Cotton (Gossypium spp.) requires significant mechanical handling and processing from harvesting through conversion to an end-product such as apparel and other textile goods. Advancements in processing rates and spinning speeds increase the mechanical stresses on cotton fiber. Mechanical processing can degrade the quality of the cotton through fiber breakage. Fiber breakage reduces fibers' length uniformity, resulting in a reduction in yarn strength and quality. Some fiber breakage during ginning is unavoidable to separate the lint from the seed and remove non-lint content; however, fiber breakage should be minimized as much as possible (Dever et al., 1988; Griffin, Jr., 1979; Hughs et al., 2013). Processing of cotton from fiber to finished goods will also result in fiber breakage (Krifa, 2008; Robert and Blanchard, 1997). Fibers are also broken during the testing processes, further confounding the characterization and understanding of fiber quality (Krifa, 2006).
Cotton samples with relatively long fiber length and high length uniformity result in more uniform yarns with higher strength (Smith et al., 2010; Wakeham, 1955). The importance of fiber length in textile processing has made length one of the most critical fiber parameters in establishing cotton quality. Fiber strength has been assigned high importance because strong fibers will generally suffer less breakage and preserve the length parameters of the sample, resulting in yarns with improved uniformity and tensile properties (Farag and Elmogahzy, 2009). The relationship between yarn strength and fiber strength is not simple (Pan et al., 2001). The failure of a yarn made by spinning staple fibers is normally due to fiber slippage and not fibers breaking within the yarn. This mechanism of failure means that several fiber properties interact to determine yarn strength: length, length uniformity, and fineness; as well as yarn parameters such as count, twist, and spinning system used (Fiori et al., 1954; Furferi and Gelli, 2010; Zurek et al., 1987). Longer and finer fibers result in stronger yarns due to the increased number of fibers within the cross-section of a yarn as well as the increased surface area of contact between fibers, which increases the adherence between fibers due to friction (Campos et al., 2003).
Fiber bundle strength, as measured by the High Volume Instrument (HVI), is currently the only tensile property reported during cotton classification by the United States Department of Agriculture (USDA), therefore it is the most common tensile property used throughout the cotton industry (Hequet et al., 2014). Alternatively, the Stelometer (Rouse, 1964) and Favimat (Foulk and Mcalister, III, 2002) may also be employed to measure the tensile properties of cotton. All three methods are able to report both fiber strength and elongation, although elongation has not been widely utilized in the determination of fiber quality (Benzina et al., 2007; Mathangadeera et al., 2020).
As with most properties, the measurement of bundle strength and elongation is impacted by the distribution of tensile properties within the sample. Numerous studies have examined the complex relationship of single fiber tensile properties to bundle tensile properties and, ultimately, to yarn properties (Frydrych, 1995; Koo and Suh, 1999; Nachane and Krishna Iyer, 1980; Orr et al., 1955). The Favimat single fiber tester allows for the distribution of tensile properties to be measured, although it is a time-consuming process. The Stelometer (ASTM D-1445, 2021) uses a flat bundle of fibers secured across two clamps with a 3.2 mm (1/8 inch) gauge length, while the HVI (Taylor, 1986) uses a tapered bundle of fibers with a 3.2 mm gauge length. Replicated bundle tests can be used to measure some level of variation of tensile properties within a sample; however, the true distribution of tensile properties is only able to be measured with single fiber testing.
In recent years, there has been a renewed focus on using elongation in breeding programs (Benzina et al., 2007; Kelly et al., 2019) and the role of elongation in processing (Mathangadeera et al., 2020). Much of the renewed interest is due to the development of calibration procedures for the HVI elongation measurement (Delhom et al., 2020; McCormick et al., 2019). However, questions remain about how to apply an elongation measurement to provide the most utility and how to interpret elongation in light of other confounding factors such as measurement equipment, test parameters, variety, growing condition, and the interaction with other fiber properties. The HVI elongation measurement is the only practical approach to routine measurement of fiber elongation, but it must be understood how to implement the measurement. It is not known if the best approach is to utilize HVI strength and elongation independently or to consider strength and elongation together. Simply adding another measurement, elongation, would add to the existing burden of end-users being able to only manage a finite number of parameters. Fiori (1956) proposed two possible approaches to utilizing both strength and elongation in understanding the response of cotton samples to processing. One approach is a toughness index, while the other is the stiffness of the fiber. Toughness index is the work-to-break, or energy required to break the fiber(s). The stiffness of fiber can be assessed by examining the elastic properties of a fiber by calculating the secant modulus, Es. The secant modulus is the slope of the line drawn from the origin to the breaking point on a force and elongation diagram for the fiber.
