★ What are industrial textiles? Definition of industrial textiles: Textiles that are specially designed with specific…
Fabric tensile properties refers to that when the fabric is subjected to external tensile force along the wrap and weft direction, the fabric appears mechanical deformation law. Commonly speaking is that a phenomenon in which a fabric is elongated and deformed, or even broken and damaged, by the action of an external force in tension.
★ Principle of fabric tensile breaking
1 When the fabric starts to be stretched, the deformation is mainly the stretching of the fibre macromolecular chains themselves, i.e. the deformation of the bond length and bond angle. Yarns that are stretched become straight, in basic accordance with Hooke’s law, while yarns that are not stretched become more curved.
2 When the external force increases further, the stretched fibres and yarns elongate and become thinner, the fabric becomes thinner, the non-stretched yarns bend, the length shortens, the deformation at the pinch is smaller, the middle is larger and the specimen gradually shrinks. At this point the macromolecular chain in the amorphous zone overcomes the secondary valence bond force, part of the macromolecular chain straightens, the taut one may be pulled off, and it may also be drawn out from the irregular crystalline part. The breakage of the secondary valence bond causes the macromolecules in the non-crystalline zone to gradually produce misaligned slip, the deformation of the fibres is more significant, the modulus gradually decreases accordingly, and the fibres enter the yield zone.
3 Continue to stretch the fabric, the resulting deformation is mainly the bond length of the molecular chain, the change of the bond angle and the destruction of the secondary valence bond, the fibers enter the strengthening zone, the modulus of the fibre increases again until it reaches the breakage of the fibre macromolecular primary bond and the large multiple valence bonds, resulting in the disintegration of the fibre and the fabric breaking.
The strength utilisation coefficient of the yarn, K
Pf: fabric breaking strength (N)
Py: yarn breaking strength by stretch (N)
K: Generally greater than 1, sometimes less than 1.
★ Assessment of the tensile properties of fabrics
Depending on the form of action of the external force on the fabric, stretching is classified as either warp or weft and can act alone or together. Test methods for the tensile properties of fabrics include the strip method with the selvedge removed, the grab method without the selvedge removed, the shear strip method, the trapezoid method and the circular strip method. The strip method with the edge yarn removed, the grab sample method without the edge yarn removed and the shear strip method are suitable for testing the tensile properties of woven fabrics, while the trapezoidal method and the circular strip method are suitable for testing knitted fabrics. The indicators assessed are breaking strength, elongation at break, load at constant elongation, elongation at constant load, etc. The breaking strength is one of the main indicators of the intrinsic quality of the fabric.
1 Indicators of primary tensile breaking: breaking force, breaking strength, relative strength, elongation at break
Breaking force: the force required to break a textile material when it is stretched by the outside world, an indicator of the absolute value of the tensile force, the basic unit is N (Newton), the derived units are: cN, mN, kN, etc. The data measured on various tensile strength machines are all breaking force values. For example, single fibre and bundle fibre strengths are the force required to stretch a fibre or a bundle of fibres until it breaks. The breaking force is related to the thickness of the fibres and yarns, so for fibres and yarns of different thicknesses, the force is not comparable.
Breaking strength refers to the maximum tensile force per tex (or per denier) of a fibre or yarn, the unit is “N/tex” or “N/D”.
Relative strength: The maximum tensile force per unit fineness of fibre or yarn, including breaking stress, breaking strength and breaking length.
Elongation at break: When a fibre, yarn or fabric is stretched, the percentage of elongation produced in relation to the original length, which is known as elongation. The elongation when stretched to break is called elongation at break and it indicates the ability of the textile material to withstand tensile deformation.
2 Elongation curves and indicators
Load-elongation curve: A curve representing the relationship between load and elongation of a textile material during stretching. The load-elongation curves are not comparable for fibres of different thicknesses and lengths.
Stress-strain curve: In a stress-strain curve, the vertical coordinate represents the relative strength and the horizontal coordinate the elongation. It allows comparison of the tensile properties of materials with different finenesses and specimen lengths.
Initial modulus: On the fibre load-elongation curve, the initial modulus refers to the ratio of stress to strain on the extension of the straighter part of the starting section. On the stress-strain curve, the initial modulus is the slope at the beginning of the curve.
The size of the initial modulus indicates the ease of deformation of the fibre under small loads and reflects the rigidity of the fibre. If the initial modulus is large, it means that the fibres are not easily deformed under small loads, and are more rigid; conversely, if the initial modulus is small, it means that the fibres are easily deformed under small loads, and are less rigid and softer.
3 Breaking work and breaking specific work
breaking work: the work done on a fabric by an external force when the fabric is tensile breaking.
Breaking specific work: the work done by an external force on a unit mass of fabric.
★ Factors influencing the tensile properties of fabrics
1 Fabric density and fabric structure
Fabric density: Increase warp density, high interweaving resistance, warp and weft strength both increase. Increase weft density, weft strength increases, warp strength decreases (warp opening times increase, stretching and friction increase).
Fabric structure: The higher the number of interlaces, the higher the strength. Under the same conditions, the breaking strength and elongation of a plain weave are greater than those of twill, which in turn are greater than those of a satin.
2 Tex and twist of yarn
The strength of the yarn is high with a high Tex; with the same Tex number, the strength of the thread fabric is greater than the strength of the yarn fabric (good dryness, small uneven twist).
For twist, the strength of the fabric starts to drop when the critical twist is approached; with the same twist direction, the strength is high (fibres mesh with each other at the yarn crossing, high resistance to interweaving).
3 Fibre varieties and blending ratio
Fibre variety is the decisive factor for the tensile strength of the fabric, the blending ratio is different, the tensile strength of the fabric is also different.
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