Factors including the extensibility and weave of a fabric, variations in crimp and tensile characteristics, and inconsistencies in yarn and transverse threads all have a significant impact on fabric strength.
2.4.1 Nature of stress and time of reaction
The nature of the stress-inducing object and the intensity of force working on the fabric over time can be used to predict the nature of a tear. However, this can never be forecast with surety in practice. Despite possessing adequate extensibility and tensile strength, a fabric must have adequate time to react if it is to suppress the incoming force by elongation of the yarn. However, under practical circum- stances, tearing actions are instant and cause damage even if the fabric possesses adequate breaking strength. The lower the symmetry of the object, the more severe is the stress exerted, the shorter the time available for response, and the more severe the nature of tear, devaluing the fabric for further use.
2.4.2 Fabric crimp
Crimp height plays a crucial role in predicting the sustainability of woven textiles.
It is inversely proportional to the abrasion resistance of the fabric; the higher the crimp percentage, the lower the abrasion resistance, serviceability and durability will be (Backer and Tanenhaus, 1951). It is the crimps that face the brunt of the abrasion rather than the rest of the fabric during use; the latter faces the destructive abrasion only when the crimps have worn out completely. Variation in crimp at different places in fabric (which arises from lack of control over tension on individual ends or picks) reduces its strength. The fall in strength is due to the differential load behaviour of yarns in the strip. Washing or wet treatment of woven fabrics permits shrinkage due to swelling, thus increasing the crimp height.
The weaving particulars of the grey fabric should be adjusted to achieve a lesser crimp and a lower yarn density in the finished fabric, as a fabric with lesser warp and weft density retains strength with greater ease.
46 Understanding and improving the durability of textiles
2.4.3 Tensile characteristics of fibre/filament
The stress–strain behaviour of a fabric reflects the characteristics of the constituent fibres, though the twist factor, weave geometry and various finishes also play a vital role. The stress–strain pattern of the constituent fibres depends upon their molecu- lar structure. In general, the longer the fibre, the more efficient its tensile characteristics will be. Applied stress initially disturbs the amorphous area through stretching of the primary and secondary bonds, enforcing elongation. Under a progressively increasing load, deformation starts by breaking these bonds, leading to the subsequent breaking of the specimen. Fibres possessing increased elasticity respond more quickly to stress, extending to prevent tearing of the fabric (Booth, 1968). The tensile strength of cotton fabric is directly proportional to the constituent fibres; any weakness in the latter weakens the final fabric strength, especially if the fabric includes fire retardant (FR) and durable press (DP) finishes.
Although fabrics made from cotton types easily meet most civilian standards, military fabrics, which have extremely high strength specifications, suffer from the loss in strength after finishing. The problem is worsened by any weak fibres in the construction of the cotton, leading to increased focus on the use of cotton–
polyester or all-polyester fabrics in the construction of military personnel apparel (Ruppenicker et al., 2007).
2.4.4 Irregularity in fibre, yarn and fabric structure
Fabric strength is highly influenced by evenness in fibre and yarn structure. The greater the unevenness, the higher the number of weak points inherited by the fabric, resulting in poor strength and prompting breakage at the weak areas, due to the ‘weak link effect’. It is important for the fibres and yarns to have uniform diameter in order to adequately share the applied load thoroughly, restricting the initial breakage. Regular orientation of the polymer chains also enhances fabric performance.
Though testing for tensile strength shows higher strength in a large sample size featuring a regularity of structure, the unpredictable nature of tearing and localisa- tion of weak points may also lead to individual results suggesting such strength in fabrics made of irregular yarns or fibres too. The percent of immature natural fibres or fibres of a lower degree of polymerisation in the blended yarn, along with the homogeneity of such fibres in the blend, are also responsible for uniform shearing under stress.
2.4.5 Impact of transverse threads
Under a progressively increasing load, the crossing or transverse threads interrupt the process of extension and increase the load required to break the specimen. This is referred to as ‘fabric assistance’. In the absence of any crossing threads, a series
Strength properties of fabrics 47
of threads will break in proportion to their extensibility, with the least extensible thread breaking first. The presence of crossing threads shifts the mode of break to the weakest link, facilitating ‘localised rupture’ (Taylor, 1959). The crossing threads grip the interlaced longitudinal threads together due to their rigidity, which in turn develops friction and prevents the free extension of the individual threads, even at the weakest link. The extension distribution does not remain uniform across warp or weft, and distorts the band as it approaches a complete break. As one thread breaks, the average load placed on the other threads increases, facilitating the breakage of another thread, and so on. Thus the tearing starts and damage spreads.
The calculated extension, which is expressed as the sum of the average exten- sion of individual yarns under test in the absence of transverse threads, remains somewhat lower than that of the fabric specimen under test. This is due to the exclusion of crimp, which is released only during the final extension, causing a higher practical extension overall. The ‘cloth strength ratio’, which is the ratio of the fabric test strength to the total strength of threads (average strength of individual thread multiplied by number of threads under stress), decreases by 1%
for each percentage increase in the crimp of the threads under stress (Taylor, 1959).
An increase in the stress placed on longitudinal threads forces a transfer of crimp onto the transverse threads, narrowing the strip into a ‘waisted’ shape. The central width of the strip reduces proportionately with any increase in longitudinal thread density, resulting in non-uniform stressing of the threads and reduced fabric strength. A balanced thread density with lower crimp increases the overall fabric strength by producing a more equal placement of stress on individual threads, minimising ‘waisting’.
