy RC Compressional resilience Surface characteristics SMD Geometrical roughness MIU Coefficient of friction MMD Mean deviation of MIU Fabric construction W Fabric weight per unit area



In order to completely validate this new approach, it was of course necessary to correlate the hand values measured using KES-F with those arrived at traditionally by experts. This was successfully accomplished over time. The following is an example of the combination of hand values that might be expected of a fabric intended for summer suiting (from BP Saville, Pg 288):

Total Hand THV 3.5 Primary Hand Koshi 6.1 Shari 6.5 Fukurami 3.5 Hari 6.8

2.1 Measurement of the KES-F parameters

Saville gives a detailed discussion of the methods for determining the various parameters on the KES-F system, and the following summary is based on his description.

2.1.1 Tensile

For the tensile determinations, a standard-sized rectangular fabric sample is extended at a constant rate, whilst the load (per unit width) is monitored up to a maximum of 500 gfcm-1. Both loading and unloading processes are carried out so as to determine the recovery behaviour. Figure 1 illustrates how the various parameters are determined from the test. EMT is simply the relative extension (ie strain) at the maximum load; it is normally expressed as a percentage, WT is related to the energy needed to stretch the sample to the maximum load, and is defined as the area under the "increasing-load" – ie stretching - curve. This is the larger area under the experimental curves. Note that the load is specified as force per unit width, and the "extension" is in fact strain (ie no units). Therefore, WT also has the dimensions of force per unit width. In the SI system, it could be expressed in Nm-1, for example, which is exactly equivalent to Nm.m-2, or Jm-2; so it is in fact an energy per unit area. LT specifies how linear the extension curve is. This is done by comparing the measured area (WT) with that which would be achieved for a perfectly linear sample. This is essentially the area of the triangle AOB, so that LT=WT/(Area AOB).



Figure 1. Schematic force-extension plot from a KES-F tensile test

The resilience, RT, is related to how recoverable the fabric is. It is the area under the "load-decreasing" curve (shown grey in Figure 1) expressed as a fraction (or %) of the area under the "load-increasing" curve. That is:

%xWT

curve unloadingunder AreaRT 100====

If the tensile hysteresis is required, this can be obtained by subtracting the area under the unloading curve from that under the loading curve.

2.1.2 Shear

The essential features of the shear test are shown in Figure 2. The tensile force of 10 gfcm-1 (98.1 mNcm-1) is applied in order to counteract the buckling tendency which generally arises from the diagonal compression accompanying shear.



10 gfcm10 gfcm--11

θθθθθθθθ 5 cm5 cm

20 cm20 cm

10 gfcm10 gfcm--11

θθθθθθθθ 5 cm5 cm

20 cm20 cm

Figure 2. The essential features of the Kawabata shear test

Figure 3 illustrates the kind of response that might be encountered, and indicates how the shear parameters are defined. Note that generally the horizontal axis represents the angle of shear: ie θθθθ in Figure 2. This is not

strictly the same as the shear strain - which is tan θθθθ - but since the

deformations are so small it can be taken as being proportional to it (or equal, if θθθθ is expressed in radians rather than degrees).

Figure 3. Typical KES-F plot of shear stress vs strain

2.1.3 Bending

In this test a fabric sample is bent first one way and then in the opposite direction, as indicated in Figure 4. The instrument applies a linearly increasing bend curvature (which is the reciprocal of the bend radius) to a sample of fabric, whilst at the same time monitoring the bending moment developed.



Figure 4. Principle of the KES-F bend test, and schematic plot of data

The test is performed between the maximum and minimum curvature limits of +2.5 cm-1 and -2.5 cm-1. The bending rigidity, B, is defined as the slope of the bending moment vs curvature plot at the point where the line crosses the vertical axis. In practice most fabrics show reasonably linear behaviour between the two extreme limits of curvature; this therefore amounts to the slope of either of the parallel lines representing the bending and unbending stages. In common with most other physical properties of textile materials, the bending behaviour usually displays a degree of hysteresis, as can be appreciated from Figure 4. This is quantified in terms of the parameter 2HB which is quantified as indicated.

2.1.4 Lateral compression

The compression test uses the "anvil and presser-foot" principle, in which the fabric sample is sandwiched between two flat, parallel metal plates. One plate is connected to a force transducer from which the applied



pressure can be determined, via the sample dimensions. The thickness change is monitored, and this would normally be converted to a compressional strain. The pressure is varied up to a maximum of 50 gmcm-2 (0.49 Ncm-2) Figure 5 shows the type of data plot that might be obtained. The actual shapes of the curves will be very sensitive to the type of fabric being tested, of course.