Most materials can be categorized as either brittle or ductile. Ductile materials experience both elastic and plastic deformation under loading. Elastic deformation is completely recoverable when the load is removed, while plastic deformation occurs when the yield stress has been exceeded, and plastic deformation results in permanent deformation of the material after the load is removed. Cotton does not exhibit plastic deformation during loading. Brittle materials do not exhibit plastic deformation and are generally considered to exhibit low rates of elongation under loading. Cotton fibers have a wide range of elongation, with approximately 10% elongation being near the upper end of expected elongation. However, the force-elongation loading of cotton is linear and can therefore be modeled as an elastic material complying with Hooke’s Law (Eq. 1) (Benzina et al., 2007; Hearle and Sparrow, 1979). Hooke’s law shows that a force applied to an elastic material will result in a change in length governed by a constant, k. The constant k can be calculated by dividing the force, F, by the change in length, x, which is also referred to as strain. The modulus of a material can be calculated by dividing stress (force divided by the cross-sectional area of the material being tested) with strain (Eq. 2). It is seldom practical to directly measure modulus for cotton fibers or fiber bundles due to challenges with accurately measuring stress or strain due to the variation in fiber cross-section and structure. However, because the stress-strain curves of cotton fibers are linear, tenacity and elongation at break can be used to approximate the secant modulus (Fiori et al., 1956).
\(F=k*x\) (Eq. 1)
F = Force
x = displacement (change in length)
k is a material dependent constant.
\(E=\frac{\sigma }{\epsilon }\) (Eq. 2)
E = Young’s modulus
σ = uniaxial stress (force per unit surface area)
ε = strain (change in length divided by original length)
Work-to-break is determined by the area under the force-elongation curve. The tenacity-elongation curve can be used in lieu of the force-elongation curve to account for the influence of fiber fineness. Since the tenacity-elongation curve is essentially linear, the area under the loading curve is calculated by dividing the product of tenacity and elongation in half.
The introduction of more fiber quality parameters may be useful in a breeding program where the improvement of specific traits is being pursued. The utility of elongation as an independent property in a breeding program has been demonstrated by multiple researchers (Benzina et al., 2007; Kelly et al., 2019). However, for practical use by processors, the introduction of an additional trait adds to the complexity of fiber selection and modeling the expected yarn properties from fiber properties (El Moghazy et al., 1990; Frydrych 1992; Yang and Gordon 2016).
It is proposed that normalizing breaking strength (tenacity) by breaking elongation to calculate the secant modulus would provide a measure with the combined utility of strength and elongation without resulting in additional parameters for consideration by end-users.
The reported study examines the use of both secant modulus and work-to-break when calculated using the breaking tenacity and elongation as an indicator of textile processing quality. Work-to-break has been studied and proposed as a combined measure of strength and elongation (Hsieh et al., 2000; Kelly et al., 2019; Mathangadeera et al., 2020; Sasser et al., 1991), but there have been no modern attempts to revisit the use of secant modulus. The study compares the distribution of single fiber testing results to bundle tests, examines the test results as an indicator of textile processing, and investigates approaches to incorporate elongation into fiber quality assessments.
This study was undertaken using cottons grown as part of the National Cotton Variety Trials (NCVT) over two crop years and in multiple locations (Zeng, 2021), as well as the elongation calibration materials reported by McCormick and colleagues (2019). The samples have been characterized with a wide array of techniques for measuring tensile properties of strength and elongation and subjected to the stresses of textile processing via aggressive opening and cleaning. A large set of the 2018 NCVT samples were processed into ring-spun yarns and tested for skein strength.