GATE Textile Engineering Textile Testing Syllabus

What is AFIS and how does it differ from HVI?

AFIS (Advanced Fibre Information System) tests individual fibres after aeromechanical opening, giving distributions of length, fineness, maturity, neps and trash on a single-fibre basis. HVI tests fibres in bundles/beards and reports averaged classing values; AFIS gives more detailed distribution data but is slower.

Define twist of a yarn and the unit Twist Multiplier (TM) / Twist Factor.

Twist is the number of turns per unit length (TPI = turns per inch, or TPM = turns per metre). Twist factor / multiplier relates twist to count: in the English system $$TM = \frac{TPI}{\sqrt{N_e}}$$ and in tex $$\alpha_{tex} = TPM \times \sqrt{\text{tex}}$$ giving twist hardness independent of count.

State the twist-contraction (retraction) relationship and define twist contraction.

Twisting shortens a yarn because surface fibres follow a helical path. Twist contraction $$C = \frac{L_0 - L}{L_0}\times 100$$ where $L_0$ is untwisted length and $L$ the twisted length; the twisted helix angle increases with twist, increasing contraction.

Name the standard methods for measuring yarn twist and state their principles.

Untwist (direct counting) method: untwist a known length and count turns — used for plied yarns. Untwist–retwist (twist contraction) method: untwist then retwist in the opposite direction until original length is restored, reading twice the twist — used for single spun yarns, automatically compensating for contraction.

What is yarn hairiness and name two ways it is quantified.

Hairiness is the amount of fibre ends and loops protruding from the main yarn body. It is quantified by the Uster Hairiness Index $H$ (total length of protruding fibres per centimetre of yarn) and by Zweigle S3 (count of hairs $\geq 3\,\text{mm}$ per unit length).

Define the Uster Hairiness Index $H$.

$H$ is the total length (in cm) of all protruding fibre ends/loops measured optically within a $1\,\text{cm}$ length of yarn: $$H = \frac{\text{Total length of protruding fibre}}{\text{1 cm of yarn}}$$ A higher $H$ means a hairier yarn.

What is the 'lea strength' test and define Count-Strength Product (CSP / Lea CSP)?

A lea is 120 yards of yarn wound into a skein and broken on a lea tester. Count-Strength Product $$CSP = \text{Lea strength (lbf)} \times \text{English count } N_e$$ It is a count-independent index of yarn quality; higher CSP indicates stronger yarn for its count.

Why is single-yarn tenacity from a single-strand test usually higher than the lea (skein) tenacity?

In a lea/skein, many threads share the load but break progressively because of strength variation and uneven load sharing (weak-link and load-sharing effects), so not all threads carry maximum load simultaneously. Hence skein strength per thread is lower than the average single-thread strength.

Define yarn evenness (irregularity) and the two common measures $U\%$ and $CV\%$.

Evenness describes variation in yarn mass per unit length. Mean linear irregularity $$U\% = \frac{\text{mean absolute deviation of mass}}{\text{mean mass}}\times 100$$ Coefficient of mass variation $$CV\% = \frac{\sigma}{\bar{x}}\times 100$$ For a normal distribution $CV\% \approx 1.25 \times U\%$.

What is the limit (ideal) irregularity of a yarn and its formula?

The limit irregularity is the minimum possible $CV$ from random fibre arrangement (Poisson distribution), given by $$CV_{lim}\% = \frac{100}{\sqrt{n}}$$ where $n$ is the mean number of fibres in the yarn cross-section.

Define the Index of Irregularity $I$ for a yarn.

$$I = \frac{CV_{actual}}{CV_{limit}} = \frac{CV_{actual}\%}{100/\sqrt{n}}$$ It compares the actual measured irregularity to the theoretical limit; $I = 1$ is ideal random, and practical yarns have $I$ in the range of about $1.1$ to $2$.

What measurement principle does the Uster Evenness Tester use?

It uses a capacitive sensor: the yarn passes between two parallel plates of a capacitor, and the change in capacitance is proportional to the mass of material between the plates. Mass variations along the yarn produce a signal giving $U\%$, $CV\%$, spectrogram and imperfections (assuming uniform moisture and density).

What is a spectrogram in evenness testing and what does a 'chimney' (peak) indicate?

A spectrogram plots the amplitude of mass variation against wavelength. A pronounced peak (chimney) at a specific wavelength indicates a periodic (mechanical) fault, e.g., from an eccentric or damaged roller/gear; the fault's wavelength helps locate the defective machine element.

Define the three imperfection categories (IPI) counted by an evenness tester and typical sensitivities.

Thin places (e.g., $-50\%$ cross-section), thick places (e.g., $+50\%$), and neps (e.g., $+200\%$), each counted per kilometre or per $1000\,\text{m}$. The Imperfection Index (IPI) is usually the sum of thin places, thick places and neps per kilometre.

What is the variance–length curve (B(L) curve) used for in evenness analysis?

The variance–length curve plots the coefficient of variation (or variance) of mass against the length over which mass is averaged. It separates short-term, medium-term and long-term variation, helping diagnose drafting waves and quantify between-bobbin/within-bobbin variability.

What is yarn 'classification' of faults (e.g., Classimat) and what does it measure?

Yarn fault classification (Uster Classimat) groups seldom-occurring faults by their cross-sectional size and length into a matrix (e.g., A1–D4 for short thick faults, plus long thick and long thin classes). It distinguishes objectionable faults to set clearer settings for yarn clearers.

Define fabric thickness and how it is measured, including the role of pressure.

Thickness is the distance between the two faces of the fabric under a specified pressure, measured with a thickness gauge (precision micrometer with a defined presser foot area). Because fabrics are compressible, thickness must be quoted at a stated pressure; a thickness–pressure curve characterises compressibility.

Define fabric compressibility and how it appears on a thickness–pressure plot.

Compressibility is the reduction in thickness when pressure is applied. It is the slope of the thickness–pressure curve; fabrics typically show a large initial decrease (removal of surface hairiness and air) followed by a smaller, near-linear region as the yarns themselves compress.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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 $\text{m}^2\text{K/W}$.

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.