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Tensile Properties
Stress:
It is the ratio between force applied and cross-sectional area of specimen.
Stress = Force applied / cross-sectional area, f = F/A, Unit = Dyne/cm2
Mass stress (Specific stress):
Mass stress is the ratio of the force applied to the linear density mass per unit length.
Mass stress = Force applied / linear density = F/P, unit = gm/denier
Relation between stress f, specific stress α and density p
f = α × p
Strain:
It is used to relate the stretched or elongation with the initial length.
Stress = elongation / initial length
Tensile strength:
Force applied to break the fibers to cross-sectional area is known as tensile strength.
Tensile strength = force required to break / linear density
Breaking extension:
The extension to break the material to the initial length of that material is known as breaking extension. It is expressed as %
Breaking extension = elongation at break / initial length × 100%
Breaking load:
The load at which the material break is called breaking load. It is usually expressed in gm/wt or lb/wt.
Load:
The application of a load to a specimen in its direction causes a tension to be developed in the specimen. The load is usually expressed in gm-wt or lb-wt.
Breaking length:
The breaking length is the length of the specimen which will just break under its own weight when it is hung vertically. Its unit is kg.
Elasticity:
Tendency to recover the extension of material when stress is removed .
Plasticity:
After elastic limit when stress is increased the material can’t recover the extension it stress is removed is known as plasticity.
Elastic recovery:
It is a property of material by which it tends to recover original size and shape.
Elastic recovery = elastic extension / total extension
Stress Strain Curve:
Y
B Breaking point
Yield point
Stress A
O Strain X
Fig. Stress strain curve
When external force is applied to a material, it is balanced by internal forces development in the molecules structure of the material. By increasing the stresses, material will deformed and follow the stress –strain curve.
Region O to A: In this stage, elongation is mainly concerned with the deformation of the amorphous region in which primary bonds of molecules structure and removed at this stage most of extension be recovered and material would show elastic region and deformation is called elastic deformation.
Region A to B: The long chain molecules would rearrange with breakage of secondary bonds of molecules structure. After deformation in this stage, material would not come back to its original form i.e. material shows plastic properties in this region. This deformation is called plastic deformation.
For further increasing the stresses the mtl will break down in the point B, which is called breaking point.
Yield point:
The point A up to which a mtl shows elastic properties (up to elastic deformation) after which the mtl shows plastic properties (plastic properties) is called yield point.
Y
Yield point
Stress A
O Strain X
Fig. Yeild point
Meredith yield point:
Meredith defines yield point as the point at which the tangent to the curve is parallel to the line joining the origin to the breaking point.
Y
Stress
O Strain X
Fig. Meredith yield point
Coplan yield point:
Coplan defines the yield point as the point at which the intersection of the tangent at the origin with the tangent having the least slope.
Y
Coplan yield point
A
Stress
O Strain X
Fig. Coplan yield point
Work of rupture:
Work of rupture is defined as the energy needed to break a specimen. If we may consider a fiber under a load F, increasing in length dl.
Then, work done Force × displacement
F × dl
Total work of break ʃ F× dl
Work of rupture can also be calculated by ½ × Braking load × Breaking elongation
Y Break
Work of rupture
Stress
O Strain X
Fig. Work of rupture
Specific work of rupture:
It will be proportional to its mass per unit length and to its initial length (because of the effect on the elongation) to compare different mtl. We may therefore use the term specific work of rupture,Specific work of rupture = Work of rupture/ Mass per unit length × Initial length
Work factor:
If the fiber dyed Hook’s law the load elongation curve would be a straight line anf the work of rupture would be given by
Work of rupture = ½ × Breaking load × Breaking extension
So, the work factor is defined as the ratio of work of rupture to the multiply of breaking load & breaking extension.
Work factor = Work of rupture/ Breaking load × Breaking extension
= ½ × Breaking load × Breaking extension / Breaking load × Breaking extension
= ½
In ideal state the work factor will be ½ if the load elongation curve lies mainly above the straight line the work factor will be more than ½. If the load elongation curve lies mainly below the straight line, the work factor will be less than ½.
