Ass 05 Tension in Circular Roods

8/2/2019 Ass 05 Tension in Circular Roods

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Miaaza Hussain (10/CE/61) Department of civil engineering

NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPURDEPARTMENT OF CIVIL ENGINEERING

MATERIAL TESTING LABORATORY: GROUP C

EXPERIMENT 05:TENSION IN CIRCULAR ROD

NAME: MIAAZA HUSSAIN

ROLL NO.: 10/CE/61


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Experiment No: 05

UNIVERSAL TESTING MACHINE

OBJECTIVE: To observe the behaviour of a specimen while being tested and to determine:

1. Upper and lower yield points2. Ultimate strength3. Breaking strength4. Percentage elongation of length5. Percentage reduction of cross- section

APPARATUS REQUIRED:

Universal Testing Machine ZD-20, Capacity 20

ton Micrometer, Dividers, Steel Rule, Centre

Punch, etc.

PROCEDURE:

1.Make sure the diameter of the specimenwith the micrometer. Mark the 10 cm gauge

length with the centre punch in the central zone

of specimen.

2. Mount the approximate jaws in themachine.

3.Placed the fixed yoke in an appropriateposition by moving he frame guide spindles.

Start the pump to keep the movable yoke in a floating condition.

4. Bring the dial pointer to zero operating the zero adjuster.5. Bring the movable yoke in a suitable position and stop the pump.6. Fix the specimen between the jaws taking care that the grip is perfect and full. And now put

the scale to zero position.

7. Start the pump.8. Take the readings up to destruction.9. Stop the pump and take out the two pieces of broken specimen.

PRECAUTIONS

1. After fixing the accessories check the zero position of the indicator.2. During the test the rate of loading must be kept constant.


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THEORY

Universal Testing Machine otherwise known as a materials testing machine/ test frame is used to

test the tensile and compressive properties of materials. The Universal Testing machine is named

so because it can perform all the tests like compression, bending ,tension etc to examine the

material in all mechanical properties Such machines generally have two columns but single column

types are also available. Load cell and extensometer measure the key parameters of force and

deformation as the sample is tested.

A typical testing system consists of a materials testing machine/test frame, control and analysis

software, and critically, the test fixtures, accessories, parts and devices used to hold and support

the test specimen.

A tension test is a destructive test in the sense that the specimen is finally broken or fractured into

two pieces. To perform the tensile test, the universal testing machine should be capable ofapplying that load which is required to break or fracture the material. The test piece or specimen

of the material is generally a straight piece, uniform in the cross-section over the test length and

often with enlarged ends which can be held in the machine holders. However, the machine can

hold the specimen without enlarged ends also. Two fine marks are often made near the end of

uniform test section of the specimen and the distance between these points is termed "gauge

length". The gauge length is that length which is under study or observation when the experiment

on the specimen is performed. The gauge length of a specimen bears a constant standardized ratio

to the cross-sectional dimension for certain reasons. The specimen is placed in the machine

between the holders and any measuring device to record the change in length is fitted on to thespecimen between the gauge points. If such a device, generally extensometer, is not fitted, the

machine itself can record the displacement between its cross heads on which the specimen is

held. Once the machine is started it begins to apply a slowly increasing load on specimen. At

preset interval, the reading of the load and elongation of specimen are recorded. Finally, the

specimen breaks in the form of cup and cone shape at the fracture point(for ductile

metals).Before breaking, the area of cross section becomes very small, so a large stress is being

produced. The maximum stress which the specimen can bear is the "ultimate stress". We can also

find the modulus of elasticity for the specimen.

Salient Features:

Loading accuracy as high + 1%. Speeds: Straining at variable speeds to suit wide range of materials. Facilities for tests: Motor-driven threaded columns for quick and convenient adjustment of lower

cross head to facilitate rapid fixing of tear specimen.

Autographic recorder: Simultaneous roll autographic recorder supplied as standard to enablestudy of the behaviors of materials.


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Ideal Dial: High reading accuracy due to large size ideal design of dial. Large columns: Large effective clearance between columns enables testing of standard specimen

as well as structures.

Easy Changeability: Easy change from plain to threaded and screwed specimens. Simple and Safe: Simple to operate. Robust construction. Chrome plated metal components

Fig.: a labeled diagram of a universal testing machine


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Tensile Test

In tensile testing, material to be tested is machined to standard dimensions. Generally 12.5 mm

round test specimen as shown below is used for the test.

Where,

G Gauge length = 5 cm

D Diameter = 12.35

R Radius of fillet

A = Length of reduced section

The specimen is gauge marked with a center punch, scribe marks or drawn with ink. The purpose

of these gage marks is to determine the elongation. The elongation is measured by an

arrangement consisting of dial gauge and clamps called the extensometer. The testing machine

(Universal Testing Machine) is equipped with a loading system of mechanical (screw power) or

hydraulic type.. The unit stress (intensity of stress) is the load per unit area of the cross section of

the specimen and is plotted as the ordinate in stress strain diagram. The unit strain (unit

deformation) at any stress is the extension of the gauge length undergone by the specimen per

unit length (measured elongation divided by gauge length) under that stress. Unit strain is plottedas the abscissa in stress strain diagram. The relation between unit stress and unit strain found

experimentally is represented by the stress strain diagram. These changes have a negligible

effect except during the final stages of the test. The stress strain diagrams for a ductile and a

brittle material are shown below.

Why Perform a Tensile Test or Tension Test?

You can learn a lot about a substance from tensile testing. As you 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. The point of failure is of much interest and is typically called

itsUltimate Strengthor UTS on the chart.
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Tensile Properties

The properties which may be determined by a tensile test (with reference to stress strain

diagrams for a ductile and a brittle material shown above) are as under.

