Chapter 5 Tensile Strength Testing of textiles

tensile strength testing of textiles in detail with the explanation of testing machine

1. Chapter 4 Tensile testing of Textiles

2. . Normally the strength of fibres or fibre structures is commonly regarded as the criterion of quality. . Although strength in testing is of great importance, but properties such as flexibility, resilience, moisture absorption, dye affinity etc are necessary to be considered in the assessment of goodness or quality of a textile material. . It is important to note that the Relative importance of one particular property will depend mostly on the end use. . Most of the properties are interrelated. . Before understanding testing of materials, it is essential to know the different units and terminologies employed.

3. Load: This refers to the application of pressure to a specimen in it’s axial direction. It causes a tension to be developed in the specimen. The load is usually expressed in grams weight or pounds weight like gravitational units of force. (Axial: Located on, around, or in the direction of an axis.)

4. Breaking Load: This is the load at which the specimen break usually expressed in grams weight or pounds weight. Stress: This is the Ratio between the force applied and the cross-section of the specimen. That is an effort is being made to compare the fine and coarse structure of the specimen. Stress: Force Applied Cross sectional Area. The force is accurately expressed in dynes or poundals . Hence, the stress on a fibre may be given in terms of Dynes per sq centimetre ( dyn / cm2).

5. Mass-Stress: The cross section of many fibres and fibre structures are irregular in shape and difficult to measure. To simplify matters, a dimension related to cross section is used, known as the linear density ( the amount of mass per unit length))of the specimen. The Linear density may be expressed in denier or tex –count and the mass-stress then becomes the ratio of the force applied to the linear density ( Mass per unit length). Mass Stress = Force Applied Linear Density The Units of the Mass stress therefore become grams weight per denier or grams weight per tex . The abbreviated for of “Mass Stress” is ‘Stress’ and quoted in grams per denier or grams per Tex.

6. Tenacity or Specific Strength: The Tenacity of a material is the mass stress at Break. The units being, gms per denier or gms per tex . The value used for the denier of a specimen is usually the nominal denier before testing, no account being taken of the decrease in denier as the specimen is stretched and becomes finer. For some purposes this change is noted and suitable correction made. It is useful to note that by expressing the breaking strength of different materials in terms of tenacity, comparisons can be made directly between specimens of varying fineness.

7. Breaking Length: The ‘ Breaking length’ is the length of the specimen which will just break under its’ own weight when hung vertically. The unit for breaking length is the kilometre. Eg : If, for example, a 100 Denier viscose yarn broke at a load of 185 gms , the breaking length would be; 185 x 9000 =16.65 Km 100 x 1000

8. Strain: When a load is applied to a specimen a certain amount of stretching takes place. The amount will vary with the initial length of the specimen. The ‘Strain’ is the term used to relate the stretch or elongation with the initial length. Strain = Elongation Initial Length. Extension By expressing the strain as a percentage, we obtain the extension, Extension = = Elongation x 100 Initial Length. The extension is sometimes referred to as the ‘ Strain Percent’.

9. Breaking Extension: The Breaking extension is the extension of the specimen at the breaking point. Load Elongation Curve: An extremely important curve is produced when the load on a specimen is plotted against the elongation. This curve describes the behaviour of the specimen from Zero load and elongation up to the breaking point. . From this load elongation curve much important information can be gained, initial young modulus , work of Rupture, yield point etc.

10. The Stress-Strain curve: For some purposes, it is more convenient to convert the load-elongation curve to a stress-strain curve, there by enabling more direct comparison to be made between different types of materials and structures.

11. The Load elongation curve of 250 Denier viscose rayon and a 30 Denier Nylon yarn is shown. The test length in each case was 20 Inches and yarn was tested on a Scott Serigraph which operates on the inclined plane principle to give a constant rate of loading. To convert the curves to stress strain curves we must draw up a table of values of stress strain derived from measurements on the load-elongation curves. Stress values are obtained by dividing load values by the denier; strain values are calculated by dividing the actual elongation by 20 in the test length. For instance, at 1” elongation the load on the viscose was 272 gms . The stress at this point was therefore 272 / 250, 1.08 g / denier. The corresponding strain was 1/20 or 0.05.