Break
w.f>½
Load w.f=½
w.f<½
Elongation
Fig. Work factor
Initial modulus:
It is equal to the slope of the stress-strain curve at the origin (after the removal of any crimp). This slope usually remains constant over the initial portion of the curve.
Initial modulus, tanα = Stress/Strain
The effect of initial modulus in textiles:
- Chemical structure: High ring structure of molecular chain in fiber, higher initial length.
- Orientation: For higher orientation modulus, higher initial modulus.
- Crystallinity: For higher Crystallinity fiber, higher initial modulus.
- Extensibility: For lower extensibility of fiber, higher initial modulus.
- Stiffness: For lower stiffness of fiber, higher initial modulus.
Crimp:
It is generally assume that the fiber is in initially straight, however in many causes fibers are crimped. The crimp is normally pulled out by a small tension measuring linear density O it can also be removed by a pre-tension at the start of a tension test. When a crimped fiber is inserted in the tester without any limited tension, the load-elongation curve will have the form shown in the figure. The origin of the curve may put A, where it drives from the zero line, but this point is different to locate precisely. A better procedure is to put the origin at O. The exploded point is corresponding to a hypothetical straight fiber. The crimp is given by AO and may be expressed as percentage of initial length.
Load
Crimp
A O Elongation
Creep:
We know that elastic mtl follow the Hook’s law (stress α strain). But textile mtls follow Hook’s law up to a certain point. After that point it doesn’t follow Hook’s law. So, when a load is applied on a textile then instantaneous strain occurred in the fiber and after release the stress the fiber strain will be lower with passing time i.e. slow deformation will be occurred. This behavior of textile mtl of fiber is called creep.
There are two types of creep. They are given below:
- Primary creep
- Secondary creep
Primary creep:
- It is fully recoverable in times.
- Fiber will come back to its origin position after removing applied force.
- Elastic deformation is occurred.
- Molecular chain is stretched slightly.
Secondary creep:
- It is non-recoverable in times.
- Fiber can’t be come back to its origin position after removing applied force.
- Plastic deformation is occurred.
- Molecular chain break.
Strain
Strain
Time Time
Primary Secondary
Creep behavior:
On application of load to a fiber an instantaneous extension continue to extend as the time goes on. On removal of load, the recovery will not be limited to the instantaneous recovery but will continue to take place as the time goes on. This behavior is known as creep and creep recovery.
C d
e
Load
Extension b f
g
h
Time Time
Fig: Creep under constant load & recovery under zero load
Here instantaneous extension a-b, d-c.
Total creep: b-c
Primary creep: e-f
Secondary creep: g-h.
Extension
Stress
Time Time
Fig: Relation of stress under constant extension
Basic method of tensile experiment:
- Constant Rate of loading (CRL)
- Constant Rate of Elongation (CRE)
Constant Rate of Loading (CRL):
J1
A J2
A specimen A is griped in a fixed jaw J1 & bottom jaw J2 which is removal. A force F is initially zero but increases at constant rate of water in a container which is attached to jaw J2, may increase the load gradually. Constant rate of flow gives constant rate of loading. The function of this applied force is to extend the specimen until it eventually breaks. Hence loading causes elongation.
Constant rate of elongation (CRE):
J1
B
J2 Screw mechanism
A specimen B is gripped in a fixed top jaw J1 & bottom jaw J2 which is removal to downwards at a constant velocity by a screw mechanism. Initially the tension B is zero but when the bottom jaw J2 moves at a constant rate. The specimen is extended & can increasing tension is developed until the specimen break down. In this case the extension causes loading.
Deference between constant rate of loading & constant rate of elongation:
It will show that, creep in fibers under load results in deference between tests at constant rate of elongation & constant rate of loading. The curves may also have quite a deference shape sever in a constant rate of elongation test. It is possible for the load to decrees will elongation increase as in fig. this is not possible in CRL test where the load must increase throughout the test.
