Introduction:

Tensile tests are simple, relatively inexpensive, and fully standardized by pulling on something; you will very quickly determine how the material will react to forces being applied in tension. As the material is being pulled, you will find its strength along with how much it will elongate. Tensile testing is used to determine the breaking force, elongation, and toughness properties of the textile materials. Breaking tenacity, a ratio of the breaking force to yarn linear density is also a common property for evaluating the strength of a yarn material and for comparison and validation purposes. Tensile testing of yarn to determine properties such as breaking strength and elongation includes the use of a tensile testing machine and unique grips to hold the yarn.

Tensile properties:

Tensile properties indicate how the material will react to the force being applied in tension. A tensile test is a fundamental mechanical test, where a carefully prepared specimen is loaded in a very controlled manner, while measuring the applied load and the elongation of the specimen over some distance. The measurement of the tensile properties of textile is an important branch of textile testing.

Tensile properties of different fibers:

Fiber Tensile(N/Tex) Breaking extension (%)Cotton 0.19-0.45 5.6-7.1Jute 0.31 1.8Silk 0.38 23.4Nylon 0.47 26Polyester 0.47 15Wool 0.11-0.14 29.8-42.9

Tensile properties include:

Load: The application of a load to a specimen in its axial direction causes a tension to be developed in the specimen. The load is usually expressed in grams weight or pound weight. In other hand it is common practice to leave out the word weight and quote load in grams of pounds.

Breaking load: This is the load at which the specimen breaks usually expressed in grams weight or pound weight.

Stress: This is the ratio between the force applied and cross section of the specimen. This is defined as how much pressure the material can stand without undergoing some sort of physical change.

Where σ is stress (in Newton’s per square meter but usually Pascal’s, commonly abbreviated Pa), F is force (in Newton’s, commonly abbreviated N) and A is the cross sectional area of the sample.

Strain: When a load is applied to a specimen a certain amount of stretching takes place .The strain is the term used to relate the stretch or elongation with the initial length.

Strain can be defined: l/L Where, l= Elongation and L= Initial length.

Tenacity: The ‘tenacity’ of a material is the mass stress at break, the units being, of course, grams per tex.An alternative term for tenacity is ‘specific strength.’ The ratio of load required to break the specimen and the linear density of that specimen is called tenacity.

Tenacity = Load required breaking the specimen / Linear density of the specimen

Unit: gm/denier, gm/Tex, N/Tex, CN/Tex etc.

Breaking extension: The ‘breaking extension’ is the extension of the specimen at the breaking point. It may be expressed by the actual percentage increase in length and is termed as breaking extension.

Breaking extension (%) = (Elongation at break / Initial length) × 100%

Initial modulus: In the stress-strain curve the slope at the first linear part at the curve up to the yield point is called initial modulus .up to the yield point the ratio of stress to strain is called initial modulus.

Initial modulus, tan α = stress / strain Tan α ↑↓ → extension ↓↑

Work of rupture: Work of rupture is defined as the energy required breaking a material or total work done to break that material.Unit: Joule (J)

Work factor: If the textile material follows the Hooks law then the load elongation curve will be straight line. So work factor is defined as the ratio of work of rupture to breaking load × breaking extension.

Work factor = work of rupture / (breaking load × breaking elongation).

In the ideal state, the work factor will be 0.5. If the load–elongation curve lies mainlyAbove the straight line, the work factor will be more than 0.5; if below, it will be less than 0.5.

For materials breaking at the same point, the work of rupture will be greater the higher the work factor. Since the work factor will not very much in different specimens of the same material, the values given later or other available values may be used to estimate the work of rupture from measurements of the breaking load and elongation.

Work recovery: The ratio between work returned during recovery and total work done in total extension is called work recovery.

Total extension = Elastic extension + Plastic extension Total work `

Elastic recovery: Elasticity may be defined as that property of a body by which it tends to recover its original size and shape after deformation .The power of recovery from a given extension is called elastic recovery. Elastic recovery depends on types of extension, fiber structure, types of molecular bonding and crystalline of fiber.

Here, AB = Original length of the specimenBD = total extensionCD = elastic extensionBC = plastic extension

Total extension = Elastic extension + Plastic extensionHook’s Law: When a material is subjected to load, within its elastic limit, the stress is proportional to the strain. Mathematically, Stress ∞ Strain

Stress-strain curve

Here, A to B → linear region. This region follows Hook’s law (stress ∞ strain). So, fiber comes to its original position after removal of load. So, the region is called elastic region and the deformation is called elastic deformation.

