The durability and serviceability of a fabric are mainly judged by its mechanical/
physical and aesthetic parameters, and the services it has to cater for with a progressive decrease in overall quality during wear, as depicted in Fig. 2.1. With extended use, the fabric’s mechanical and physical properties deteriorate progres- sively, with a consequent fall in strength and performance parameters; its appearance and feel deteriorate with time under differing conditions of use. Deficiency in any of these criteria restricts further wear, even if the apparel is otherwise in good condition.
Any specific application requires a minimum fabric strength. This level is defined as the strength required during practical use, with some additional excess over the minimum level. Enhancement of other aspects can add such excess, with better drape, for example, enhancing abrasion resistance and durability. The minimum strength factor, along with the appearance and handling of the fabric, comes into effect from the stage of marketing, meaning that the basic grey woven fabric produced before this must retain a strength level at least 10% higher than the value required in practice. A fall in strength noted during routine checks may be regarded as a warning of a change in the quality of the raw material or yarn, or of a deterioration of the fabric during various processes.
The strength parameters are key deciding factors in ensuring the durability and serviceability of the final product. When serviceability refers to the ability of a fabric to be used with ease and comfort, its durability facilitates trouble-free servicing for an extended span of time. Though tests to assess strength can provide a good indication of the behaviour of the fabric in practical circumstances, miscellaneous tests to ascertain durability have virtually no relationship with the practical stresses fabric is required to withstand in adverse situations, as the range of stresses a fabric or garment may face are infinite in number.
Adequate drape, air permeability and crease recovery are essential physical attributes of a fabric, but these are of no use unless the fabric is strong enough to face the abrasion and stress encountered during everyday ‘wear and tear’. The flexing of body parts puts multi-directional stress on wear apparel, with every movement of the body forcing apparel to change shape or extend in new directions.
Similar demands placed on other fabrics can reveal the strain to a greater extent, with the positioning and fixing of a tent to the ground providing a particularly good example.
Pattern pieces of apparel are stitched under tension, necessitating adequate
Strength properties of fabrics 35
2.1 Changes in fabric properties during cyclic wear.
fabric strength. Increased use of technical textiles in recent times has necessitated the recording of measurements to enhance strength factors. Electrotextiles, a range of smart fabrics developed for numerous military and civilian applications, are used to sense and respond to pressure, temperature or electrical charges in their environment. These have other important applications, too, in areas such as healthcare, safety, communication and entertainment. They are highly flexible, comfortable, air permeable, dimensionally stable, lightweight and non-conduct- ing. However, even with all of these attributes the fabrics must possess high uniformity, tensile/tear strength and abrasion resistance if they are to be optimally used in their respective fields (Karthik, 2004).
2.2.1 Tensile strength
Tensile strength, or breaking strength, is one of the major attributes of woven fabric. It is a measure of the degree of coherence of a fabric, and without it, other properties have little importance.
Tensile strength is the breaking strength of a specimen under exertion of a force capable of breaking many threads simultaneously, at a constant rate of extension/
load. It may be more readily interpreted in terms of the properties of the component parts of the cloth structure, and the way in which those are assembled. Tensile strength quantifies the force needed to stretch a fabric to the stage where it breaks;
in other words, it is the maximum amount of tensile stress that a fabric can withstand before failure occurs.
Yield strength is defined as the point of stress a fabric can withstand before it is deformed by 0.2% of the original dimension. A fabric is distorted elastically prior to reaching the yield point, and returns to its original shape with the removal of stress. Beyond the yield point, the deformation developed remains irreversible.
In practice, a fabric or garment is extremely unlikely to experience situations as adverse as those used during testing, meaning that the instrumentally predicted breaking strength of a fabric does not hold a direct relationship with its serviceabil-
WEAR
Change in cyclic performance behaviour
Aesthetic Mechanical Physical
Appearance
Compression Air permeability Thermal conductivity Absorbency Colour fastness Flexion
Pilling
Distortion
Handle Tensile Shrinkage
36 Understanding and improving the durability of textiles
ity. However, it does help in predicting the maximum stress the fabric can withstand being regularly placed upon it, and can help reveal the extent of any deterioration that may occur during processing.