Proportional Limit and Modulus of Elasticity

It is found that the initial portion of the stress strain diagram is a straight line OP for most

materials used in engineering structures/components. In this range, the stress () and strain ()

are proportional to each other. Therefore we can write, =E x . This relationship is known as

Hookes Law.

E, the slope of the straight line portion of the stress strain diagram is called the Modulus of

ElasticityorYoungs Modulus.

Proportional limit is the maximum stress under which a material will maintain a perfectly uniform

rate of strain to stress. Thus the stress at the limit of proportionality point P is known as the

proportional limit.

Elastic Limit

If the load is increased after proportionality point P, then released after each increment and the

extensometer checked, a point will be reached at which the extensometer needle will not return

to zero. This indicates that the material now has permanent deformation. The elastic limit may

therefore be defined as the minimum stress at which permanent deformation first occurs.

For most materials the elastic limit has nearly the same numerical value as the proportional limit.

Yield Point

As the load in the specimen is increased beyond the elastic limit, a stress is reached at which the

material continues to deform without an increase of load. The stress at point Y in stress strain

diagram for a ductile material is known as Yield Point. This phenomenon occurs only in certain

ductile materials.

Since yield point is relatively easy to determine and the permanent deformation is small up to

yield point, it is very important value in design of machine members.

Yield Strength

Yield strength is the stress at which a material exhibits a specified limiting deviation from the

proportionality of stress to strain. This value is usually determined by the offset method. As shown

in stress strain diagram for brittle a material, the specified offset OX is laid off along the strain

axis. Then XZ is drawn parallel to OP, and thus Y, the intersection of XZ with the stress strain


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diagram, is located. The value of the stress at point Y gives the yield strength. The value of offset is

generally 0.20 percent of the gauge length.

Ultimate Strength

The ultimate strength or the tensile strength is therefore the maximum stress developed by thematerial based on the original cross sectional area. On loading further, a ductile material will

continue to stretch and will fracture at point B. In case of a brittle material, it breaks when

stressed to the ultimate strength at point B as shown in stress-strain diagram for a brittle material.

Ultimate tensile strength is the value most frequently used from tensile test results. It is used for

specification and quality control. However, in engineering design, safety factor shall be applied.

Ultimate strength = (kgm-1

sec-2)

Breaking Strength

The breaking strength (point B in stress strain diagram for a ductile material), which is

determined by dividing the breaking load by the original cross sectional area, is always less than

the ultimate strength. For brittle material, the ultimate strength and breaking strength coincide.

Breaking strength = (kgm-1

sec-2)

Ductility

The ductility of a material is indicated by the

amount of deformation that is possible until

fracture. This is determined in a tension test by

two measurements.

Maximum load x g

Final cross-sectional area

Breaking load x g

Final cross-sectional area


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Figure: Stress Strain Curve for

Brittle material

Percent Elongation

This is determined by fitting together, after fracture, the parts of the specimen and measuring the

distance between the original gauge marks.

Where,

Lf= final gauge length and

Lo = original gauge length, usually 2 inch.

Reduction in Cross-Sectional Area

Reduction of area, like elongation at break, is a measure of ductility and is expressed in percent.

This is also determined from the broken halves of the tensile specimen by measuring the

minimum cross sectional area and using the following formula.

Where,

Ao = original cross sectional area and

Af= final cross sectional area

Brittle materials

Brittlematerials such asconcrete andcarbon fiberdo not have

a yield point, and do not strain-harden which means that the

ultimate strength and breaking strength are the same.

One of the characteristics of a brittle failure is that the two

broken parts can be reassembled to produce the same shape

as the original component as there will not be a neck

formation like in the case of ductile materials.
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Figure: Necking in a tensile specimen.

True Stress Strain Diagram

In case of brittle materials, the specimen uniformly

increases in strength and at the same time decreases

uniformly in cross section until ultimate strength

(point B) is reached and fracture takes place.

The true stress is determined by the load divided by

the cross sectional area at that moment of loading.

The true strain is determined by the change in length

divided by the immediately preceding length.

Necking in a tensile material

Until the neck forms, the deformation is essentially

uniform throughout the specimen, but after necking all

subsequent deformation takes place in the neck. The neck

becomes smaller and smaller, local true stress increasing

all the time, until the specimen fails. The specimen often

fails finally with a cup and cone geometry as seen in Fig.,

in which the outer regions fail in shear and the interior in

tension. When the specimen fractures, the engineering strain at break (denoted f) will include

the deformation in the necked region and the un-necked region together. Since the true strain inthe neck is larger than that in the un-necked material, the value offwill depend on the fraction of

the gage length that has necked. Therefore, fis a function of the specimen geometry as well as the

material, and thus is only a crude measure of material ductility.

Figure: Cup-and-cone fracture in a ductile metal


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OBSERVATION CHART AND RESULTS

1. Gauge length: 5 cm2. Length between gauge marks after failure: 6.7 cm3. Mean Initial diameter of the specimen: 12.35mm4. Mean Final diameter of the specimen: 8.24 mm5. Final cross sectional area: 5.33 x 10-5 m2Loads (Tonnes) Strength (( kgm

-1sec

-2) Ductility ( %)

Upper yield load= 4.2Ultimate strength = 1.06 x 10

9% elongation = 34

Lower yield load= 4

Maximum load = 5.75Breaking strength = 8.74 x 10

8% reduction of cross-section = 33.33 %

Breaking load = 4.75

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Ass 05 Tension in Circular Roods