2.4.6 Compactness of fabric
The more compact a fabric is, the greater will be the residual stress arrested in it, reducing its overall strength due to yarn jamming (Peirce, 1937). A compact structure is comprised of a higher thread density which, in association with a typical weave pattern, restricts the movement of threads at the area of impact and forces them to directly share the full stress. In contrast, a less compact fabric hinders part of the incoming force, providing better protection against tearing. The twist level of the yarn is also indicative of a fabric’s proneness to tear. The harder the yarn, the lower its breaking strength and abrasion resistance, leading to an increased risk of tearing.
2.4.7 Fabric construction
The woven structure of a fabric, in conjunction with its thread density, predicts the possible behaviour of the threads against causes of tearing. The ability of threads to slip over each other reduces impact on the fabric, and protects it from being torn.
48 Understanding and improving the durability of textiles
A plain woven structure is the most compact and interlaced structure, making it the most prone to damage. The lack of elasticity produced by the compact interlacing of the threads does not allow for any absorption or dissipation of the shock. This is in contrast to less compact fabrics, such as matt, twill, satin and pile, in which the looser weave facilitates greater resistance to impact.
2.4.8 Weaving faults
Any imperfection in the thread results in a weaker fabric, with a missing or incomplete pick or knot reducing both the appearance and strength of the final product. The differing behaviours of filament and staple yarns are also of impor- tance. Unless woven tightly, nylon fabrics have filaments which have a tendency to slide over each other due to their highly smooth surfaces, thus spoiling the fabric geometry. Fabric produced using staple yarn does not show such tendencies (Scott, 1951).
2.4.9 Strength loss on wetting
The serviceability and durability of a fabric are influenced by its component fibres.
Viscose, an alternative to good quality cotton, is often mixed with cotton or polyester to produce a lower cost blend with increased moisture absorption and lustre. However, viscose is subject to a reduction in strength when wetted, and is slowly damaged by the typically alkaline solution of cyclic domestic washing.
This causes a gradual removal of the viscose from the blend, eventually leading to a garment or apparel of inferior strength and aesthetic value.
2.4.10 Damage during mechanical processing
Mechanical processing, such as shearing, singeing or calendaring, causes abrasion of the fabric due to the friction experienced whilst it is passed through the rollers.
Though the extent of the friction is quite nominal because of the highly polished nature of the rollers, the level of tension to which the fabric is subjected may cause damage, thus reducing the lustre, appearance and strength. In addition, polyester fabrics become caught on guide rolls due to static charge during drying, making them particularly susceptible to damage.
2.4.11 Chemical processing of fabric
Textile fabrics intended for use in the production of apparel and fashion-wear are chemically processed through pre-treatment, dyeing, printing and finishing, in order to introduce the desired aesthetic properties. The combined action of soda, heat and oxygen produces oxy- and hydro-cellulose during the scouring of cotton, whilst the reducing and oxidising chemicals applied in dyeing and printing attack
Strength properties of fabrics 49
the polymer chains in the fibre, weakening the fabric. Re-dyeing further intensifies the damage. Resin and catalysts used in functional finishing also lead to damage, as they facilitate the formation of cross-links within the fibre. These cross-links restrict the movement of individual threads, reducing the fabric’s resistance against tear. A fabric with a structure that is less interlaced, such as twill or rib for example, shows greater tear resistance, but fails to perform at the same level if functionally finished.
2.4.12 Seam strength and fabric damage
The sewing thread and needle selected to stitch specific fabrics should be chosen carefully, in order to produce apparel of increased service and usability. Use of finer threads to join coarse fabric patterns and vice versa can disturb the appear- ance, seam strength and durability of a garment. The use of finer threads for the joining of coarser fabrics reduces the durability of a seam, whilst use of coarser threads on finer fabrics results in yarn slippage and fabric damage.
Stitch density should be optimised for enhanced cohesion among joined pieces, in order to produce the greatest seam strength and lowest seam slippage. Cuts or damage to the yarn during needle penetration can instigate a hidden tear, which may be realised only later, under the impact of friction, producing a key fault even with otherwise benign abradants. Knits are less prone to such damage (Scott, 1951;
Dorkin and Chamberlain, 1952a, 1952b). Sewing threads should thus possess structural stability for the most efficient seam performance and, in addition to selecting appropriate thread initially, man-made fibre sewing threads may be heat set under controlled conditions in order to avoid loss in strength (Hall and Knoff, 2008).
2.4.13 Yarn slippage
Open fabric structures with a low yarn density face an increased risk of yarn slippage, reducing both strength and appearance. This happens when some or all of the yarns in such fabrics are abraded by the body during cyclic wear. However, in spite of having such open structures, the nature of yarn interlacement may help inhibit slippage, as demonstrated by the side crossing of yarns in the structure of fabrics such as gauze and leno. Furthermore, this use of interlacement can ensure that yarn slippage does not become severe in compact fabrics while under stress.
2.4.14 Handle of fabric
The softer the fabric, the easier it is to handle and the lower the risk of damage from abrasion. The use of less twisted yarns protects the surface layer from loss or damage. All natural fibres inherently remain embedded with oils or hydrophobic materials, which are removed during pre-treatment to develop absorbency for
50 Understanding and improving the durability of textiles
colouration and finishing in subsequent stages. In order to improve the handling of the fabric, this loss should be compensated through decatising or finishing with various types of softening chemicals (Hall, 1966; Vaidya and Trivedi, 1975).
2.4.15 Structural stability during use
Any apparel or garment must be handled with adequate care to maintain its structural and aesthetic integrity. The activities undertaken in its use and care, including wear, domestic washing and draping (pressing) must be performed with due attention. The label supplied by the manufacturer normally provides clear guidance on the best way to handle and care for the garment.