GATE Textile Engineering Textile Testing Flashcards

24 sample cards from the Textile Testing deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. How is fabric bending (flexural) rigidity $G$ related to bending length $c$ and fabric mass per unit area $w$ (Peirce)?

   $$G = w \, c^{3}$$ where $w$ is mass per unit area (mg/cm$^2$) and $c$ is the bending length (cm). $G$ is in $\text{mg}\cdot\text{cm}$; higher $G$ means a stiffer fabric.

2. In the cantilever (Shirley stiffness) test, how is bending length $c$ obtained from overhang length $l$ at $41.5^\circ$?

   A strip is pushed over an edge until its tip deflects to $41.5^\circ$; the overhang length $l$ is read and the bending length is $$c = \frac{l}{2}$$ Bending length is the length of fabric that bends under its own weight to that reference angle.

3. Define flexural rigidity in terms of Young's modulus and give the bending modulus expression.

   Flexural rigidity $G = \dfrac{E I}{\text{width}}$ for the fabric. The bending modulus (a thickness-normalised stiffness) is $$q = \frac{12\,G \times 10^{3}}{T^{3}}$$ where $T$ is fabric thickness (mm) and $G$ is flexural rigidity; $q$ removes the effect of thickness, giving intrinsic material stiffness.

4. Define fabric drape and the drape coefficient.

   Drape is the ability of a fabric to deform into folds under gravity. The drape coefficient (Cusick drapemeter) is $$DC\% = \frac{A_s - A_d}{A_D - A_d}\times 100$$ i.e., the projected shadow area of the draped sample minus the support disc, divided by the undraped annulus area; higher $DC\%$ means stiffer, less drapeable fabric.

5. Define crease recovery angle and the principle of its measurement.

   Crease recovery angle (CRA) is the angle to which a creased fabric recovers after a folded specimen has been held under a standard load for a set time, then released for a recovery period. It is measured on a crease-recovery tester; a higher CRA indicates better wrinkle/crease recovery.

6. What is fabric shear and how is shear rigidity defined?

   Shear is the angular distortion of a fabric when the warp and weft are deformed from their perpendicular configuration. Shear rigidity $G$ is the slope of the shear force per unit length versus shear angle curve at low angles; it governs the fabric's ability to conform to compound (double) curvature, important in tailoring.

7. State the two main principles of fabric tearing-strength tests.

   (1) Ballistic / pendulum (Elmendorf) method: a notched specimen is torn by the energy of a falling pendulum, and the energy lost (tear force) is read from a scale. (2) Single-rip (tongue/trouser tear) method: a tongue-shaped or wing specimen is torn on a tensile tester at constant rate, recording the tearing force trace (a series of peaks).

8. Why is tear strength often a better indicator of serviceability than tensile strength, and why are tear forces much lower than tensile forces?

   Tearing concentrates stress on the few yarns in the 'del' (the small region at the apex of the tear), so only a handful of yarns bear the load at any instant rather than the whole width. This localised loading means tear forces are far lower than breaking strength, and tear better reflects real failures that start at a nick or cut.

9. Define bursting strength and state the basic relation for the Mullen-type diaphragm test.

   Bursting strength is the multidirectional (biaxial) pressure a fabric withstands before rupture when a fabric clamped over an expanding rubber diaphragm is inflated. Net bursting pressure $$P_{net} = P_{total} - P_{diaphragm}$$ It suits knitted, nonwoven and net fabrics where tensile strip tests are unsuitable.

10. What is a pneumatic bursting test and how does it differ from the hydraulic Mullen test?

    Both inflate a rubber diaphragm beneath the clamped fabric to failure, but the hydraulic (Mullen) test uses fluid pressure while the pneumatic test uses compressed air. Both report bursting pressure (kPa) and may also report bursting distension (height of dome at burst); pneumatic is common for high-strength technical fabrics.

11. Define pilling and name two standard methods of assessing pilling resistance.

    Pilling is the formation of small balls of entangled fibres on a fabric surface caused by abrasion in use. Methods include the ICI pilling box (tumbling tubes on cork-lined boxes) and the Martindale pilling test (rubbing against itself in a Lissajous figure); pills are rated 1 (severe) to 5 (none) against photographic standards.

12. Describe the Martindale abrasion test principle and how resistance is judged.

    A circular specimen rubs against a standard abradant (wool/worsted fabric) following a Lissajous (figure-of-eight) motion under a defined load. Abrasion resistance is judged by the number of rubs to produce a specified end-point: yarn breakage (e.g., two threads broken), a hole, or a defined mass loss / appearance change.