Figure 5. Schematic plot of pressure versus strain from KES-F compression test

The compressional energy, WC, is the area under the "load-increasing" curve. The linearity, LC, is the same area divided by the area of the triangle OAB (in Figure 5). LC is thus the equivalent, in compression, of the tensile linearity, LT. The compressional resilience, RC, is the equivalent of the tensile resilience, RT, and is equal to the area under the "load-decreasing" curve divided by WC, and expressed as a percentage. The two thickness parameters, To and Tm, are simply measured from the separation of the plates at the appropriate pressure levels.

2.1.5 Surface characteristic

An assessment of the "roughness" of a fabric's surface is effectively made by dragging a U-shaped wire across it, as illustrated schematically in



Figure 6. The wire (of diameter 0.5mm) is attached to a lever arm whose movement enables the fabric profile to be plotted. The arm is loaded so as to apply a normal force of 10 gf (9.81 mN). Suppose the wire is dragged a total distance X across the fabric. T is the average thickness as determined by the instrument, but the shaded area in the figure is related to how rough the surface is. The 'mean variation in surface roughness' (SMD), is specified as this area divided by X.

Figure 6. Assessing the surface roughness of a fabric

The frictional characteristics are assessed in a similar manner to the roughness, except that in this case a group of 10 wires (each having a diameter of 0.5 mm) is used, as depicted in Figure 7. Additionally, a higher normal force (50 gf; 0.49N) is applied. However, the main distinction is that instead of determining the fabric profile, the friction test monitors the force necessary to drag the wires over the surface. Dividing this force by the normal force gives the dynamic friction coefficient. This is then plotted as a function of the distance travelled across the fabric. Its mean value (MIU) is determined, together with the mean variation (MMD). Similarly to SMD, MMD is calculated from the total shaded area (Figure 7) divided by the total distance traversed (X).



Figure 7. Assessing the frictional characteristics of a fabric

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The system known as 'FAST' ("Fabric Assurance through Simple Testing") was developed by 'CSIRO' – the Australian Commonwealth Scientific and Industrial Research Organisation. It was initially intended for the assessment of suiting fabrics, and as such was designed specifically to aid tailors & worsted finishers. However, it can be - and is - applied more generally. The approach is in some ways similar to that of the Kawabata system, but the assessment is based on fewer measured properties. The method is also claimed to be quicker, simpler & more robust than KES-F. It is certainly less expensive. As the name implies, FAST was developed to be a quality-assurance tool for fabrics – a means of ensuring good performance during downstream operations such as sewing and making up into garments. In this respect, one of its main objectives was to enable discrimination between loosely-constructed and tightly-constructed fabrics. Loose fabrics tend to distort very easily, and this can cause a multitude of problems during making-up. On the other hand, excessively tight fabrics can cause problems associated with moulding, over-feeding of seams, etc.



The complete set of instruments consists of four sub-systems, as follows:

FAST 1 Compression meter FAST 2 Bending meter – bending length test FAST 3 Extension meter FAST 4 Dimensional stability test

3.1 Measurement of the FAST parameters

Saville provides a full description of the methods used in the FAST system, from which the following is a summary.

3.1.1 Compression

The compressional characteristics of the fabric are assessed by measuring the thickness of a 10-cm2 area in response to two different normal pressures:

a) 2 gfcm-2 (19.6 mNcm-2) b) 100 gfcm-2 (981 mNcm-2).

The fabric is assumed to consist of a core layer, which is essentially incompressible, and a surface layer which can be compressed. The thickness of the latter is taken as the difference between the two thickness values determined as above. The measurements are repeated after steaming the fabric sample on a Hoffman press for 30 seconds, which imparts stability to the surface layer.

3.1.2 Bending length

The method for determining the flexural characteristics on the FAST system is effectively the Shirley bending-length test (BS 3356), but using a fabric specimen width of 5 cm. The bending rigidity is calculated from the bending length and the fabric weight per unit length.

3.1.3 Extension testing

For this test rectangular specimens are used (100 mm x 50 mm), the longer dimension being in the direction of stretching. The extension of the fabric is measured in both warp and weft directions at three fixed forces: 5, 20 and 100 gfcm

-

1 (49, 196 and 981 mNcm

-1); and in the bias direction at one force only: 5 gfcm

-1.