CRL Break
Load
CRE
Extension
Hysteresis loop:
B
Stress Strain
A
Elastic recovery that is the behavior on removal stress is only a special case of the general phenomenon of hysteresis. In a cyclic change of stress or strain, the result will fall on a loop. This means that energy is used up by internal friction & may tend to dry out.
Factor determining the result of tensile experiment:
The material & its condition:
The behavior of a material depends on the nature & arrangement of which it is composed & these will vary not only one type of fibers to another but also from one to another in a given sample.
The conditions are:
- The chemical treatment to which it has been subject.
- The mechanical treatment that it has received.
- The amount of moisture that it contains.
- The temperature.
The arrangement and dimension of the specimen:
The dimension of the specimen will have a direct effect on the result of test .The breaking load of fiber will increase in portion to its cross-sectional area & it’s elongation will increase in proportion to
The nature & timing of the test:
The result of experiment will be affected by time allowed & by the way on which the load is applied by (CRL) & (CRE) reducing from a higher load or any other sequence of events.
Factor affecting the tensile properties of textile:
Test specimen length:
- If we tested the specimen at a gauge length AB, the strength recovered would be that of the weakest point &the value would be S1.
- If we tested the specimen in two breaking loads, S1 & S2 the mean of which would be higher than S1.
- Hence, by testing the yarn as a shorter gauge length the apparent strength has increased S2.
- This effect in as “weak link effect”.
S 2 S 1
A Length B
The capacity of m/c:
If a weak specimen is tested on a high capacity m/c, the time to break it will be short & therefore optimistic strength result will be producer. The capacity of the m/c should be chosen so that, the time required to break the specimen is close to the recommended time.
The previous history of the specimen:
- The mechanical properties of a specimen before and after straining changes mentionable. Chemical treatment may also affect the tensile properties of the specimen.
- The mechanical behavior of textile fibers & fiber structure is influenced by the amount of moisture in the specimen. The moisture relationships of the various fibers type differ.
The effect of humidity &temp:
- The mechanical behavior of textile fibers and fiber structures is influenced by the amount of moisture in the specimen.
- The moisture relationships of various fibers types differ.
- Stress-strain curve for a hydrophobic such as Terylene & when tested in the dry state will be similar to curve obtained from a wet test.
- On the other hand, the curve obtained when the testing say acetate rayon dry & wet will exhibit significant difference
The form of the test specimen:
- The test specimen is a composite structure built up from individual fiber& filaments.
- Changes in the twist factors used cause changes in the yarn strength, elasticity, and luster and yarn characteristics.
- In case of fabric, the warp properties differ from wet properties.
Time of loading & breaking:
A rapid test produces a higher breaking load than a slow test.
Posted in Textile Physics-1
Crimp In Yarn And Fabric
Crimp: When warp and weft yarn interlace in fabric they follow a wavy path. According to pierce “crimp”, geometrically considered is the percentage excess of length of the yarn axis over the cloth length.
Crimp%: crimp percentage is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in cloth which is expressed as a percentage. Mathematically,
Crimp %
Where,
l= Length of yarn before weaving
P= Length or extent of yarn in fabric after weaving
Crimp amplitude: In cloth geometry, the term crimp amplitude is used. Crimp amplitude refers to the extent to which threads are defected from the central place of the cloth.
Take up%: Tae up % or crimp rigidity is a measure of the ability of a textured yarn to recover from stretch. It is related to the bulking properties of yarn.
The difference between length of yarn in the fabric after weaving, expressed as a percentage of the length of the yarn before weaving, is called take up %. Mathematically,
Take up% (T)
Crimp ratio: Crimp ratio is the ratio of the yarn length to the fabric length produced from that yarn. We know,
C
C
⇒ C+1
Here,
C+1= Crimp ratio
Relation between crimp% and Take up%:
First definition of crimp% & take up%,
C(1)
T(2)
From (1) we get,
C
⇒C
⇒
⇒
⇒ ⋯⋯⋯(3)
And from (2) we get,
T=
⇒T=
⇒T=[From (1)]
⇒T=
⇒T=
Equation (4) indicates relation between take up & crimp %. From (4) we get,
T=
⇒ 100C=100T+TC
⇒ 100C-TC=100T
⇒ C(100-T)=100T
∴C=
Equation (5) indicates the relation between crimp% & take up %.