B to C → plastic region. In this region chain breaks but fiber do not break. Here the deformation is known as plastic deformation.C → breaking point. The fiber will be break at this point.B → Yield point. The point up to which a fiber behaves elastic deformation and after which a fiber shows plastic deformation is called yield point.Yield stress → the stress at yield point is called yield stress.Yield strains: The strain at yield point is called yield strain.

Yield Point: In ductile materials, at some point, the stress-strain curve deviates from the straight-line relationship and Law no longer applies as the strain increases faster than the stress.

From this point on in the tensile test, some permanent deformation occurs in the specimen and the material is said to react plastically to any further increase in load or stress. The material will not return to its original, unstressed condition when the load is removed. In brittle materials, little or no plastic deformation occurs and the material fractures near the end of the linear-elastic portion of the curve.

Various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements. The yield strength is defined as the stress required producing a small amount of plastic deformation.

Elastic limit: Elastic limit is the greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load.Yield strength: Yield strength is the stress required to produce a small-specified amount of plastic deformation. The yield strength obtained by an offset method is commonly used for engineering purposes because it avoids the practical difficulties of measuring the elastic limit or proportional limit

Modulus of Elasticity: The modulus of elasticity is a measure of the stiffness of the material, but it only applies in the linear region of the curve. At the point that the curve is no longer linear and deviates from the straight-line relationship, Hooke's Law no longer applies and some permanent deformation occurs in the specimen. This point is called the "elastic, or proportional, limit". From this point on in the tensile test, the material reacts plastically to any further increase in load or stress. It will not return to its original, unstressed condition if the load were removed

Ductility: The ductility of a material is a measure of the extent to which a material will deform before fracture. Ductility is used a quality control measure to assess the level of impurities and proper processing of a material.

Creep: Creep is a time-dependent deformation of a material while under an applied load that is below its yield strength. It is most often occurs at elevated temperature, but some materials creep at room temperature.

There are two types of creep:i.      Temporary creepii.      Permanent creep

Creep data for general obtained under conditions of constant uniaxial loading and constant temperature. Results of tests are usually plotted as strain versus time up to rupture.  In the initial stage, strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes approximately constant during the second stage. This constant creep rate is called the minimum creep rate or steady-state creep rate since it is the slowest creep rate during the test. In the third stage, the strain rate increases until failure occurs. 

Toughness: The ability of a metal to deform plastically and to absorb energy in the process before fracture is termed toughness. A material with high strength and high ductility will have more toughness than a material with low strength and high ductility. Measurement of toughness is done by calculating the area under the stress strain curve from a tensile test. Material

toughness equates to a slow absorption of energy by the material.

Principles of tensile experiment or Method of tensile experiment:

CRL method (constant rate of loading): The function of applied force is to extent the specimen until it eventually breaks down. Here the loading causes the elongation. By adding constant rate of water flow in a container which is attached with the jaw J2 may increase the load gradually. Thus constant rate of flow gives constant rate of loading.

CRE method (constant rate of elongation): A specimen A is gripped between two jaws. J1 is fixed and bottom jaw J2 is movable to down-ward at constant velocity by mean of a screw mechanism. Initially the tension on A is zero. But when the bottom jaw J2 moves down-

ward at a constant rate the specimen is extended and an increasing tension is developed until it eventually breaks down. Here the elongation /tension causes loading.

Difference between CRL & CRE methods:

1. CRL means Constant rate of loading. 1. CRE means Constant rate of elongation.2. This method contains container and water flow used to increase load gradually.

2. This method contains screw mechanism.

3. In this method loading causes elongation.

3. In this method elongation/ extension causes loading.

Fabric tensile strength depends upon

1. Raw material.2. Yarn strength (twist: more twist for more strength)3. Fabric construction (weave: plane weave is stronger than floats-satin, sateen which are weaker, Density: low density cause weaves slippage which results in seam slippage).4. Finish applied (resin finish improves weave slippage).5. Adverse of “finishing” process

Stelometer (CRL)

1. Capable of measuring strength as well as elongation of fibre bundle.

2. Works with Pendulum lever principle.

3. The loading of the specimen is carried out by a pendulum system, which is mounted in such

a way that it rotates about its C.G.

4. It eliminates the inertia effects associated with normal pendulum principle.

5. The beam and pendulum start in a vertical position but the C.G. of beam is such that when it is

released the whole assembly rotates.