Both mechanical and chemical processing can cause fabric deterioration. The extent of this deterioration may be evaluated by comparing the breaking strength of processed and unprocessed fabric specimens. However, such testing is problem- atic. Measured tensile strength remains high, as the test evaluates the collective strength of a number of ends or picks, rather than individual yarns, meaning that the severity of weak spots is not necessarily reflected in the results. Furthermore, the test includes application of the stress parallel to the yarn axis, which does not mirror the stress placed on a fabric to result in a break during practical use. The level of strength in a fabric is influenced by transverse threads, with the binding effect they produce demonstrated by the behaviour of the threads under test conditions. The extent of the binding effect increases with the number of trans- verse threads present, and the strength of the individual threads in strip form can be up to 1.8 times the strength of the single threads if tested individually (Booth, 1968).
The possibility of applying results obtained from fabric testing to the assessment of fabric behaviour in reality is further diminished by the effects of crimp interchange, which allows flexibility and extensibility of the fabric. When a strip of fabric is put under force, the gripped ends are straightened and crimp inter- change occurs, with the crimp transferred to the transverse threads (Booth, 1968).
Progressive stretching of the strip results in ‘waisting’, during which the rectangu- lar shape of the strip contracts in the middle. The initial extension of the entire fabric sample occurs at a very high rate, in order to produce yarn straightening under force and, as such, the observed behaviour of the fabric does not produce results reflecting the realistic impact of everyday stress on fabric. It is therefore difficult to predict the practical failure of a garment from such results. During use, a fabric is more likely to face incoming stress locally or on stitch lines, so failure cannot be predicted by the magnitude of tensile strength. However, tensile strength does still act as a useful parameter by which to judge a fabric, and a higher value undoubtedly endorses a product as stronger in comparison to another of lower tensile strength.
2.2.2 Tearing strength
Whilst the tensile strength of a fabric provides a potential parameter for basic strength judgement, the tearing strength predicts the actual serviceability, as well as durability, of a fabric. Tensile strength is an instrument-based parameter, and hence question does not arise regarding its impact on usability or serviceability. In contrast, tearing is a natural, undesired and destructive phenomenon; it does not have any match with laboratory practices.
The nature and causes of tears are unpredictable, and produce a range of
Strength properties of fabrics 37
problems in garments and apparel. A hole or slit developed as a result of an accident or carelessness gradually develops into a tear, and the stresses of normal use are quite capable of causing an extension of such damage. Typical tearing processes result from the impact of a large and uneven placement of stress on the fabric, and may render the garment unserviceable by a single tear. Tearing is thus a much more common mode of failure than breaking. It is mainly dependent on the spacing and strength of the threads being torn, and the force required to make them slip over the crossing threads.
The possible sources and types of tear are infinite in number, and once the damage has occurred, further use of the fabric ceases. Office work or high fashion apparel are scrapped from further use, even if the appearance is affected only with a minor tear. Single thread strength may help in developing some idea of the vulnerability of the garment to tearing, but the results are still unpredictable, as the differing proportionately of incoming shock may cause a variety of peculiar tears which cannot be forecast.
The tearing strength of a fabric may vary based on the location of the impact, along with factors such as the fabric geometry, design and slight variations in the action of chemicals used in processing. Tearing strength is a less reliable indication of cloth quality than tensile strength. This has led to the development of several tearing test methods, including, for example, the tongue tear (single rip) and falling pendulum (Elmendorf type) methods, with a vast range of parameters and test details used in an attempt to produce a closely matching performance criteria.
Certain products, such as medical bandages and adhesive tapes, are precisely designed for fast use, and require a lower strength than is usually accepted, in order to allow for tearing-off without use of a cutter. A tear test gives an idea of the ability of the combined action of warp and weft yarns to work in synergy to resist a tear propagating. The tearing strength of any fabric remains far below its breaking strength, and is often at least ten times lower, meaning that in practice it provides no reflection of the tensile strength of the same specimen (Witkowska and Frydrych, 2004).
2.2.3 Bursting strength
The level of multi-directional pressure at which a film or sheet bursts is a measure of its resistance to rupture. Bursting strength depends largely on the tensile strength and extensibility of a fabric, and is expressed in pounds per square inch (psi). It is essential that certain textile products, such as swimming costumes, sacks, filter fabrics, nets and parachutes, are tested to assess their bursting behaviour, as they are likely to come under multi-directional stress internally or externally under the force of air, water or solid contents. Local degradation or deformation of fabrics that may arise from missing ends or picks, knotting, the localised action of chemicals in the scouring and bleaching of cotton, discharge printing, or catalysts in resin finishing, for example, can also be assessed. The
38 Understanding and improving the durability of textiles
laboratory methods used to assess such degradation involve the application of force on the specimen from an enclosed container of air or water. The stretch behaviour, uniformity in fineness of threads and crimp, and design of the weave or knit, can predict the phenomenon. The limited extensibility of strong threads causes these to burst first; a higher crimp in both warp and weft reflects excellent extensibility through crimp release (Booth, 1968). The elastomeric nature of the main polymer in a fabric can also play a key role in controlling the bursting phenomenon.