Difference between crimp% and take up%:
| Crimp% | Take up% |
| 1. crimp percentage is defined as the mean difference between the straightened thread length and the distance between the ends of the thread while in cloth which is expressed as a percentage. | 1. The difference between length of yarn in the fabric after weaving, expressed as a percentage of the length of the yarn before weaving, is called take up %. |
| 2. It is not related to the bulking properties of yarn. | 2. It is related to the bulking properties of yarn. |
| 3. It is denoted by C. | 3. It is denoted by T |
| 4. Crimp % | 4. Take up% (T) |
Influence of crimp on fabric properties:
Warp and weft crimp percentage are two factors which have influence on the following fabric properties:
Resistance to Abrasion:
The abrasion resistance of a fabric will be more, if the crimp in the yarns is more. The yarns will high crimp take the brunt of abrasion action. This is because crowns, formed as the yarn bends round a transverse thread, will produce from the surface of the fabric and meet the destructive abrasion agent first. The other set of yarns lying in the center of the fabric will only play their part in resisting abrasion when the high crimped threads are nearly worn through.
Shrinkage:
When the yarns are wet, they swell and consequently say a warp thread as longer bending path to take a swollen weft thread. The warp thread must either increase in length or alternatively the weft threads must move closer together. An increase in length of warp requires the application of tension is absent, equilibrium conditions will be attained by the weft threads moving closer together.
The largest amount of shrinkage is that represented by increase of crimp. Yarn shrinkage takes a second place and generally it is much less than increase in crimp. Since shrinkage is mainly due to yarn swelling and the resulting crimp increase, mechanically means of controlled pre-shrinkage have been developed such as-Sanforizing and Rigmel processes.
Fabric behaviour during strength testing:
When a stripe of fabric is extended in one direction, crimp is removed and the threads are straightened. This causes the threads at right angles to the loading direction to be crimped further, i.e. when the load is applied along the warp threads, crimp in the warp is removed and that in the weft threads is increased. This is known as crimp interchange. The sample loses its original rectangular shape the middle position to the stripe contracts. This is known as wasting. Due to the removal of crimp, the load-elongation curve will show relatively high extension per unit increase in load in the early stages of strength testing of a stripe of fabric.
Faults of fabric:
Variation in crimp can give rise to faults in fabric, eg, reduction in strength, bright picks, diamond bars in rayon’s, stripes in yarn dyed cloth etc. The crimp variation is mainly due to the improper tension on the yarn during yarn preparation and weaving.
Fabric design:
Control of crimp percentage is necessary when a fabric is design to give a desired degree of extensibility. Some fabrics require control of crimp in the finishing process to give the correct crimp balance between warp and weft so that the finished appearance is satisfactory. Therefore, the tension applied must be carefully controlled.
Fabric costing:
Since crimp is related to length, it follows that the quantity of yarn required to produce a given length of fabric is affected by the warp weft crimp percentage. Therefore in calculating the cost and the yarn requirements, the value of crimp play an important role.
Measurement crimp percentage:
From the definition of crimp, two values must be known, the cloth length from which the yarn is removed and the straightened length of the thread. In order to straighten thread, tension must be applied, just sufficient to remove all the crimp without stretching the yarn.
The principle of yarn crimp determination is very simple. With a fine pen and rule, lines are drawn on a piece of cloth at a known distance. Some of the threads are raveled out, the yarns are straightened without stretching and the stretched length is noted and form that the crimp is calculated. The difficulty lies in the straightening of the yarn without stretching it. To do this, the following three methods are available:
- Straighten by hand: This is inaccurate since we do not know force applied.
- Straighten by a standard weight: This is satisfactory it we know what weight to use.
- Determine the straightened length from the load-elongation curve: This most accurate method.
Posted in TTQC-II
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