6. The speed of rotation is controlled by adjusting the dashpot.

For Extensible Material

Example Let, extension at breal - 7% Gauge length -8inch Breaking load - 220gm Std. Machine rate of loading, µ0 -1100gm/in Rate of traverse - 2.28 inch/min Calculate time of breakT= [{(220/1100) +8 x 0.07}/2.28] x 860 sec = 20sec

The beam balance principleP x BC = F x AC The load on the specimen ‘P’ can be varied by changing F, or by changing the distance from fulcrum, keeping F constant.

Pressley Fiber strength tester :

1. The beam AB is pivoted at O.2. When B rises, the clamp C1 moves upwards. 3. Initially the beams have a slight inclination of a few degree to the horizontal. 4. The heavy rolling weight (W) when released from the catch, it rolls down the beam. 5. A 'O increases until the fibres break. 6. As soon as the break occurs, the arm AO drops and the brake arrangement stops the carriage instantly.7. The distance A'O is the measure of breaking force. The scale is directly graduated on the beam AB.

Fig: Schematic diagram of pressely fiber strength tester8. If we can control the velocity of rolling wt. by a specially made device, we can achieve CRL test condition. 9. In HVI, this principle is used.

Pendulum Lever Principle (CRT)

M=Mass of pendulum and its C.G. is at R distance from the pivot For extensible material, v >u

Assuming the specimen is in extensible, Taking moments about pivot,

Machine rate of loading (µ)Increase in the load per unit increase in the displacement of upper jaw (J1) The displacement of upper jaw (J1) =rdθdF/dθ =(MgR/r)cosθ , dF/ rdθ = µ = (MgR/r2)cosθ.

MgR/r2 is constant for a particular m/c and known as “standard machine rate of loading” or µ0.

Time rate of loading (L):

Loading by spring (both CRL and CRE)

The instrument is used for single fiber or fine yarns

To test with CRL condition:

1. Motor M1 runs continuously2. As motor M1 starts, the H1 moves upwards at a constant speed. 3. The spring extends and load is applied on the specimen at a constant rate. 4. The extension of specimen will cause the leaf spring to touch upper contact C2, which starts the motor M2 and H2 moves down for a short period. This cycle continues until the specimen breaks5. S is helical spring.

6. G1 and G2 are the sample grips 7. The upper grip G1 is connected to a leaf spring, which has a restricted movement between the electrical contacts C1 ad C2.

To test with CRE condition1. Motor M2 runs continuously 2. The M1 starts and stops intermittently, as described above. 3. Used for single fiber and fine yarns.

Inclined plane principle (CRL)

So, CRL condition is achieved.Extension of the specimen will not affect the rate of loading, the carriage merely rolling further down the plane.

Ballistic or impact principle

1. Measures ‘work of rupture’ of a specimen instead of max breaking force. 2. Potential energy at point 1, W x h1.3. When pendulum is released, it swings downward and when it is nearly vertical, it begins to pull on the specimen (at 2) 4. Breaks the specimen and rises to position 3.

5. Work of rupture = W (h1 – h2) in lbs 6. ‘K’ is known as centre of percussion of the pendulum. it is a point on the axis of pendulum where a force may be applied without causing a reaction about the fulcrum

Strain gauge principle (Load Transducer)

1. Most of the modern tensile testers work on this principle.2. When the beam bends the length of upper face (AB) increases and lower face (CD) decreases and (NL) remains unchanged.3. Resistance wire (R) cemented on AB also expand and thus the value of resistance changes. 4. Convert this value of change in resistance to load value (applied on specimen)Two resistance wires are placed on upper and other two are on lower surface. (To form a Wheatstone Bridge).

5. with the beam un-deflected, no voltage across CD, when a voltage is applied across AB. The bridge is ‘balanced’. 6. When load is applied, the deflection occurs and the values of the resistances change and a voltage is produced across CD, i.e., which is proportional to the load.

Advantages:1. Free from inertia errors and friction. 2. The deflection of the end of the beam is very small, and thus it is tests under ‘CRE’ condition.3. Versatility in the type of instrument (yarn, fibre, fabrics, wide speed and load range, etc.)

Disadvantages: 1. Expert technician is required for maintenance and repair.2. Chances of ‘drift’ in electronic circuits3. High initial cost.

Constant tension winding tests

1. It provides conditions somewhat similar to actual processing of yarn during winding, warping, sizing etc. 2. The test is closer to actual running condition. 3. A, B fixed pulleys and P movable pulley 4. Under static conditions the tension of the loop will be 0.5L (uniform throughout the loop)

5. The tension imposed on the yarn will cause it to stretch. “e” be the extension per unit length, v = u (1 + e) 6. Necessary means are required to adjust the input and output velocity. 7. The tension required to get the std. breakage rate

Measurement of fabric tensile strength

1. Strip Test:

1. In this method a fabric strip is extended to its breaking point by a suitable mechanical means which can record the breaking load and extension. 2. Five fabric samples both in warp and weft direction are prepared with each not containing the same longitudinal threads. 3. Samples are prepared 60mm x 300mm and then frayed to get 50mm wide specimen.

