introductiondepending upon the sources, the various types of fibres can be classified into the following three main categories :(i) animal fibres e.g. wool & silk.(ii) vegetable fibres e.g. cotton & linen.(iii) synthetic fibres e.g. nylon & polyester. besides their chemical composition and properties, most important property of these fibres is their tensile strength. tensile strength mean the extent to which a fibre can be stretched without breaking and it is measured in terms of minimum weight required to break the fibre. to determine the tensile strength of any fibre, it is tied to a hook at one end and weighted are slowly added to the other end until the fibre break.since peptide bonds are more easily hydrolyzed by bases than acids therefore wool and silk are affectedby basis not by acids. it is because of this reason that wool and silk threads breakup into fragments and ultimately dissolve in alkalines.in other words alkalines decreases the tensile strength of animal fibres (wool & silk). vegetable fibres (cotton & linen), on the other hand, consist of long polysaccharide chains in which the various glucose units are joined by ethers linkage. since ethers are hydrolised by acids and not by bases therefore, vegetable fibres are affected by acids but not by bases. in other words acids decreases the tensile strength of vegetable fibres. in contrast, synthetics fibres such as nylon & polyester practically remains unaffected by both acids and bases.

How study of effects of acids and bases on the tensile strength of fibre?

‘ The glycosidic bond in cellulose is chemically an acetal group, and acetals are hydrolyzed by acid. Treatment by acid will partially hydrolyze the polymer chains in cellulose, in effect shortening them

and thus decreasing the tensile strength. 

Acetals are stable to bases, so cellulose fibers retain their properties when treated with base. 

Animal fibers are proteins, which contain peptide bonds . The peptide bond chemically is an amide group, and amides do not readily hydrolyze in acid solution. Amides are hydrolyzed by bases, so animal fibers are hydrolyzed to shorter chains and lose their tensile strength in bases. 

IIRC, synthetic fibers like Nylon (which is also a polyamide like proteins) owe some of their chemical resistance to their more 'crystalline' structure, i.e., there is greater regularity in the molecular structure, and this regularity reduces the ability of bases or acids to approach the amide bonds to initiate a hydrolytic reaction. Additionally, synthetic fibers tend to be hydrophobic, and water thus is not able to hydrate the amide bond adequately to assist in attack on the amide bond to begin hydrolysis.

Result(i) The tensile strength of woolen fibre decreases on soaking in alkalies but practically remains unaffected on soaking in acids.

(ii) The tensile strength of cotton fibre decreases on soaking in acids but remains practically unaffected on soaking in alkalies.

(iii) The tensile strength of nylon fibres remain practically unaffected on soaking either in acids or in alkalies.

Precautions

(i) Thread must be of identical diameters.

(ii) Always take the same length of the threads.

(iii) Add the weights in small amounts very slowly.

Ultimate tensile strengthFrom Wikipedia, the free encyclopedia

Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fails. The maximum

stress it withstands before failing is itsultimate tensile strength.

tensile strength (TS)  is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. Tensile strength is distinct from compressive strength.Some materials break sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture.The UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress–strain curve (see point 1 on the engineering stress/strain diagrams below) is the UTS. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such

as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.

Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood.Tensile strength is defined as a stress, which is measured as force per unit area. For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In theInternational System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix mega); or, equivalently to pascals, newtons per square metre (N/m²). A United States customary unit is pounds per square inch (lb/in² or psi), or kilo-pounds per square inch (ksi, or sometimes kpsi), which is equal to 1000 psi; kilo-pounds per square inch are commonly used when measuring tensile strengths.

Many materials can display linear elastic behavior, defined by a linear stress–strain relationship, as shown in the left figure up to point 3. The elastic behavior of materials often extends into a non-linear region, represented in the figure by point 2 (the "yield point"), up to which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this elastic region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen does not completely return to its original size and shape when unloaded. For many applications, plastic deformation is unacceptable, and is used as the design limitation.

After the yield point, ductile metals undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress–strain curve (curve A, right figure); this is because the engineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress–strain curve, and

the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1.

The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples.[3]

The UTS is a common engineering parameter when designing brittle members, because there is no yield point.[3]

Testing[edit]

Typically, the testing involves taking a small sample with a fixed cross-sectional area, and then pulling it with a tensometer at a constant strain (change in gauge length divided by initial gauge length) rate until the sample breaks.

When testing some metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.[4] This practical correlation helps quality assurance in metalworking industries to extend well beyond the laboratory and universal testing machines.It should be noted that, while most metal forms, such as sheet, bar, tube, and wire, can exhibit the test UTS, fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, must be made into composites to create useful real-world forms. As the datasheet on T1000G below indicates, while the UTS of the fiber is very high at 6,370MPa, the UTS of a derived composite is 3,040MPa - less than half the strength of the fiber