2.2.4 Abrasion resistance
Garments are regularly subject to wear caused by rubbing during use, which leads to progressive damage to the structure, and may be sufficient to produce an initial change in appearance before finally developing into a tear. Such tears increase under further abrasive action, thus affecting both serviceability and durability. The smoother the surface and better the drape, the greater the resist- ance of the fabric to abrasion. An open weave with a lower number of ends and reduced pick density ensures adequate abrasion resistance, with the fibre type, fibre properties and yarn twist also key factors. In addition, the nature of abra- sion and the abradant, intensity of abrasion, speed, tension on the fabric during abrasion and direction of abrasion all play a major role in the level of destruc- tion (Saville, 1999).
Abrasion resistance is influenced by several factors in a complex manner. The
‘uniform abrasion test’ helps to evaluate these factors relative to the wear service- ability of the product, but does not include the factors accounting for wear performance or durability during actual use. It should also be noted that apparel made of identical fabrics may display differential durability patterns based on the mode of end use.
2.2.5 Yarn slippage
On application of stress to a seam, the yarns in the fabric slide out of the weave construction, causing seam grinning and fabric distortion known as yarn slippage.
The propensity of yarns to slip or distort results in poor fabric appearance, resulting damage sometimes referred to as ‘finger marks’ or ‘shift marks’. Yarn slippage creates a range of typical problems on open-weave fabrics such as nettings, marquisettes, gauzes, chiffons, and heavier fabrics made from slippery surface yarns. On exposure of the fabric to a specified shearing force, the degree to which the force causes yarns to shift and distort the original symmetry of the weave is expressed as the ease of yarn distortion in the fabric. This is reported in terms of the widest opening, measured in hundredths of an inch. Yarn slippage changes the load shearing pattern of the fabric, and deteriorates the overall appearance of a fabric.
Strength properties of fabrics 39
2.2.6 Seam strength
The objective of seams is to join pattern pieces in order to develop the standard appearance and performance of apparel. Stitches with adequate strength are introduced at some distance from the pattern edge. The strength of the stitching, which is designed to remain intact throughout the life of the apparel, is known as the seam strength. The seam applied may be subjected to many adverse situations leading to seam failure, and thus shortening the serviceability of the apparel. Seam failure may occur in various ways, for a variety of reasons. A seam may wear out at places, even if the fabric is in good condition, whilst seam slippage may occur due to the reduced interlacing of a fabric with either an open structure with smooth threads, or a loose structure with fewer ends and picks per unit length. Finally, sewing thread may be broken due to needle cutting (Booth, 1968).
The best seam performance can only be achieved by judicious selection of the seam/stitch type, sewing machine feeding mechanism, needle and sewing thread (Carr and Latham, 1994). The seam type, in many instances, produces a significant effect on the seam strength. Out of four seam types frequently used in the stitching of different areas of denim trousers, the descending order of seam strength was found to be as follows:
• felled seam
• 5-thread overlock + 2-needle lockstitch
• 5-thread overlock + single-needle lockstitch
• overlock stitch
For areas remaining under high tension during wear, it has even been recommended that both of the first two seam types should be used (Yesilpinar and Eylul, 2007).
2.2.7 Pile loss
Extra threads are introduced in a few classes of fabric, on one or both sides, either to form loops, such as in terry towelling, or for extra figuring, such as in carpets, velvets or warp/weft pile fabrics. Loop yarn is soft, less twisted and coarser, allowing for better moisture absorption and feel, whereas the piles are shaped by cutting for decoration. These loops and piles must remain secured and intact to retain commercial and aesthetic functions, which becomes possible only if these are retained by the fabric with adequate strength and restricted movement under abrasion. However, abrasion experienced by such fabrics can cause the complete collapse of the loop, or progressively diminish the loop height after several uses, thus reducing the feel and moisture absorptivity. Alternately, abrasion pulls out the cut pile yarn, diminishing aesthetic and wear resistance values. These extra threads form the surface layer, which faces abrasion prior to that experienced by the rest of the fabric, and hence they have to be stitched firmly during weaving by means of a suitable stitching technique (Grosicki, 1977).
40 Understanding and improving the durability of textiles