13. In tensile testing of fabrics, distinguish the strip test from the grab test.

    In the strip test the full width of a ravelled strip (e.g., 50 mm) is gripped and stretched, measuring strength of all yarns in that width. In the grab test a wider specimen is gripped only over a central narrow jaw (e.g., 25 mm), so adjacent yarns share load (assistance effect); grab strength per cm is therefore higher and represents in-use behaviour better.

14. What does KES-F (Kawabata Evaluation System) measure and which six properties does it cover?

    KES-F objectively measures low-stress mechanical properties used to predict fabric hand: tensile, bending, shear, compression, surface (friction and roughness) and thickness/weight. From these, Total Hand Value (THV) and primary hand values (Koshi, Numeri, Fukurami) are computed.

15. What is the FAST system and what does it measure compared with KES-F?

    FAST (Fabric Assurance by Simple Testing, by CSIRO) is a simpler, cheaper set of instruments measuring compression, bending, extension and dimensional stability (relaxation/hygral expansion). It produces a control chart for tailoring performance. It addresses similar low-stress properties to KES-F but is faster and aimed at making-up/garment quality rather than full hand analysis.

16. Define air permeability and the basic quantity measured.

    Air permeability is the rate of air flow per unit area through a fabric at a stated pressure difference (commonly $100\,\text{Pa}$ or $\sim125\,\text{Pa}$ / 0.5 in water). It is expressed in $\text{cm}^3/\text{cm}^2/\text{s}$ or $\text{L/m}^2/\text{s}$ and depends on fabric porosity and structure.

17. How does air permeability relate to pressure drop in the low-flow (laminar) regime?

    For laminar flow through the fabric pores, flow rate is approximately proportional to the pressure difference (Darcy's law): $$Q = \frac{k\,A\,\Delta P}{\mu\,L}$$ where $k$ is permeability, $A$ area, $\mu$ air viscosity and $L$ thickness. At higher flows the relationship becomes nonlinear due to turbulence.

18. Distinguish wetting from wicking in fabrics.

    Wetting is the initial displacement of the fibre–air interface by a fibre–liquid interface (spreading of liquid onto the fibre surface), governed by the contact angle $\theta$. Wicking is the subsequent spontaneous flow of liquid through the capillary spaces (inter-fibre pores) driven by capillary pressure. Wetting must occur before wicking can proceed.

19. State the equation governing the capillary pressure that drives wicking.

    $$P = \frac{2\gamma\cos\theta}{r}$$ where $\gamma$ is the liquid surface tension, $\theta$ the contact angle and $r$ the effective capillary radius. Smaller pores and lower contact angle give higher capillary pressure and faster wicking.

20. How does wicking height/distance in a vertical strip vary with time (Washburn/Lucas relation)?

    $$h^{2} = \frac{\gamma\,r\cos\theta}{2\mu}\,t \quad\Rightarrow\quad h \propto \sqrt{t}$$ The wicking height initially rises with the square root of time, then levels off as gravity balances the capillary rise.

21. How is water-vapour transmission (breathability) of fabrics characterised and measured?

    It is characterised by Water Vapour Permeability or Water Vapour Transmission Rate (WVTR), the mass of water vapour passing through unit area per unit time (g/m$^2$/24h). The cup (dish) method seals a fabric over a cup containing water or desiccant and measures mass change; the sweating guarded hotplate gives water-vapour resistance $R_{et}$ (m$^2$Pa/W).

22. Define the evaporative resistance $R_{et}$ measured on a sweating guarded hotplate.

    $$R_{et} = \frac{(p_s - p_a)\,A}{H}$$ where $p_s - p_a$ is the water-vapour pressure difference across the fabric (Pa), $A$ the plate area (m$^2$) and $H$ the evaporative heat flux (W). Lower $R_{et}$ means a more breathable fabric.

23. Define thermal resistance of a fabric and state the steady-state conduction relation.

    Thermal resistance is the resistance to dry (conductive) heat flow through the fabric. From Fourier's law in steady state: $$R = \frac{\Delta T \, A}{Q} = \frac{L}{k}$$ where $\Delta T$ is the temperature difference, $Q$ the heat flow, $A$ area, $L$ thickness and $k$ thermal conductivity. Units are $\text{m}^2\text{K/W}$.

24. What is the 'tog' unit of thermal insulation and how does it relate to SI thermal resistance?

    The tog is a practical unit of thermal insulation where $$1\ \text{tog} = 0.1\ \text{m}^2\text{K/W}.$$ It is used for clothing and bedding; the clo unit (clothing insulation) equals about $1.55\ \text{tog}$ ($0.155\ \text{m}^2\text{K/W}$). Still air trapped within the fabric is the main contributor to thermal resistance.

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