4. Any breaks that occur within 5mm of the jaws or at loads substantially less than the average should be rejected.5. The mean breaking force and mean extension % of initial length are reported. Samples are cut (60mm x 300mm) parallel to warp/weft.6. Frayed the threads from both sides of the width to bring down to 50mm wide. 7. For heavily milled fabrics, no fraying is done (50mm x 300mm).

2. Grab Test:

1. The grab test uses jaw faces which are considerably narrower than the fabric, so avoiding the need to fray the fabric to width and hence making it a simpler and quicker test to carry out. 2. The sample used is 100mm x 150mm jaws are 25mm square which stress only the central 25mm of the fabric.

3.A line is drawn 37.5mm from the edge of fabric to assist it in clamping so the same set of threads are clamped in both jaws. 4. The gauge length is 75mm and speed is adjusted so that the sample is broken in 20±3s.5.In this test, there is a certain amount of assistance from yarns adjacent to the central stressed area so that the strength measured is higher than for a 25mm frayed strip test.

6. Fundamentally different from strip test. 7. Jaw faces are considerably narrower than fabric. No need to fray the fabric. 8. Simpler and quicker method.

USTER TENSORAPID (CRE Principle):

1. For tensile testing of single and ply yarn.

2. Testing of slivers, leas and fabrics is also possible.

3. Force measurements up to 1000N without exchanging the force transducer.

4. The clamping force, the yarn tensioners and the suction-off of the yarn can be programmed.

5. All numerical and graphical results are displayed on a video screen. (Histogram, L-E curve,

tables, etc.)

6. Package creel for the automatic measurement up to 20 packages.

7. Calling-up of test parameters of frequently tested yarn types from the memory (up to 40).

8. Pneumatically-actuated yarn clamps ; the clamp pressure is programmable.

9. Electronic elongation measurement. 10. Test speed – Continuously adjustable between 50 and 5000mm/min.11. Test length. i) With horizontal position of clamps, continuously adjustable between 200 and 1000mm. ii) With vertical position of clamps, continuously adjustable between 100 and 1000mm. 12. Self test - Automatic calibration check for accuracy through inspection.

YARN STRENGTH

Single yarn strength: Instron , Uster500mm gauge length and speed adjusted so that the time to break is 20 ± 3sec.(ii) Skein Method: Lea Strength

Advantages:1. It tests a long length of yarn in one test.2. Yarn is expected to break at its weak spots, so give more realistic strength values.3. Same hank can be used to measure yarn count.

Disadvantages:1. Result depends on friction between yarn and also between yarn and hook.2. No measure of strength variability.

Tensile strength test of Yarn First, a specific length of yarn is taken off the spindle. This machine counts to the proper number of revolutions and stops. Thus, producing the same length of yarn for each test.

The yarn is then mounted on a device that stretches it until it breaks, measuring the strength in pounds and kilos. What they are looking for in this length of yarn is a breaking strength of about 100 kilos or almost 220 pounds of strength.

Here the gauge shows this batch broke at about 99.5 kilos – nice and close, well within accepted tolerance. This batch of yarn can be strung on a loom to have knots tied on it.

Elmendorf Type Tearing Strength TesterThe Elmendorf Type Tester determines the tearing strength by measuring the work done in tearing through a fixed length of the test specimen. It consists of a sector pendulum pivoted on anti-friction ball bearings on a vertical bracket fixed on a rigid metallic base.

Elmendorf Type Tearing Strength Tester is used to determine the tearing strength of fabrics, plastic films or other similar materials. B-Tex Engineering produced Elmendorf Tearing Testers are accurate, of low-cost and of high quality.

Features of Tearing Elmendorf Type Tearing Strength Tester:

The equipment effectively measures the tearing strength.

It is user-friendly.

It consists of anti-friction ball bearings.

It has an adjustable knife.

 A set of calibrated check weights are also supplied.

Conclusion

We can learn a lot about a substance from tensile testing. As we continue to pull on the material until it breaks, you will obtain a good, complete tensile profile. A curve will result showing how it reacted to the forces being applied. One of the most common testing methods, tensile testing, is used to determine the behavior of a sample.