12. The figure following shows the stress strain curves derived from the load elongation curves. The general shape of the curves remain the same but the relative positions have changed. The superior strength of the nylon is more clearly seen and the comparison between the two types of fiber made easier.

13. Why in the Stress-Strain curve, different types of materials have different types of curves? The stress strain curves of fiber structures such as fabrics and yarn will reflect the characteristics of the fibers are fibers from which they have been built , modified according to the methods used in their construction, eg Twist, weave, Physical and chemical finish, etc. A study of the properties of the fiber is there fore of paramount importance. The stress-strain curve of a fiber is governed mainly by its molecular structure;. When an external force is applied to a material it is balanced by internal forces developed in the molecular structure of the material.

14. The arrangement of molecules with in the fiber in a amorphous and crystalline form with varying degrees. In the early stages of stretching the material, the elongation is mainly concerned with the deformation of the amorphous regions, in which primary and secondary bonds are stretched and sheared. If the stress were removed at this stage most of the extension would be recovered and the material would exhibit elastic properties. By increasing the stress further, the stress-=strain curve bends sharply and large strains or extensions are produced by small increase in stress. The stress-strain curve begins to bend towards the stress axis un till finally the breaking point is reached. Since different materials have different molecular structures it follows that their stress-strain curves will be different.

15. The initial Young’s Modulus: A particularly important part of stress-strain curve is the initial portion starting at Zero stress and strain. The initial portion of stress-strain curve is fairly straight , indicating a linear relationship between stress and strain. In other words, the material behaves like a spring . The significance of this is that when the load is removed, the material recovers its’ original length or nearly so. The tangent of the angle between the initial part of the curve and the horizontal axis is the ratio stress;strain . In engineering science this ratio is termed “ Young's’ modulus”. It gives a measure of the force required to produce a small extension. A High modulus indicates inextensibility and a low modulus great extensibility. In textiles we use the term initial young modulus and it describes the initial resistance to extension of a textile material.

16. LEA TESTING (CSP)   YARN COUNT It indicates the weight per unit length or length per unit weight. In English system count denotes the number of hanks of 840 yards that will weight one pound. LEA A continuous length of yarn in the form of a coil of 80 loops made on the reel of girth of 1.5 yards. Ie 120 Yards  

17. YARN STRENGTH It is measured in terms of lea strength or skein strength and expressed as count strength product. The CSP indicates the number of leas required to break under their own weight when the leas are tied one over and hung vertically.   LEA TESTER The lea tester is a simple and versatile machine. They are made in different capacities upto 100,200,300 and 500 lbs. The unit records tensile strength and extension at breaking. While carrying out the test, an instrument of suitable capacity is chosen so that most of the test readings fall between 25 and 75 persent of the maximum value that can be registered on the instrument. The rate of traverse is 12” per minnute . Sampling Take 10 cops or 5 cones per sample.

18. Automatic Wrap Reel It is designed for preparing leas of 120 yards of definite length with uniform tension, it helps in preparing more number of leas in short time.   Preparation of test specimens 1.    Number the selected cones/cops and fix them on the bobbin holder of the wrap reel. 2.    Reel out the required length of 120 yards for wrap reel. 3.    Cut and tie the trailing end of the lea to its leading end. 4.    Similarly take 30 leas making a total of 40 leas from the same 10 bobbins. 5.    Condition the sample in a conditioning box for about 12 hours. 6.    Determine the mass in grams of the leas and calculate the count Cotton count = 64.80 / Weight of lea in gms

19. Lea strength test procedure 1.    Bring the hooks of the testing machine to zero position. 2.    Take a particular lea with count known and fix it on to the hooks and carefully separate the yarn to avoid overlapping on individual strands. 3.    Start the m/c and carry out the test up to rupture. 4.    The lea strength is automatically recorded in the system. The count CV%, the strength CV%, Lea CSP, maximum and minimum values of count and strength are obtained in the form of printouts. 5.    Similarly find the breaking load of the remaining leas and record them against their respective counts. 6.    Calculate the avg. breaking load of the, avg. linear density(count) of all the observations taken, coefficient of variation (CV) of breaking load and CSP 7. The product of average strength to the Avg count of the lea will give you the CSP ( Count strength Product)