This also enables an estimate to be made of the shear rigidity

3.1.4 Dimensional stability

FAST differs from KES-F in that it includes an assessment of how susceptible the fabric is to shrinkage. No special equipment is needed for this other than an oven, in which the sample is dried at 105

oC. Shrinkage is measured (for each of the warp

and weft directions) to give an initial length L1. The sample is then soaked in water

and re-measured to give a ‘wet relaxed’ length L2. It is then re-dried in the oven and

re-measured to give a final length L3. The following parameters are determined from these measurements:



• Relaxation shrinkage: %100xL

LL

1

31 −−−−

• Hygral expansion: %100xL

LL

3

32 −−−−

3.2 The wider applicability of FAST

The basic parameters measured using FAST can be utilised effectively in determining fabric performance more widely. For example, from the warp- weft- and bias-extension data the in-plane shear behaviour can be derived. In combination with the bending parameters, this can then be related, for example, to the fabric’s ability to drape.

Fabric ‘formability’ can be assessed from the longitudinal compressibility and the bending rigidity. Although the former is not measured directly, it can be estimated by assuming the in-plane compressional modulus to be equal to the extensional modulus.

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4.1 Identification of potential problems in garment manufacture

The many processes that are involved in making up fabric and other components into a final product, such as a garment, can be fraught with a range of difficulties arising from shortcomings in the fabric properties. Particularly in view of their convenience and speed of operation, KES-F and FAST have proved of great value in their ability to highlight such potential problems. Examples of property-deficiencies that might be encountered include:

• low bending stiffness This can cause ‘seam-pucker’ and problems in cutting out.

• Excessively stiff fabrics Very stiff fabrics may be more manageable in sewing, but can cause problems during operations such as moulding.

• Low values of fabric extension If the fabric is insufficiently extensible it can give rise to unwanted effects such as seam-pucker, problems associated with moulding and overfed seams.

• Excessively high values of fabric extension If a fabric is extended too easily, it can cause problems in laying-up because it may stretch during cutting, producing excessive subsequent shrinkage and pattern matching errors.

• Low values of formability



If the formability is too low there is the likelihood of puckering, especially when the fabric is made up into collars, cuffs etc.

• High values of shrinkage: This is often associated with problems in garment sizing and seam pucker during final pressing.

• High values of hygral expansion If the hygral expansion is high there can be a significant loss of appearance when the fabric/garment is stored in humid conditions, because the dimensions will increase. This may or may not be reverible.

4.2 KES-F & FAST in practical quality control procedures

The KES-F and FAST systems are considerably useful in the area of fabric quality control. In the case of KES-F, the measured values of the 16 properties are generally "normalised" by comparing them with their counterparts in an "ideal" reference fabric. The normalised value, xN, of any given property is defined as follows:

(((( ))))σσσσ

−−−−====

xxx measN

In the above, xmeas is the actual property value, as obtained from the instrument, in its appropriate units. x is the average value of the same property - obtained "historically" over many tests - for fabrics of that type (ie the "reference" value), and σσσσ is a parameter called the 'standard

deviation' which is a statistical parameter related to how variable such fabrics might be. The normalised quantity would clearly be zero if the reference and test fabric were exactly the same with regard to that particular property. The greater the normalised value (either positive or negative), then the further the test fabric lies from the reference. In any practical application there will be a "window" of acceptable values. Typically, all the normalised properties are plotted on the same scale onto a single chart, preferably along with their respective acceptable maximum and minimum values. Joining the data points with straight lines produces a ‘snake chart’. A similar normalisation process is applied to the primary and total hand values, so that these can also be incorporated. See Figure 8.



Figure 8. A typical HESC control chart, as used with KES-F; source: S Kawabata, "The Standardization and Analysis of Hand Evaluation", Second Edition (1980)

Using this kind of chart, departures from the ideal (zero) can readily be identified. The procedure is used to map the acceptable range for the various handle parameters in relation to the technical requirements for any given application or set of operations, such as cutting and sewing.



The FAST system is also often used in conjunction with a control snake-chart (Figure 9). In this case, though, it is usual to plot the actual measured property values in their original units, as distinct from the dimensionless normalised quantities. The procedure is designed specifically to aid the tailoring of worsted suiting fabrics, as previously mentioned.

Figure 9 A blank control chart, as used with the FAST system; the shaded areas indicate where problems could be expected to occur during the various making-up operations;

source: CSIRO Report No. WT92.02 (see Further Reading)



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A de Boos & D Tester. "SiroFAST – A System for Fabric Objective Measurement and its Application in Fabric and Garment Manufacture", CSIRO Report No. WT92.02 (1994). ISBN: 0 643 06025 1 JE Booth “Principles of Textile Testing”, (3rd Ed.), Butterworths (1986). ISBN: 0 408 01487 3. Chapter 7 BP Saville. “Physical Testing of Textiles”. Woodhead (1999). ISBN: 1-85573-367-6. Chapter 10

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Topic7x Kawabata FAST