Tensile testing experiment

Tensile Testing Experiment
Introduction
What is Tensile Testing
Tensile testing is a way of determining how something will
react when it is pulled apart –i.e. when a force is applied to it
in tension. In Tensile Testing, a specimen is subjected to an
increasing axial load whilst measuring the corresponding
elongation until it fractures. The test is designed to give the
yield stress, ultimate tensile stress and the percentage
elongation (an indication of ductility) for a material.

Applications of Tensile Testing
TENSILE TESTS are performed for several reasons.

If a material is to be used as part of an engineering
structure that will be subjected to a load, it is
important to know that the material is strong and
rigid enough to withstand the loads that it will
experience in service. These tests provides data on the integrity and safety of materials,
components and products, helping manufacturers ensure that their finished products are fit-for-
purpose and manufactured to the highest quality.These measures of strength are used, with
appropriate caution (in the form of safety factors), in engineering design.
Moreover, tensile properties often are measured during development of new materials and
processes, so that different materials and processes can be compared. Thus, the results of
tensile tests are used in selecting materials for engineering applications.

Some of the examples are discussed below.

1.Airframe Manufacturing
A newly designed aircraft must be
thoroughly tested before it can be
flown. Hydraulic stress testing is a
common technique for checking
the strength and flexibility of
wings, fuselage, propellers, and
entire airframes. Tens or even
hundreds of hydraulic actuators

push and pull at an airframe to test for failures and materials fatigue that might occur under flight
conditions. Strain is measured at hundreds of points to qualify test results.

2. Bolts Installation
Most fastener applications are designed to support or transmit some form of externally applied load. In
bolts testing, Tensile strength is the maximum tension-applied load the fastener can support prior to
fracture. Usually, if strength is only requirement carbon steel bolts are sufficient. For special
applications, non-ferrous metal bolts can be considered.

Theory

Tensile Specimens
The Shape of the Specimen
Consider the typical tensile specimen shown in Figure below.

It has enlarged ends or shoulders for gripping. The importantpart of the specimen is the gage section.
Thecross-sectional area of the gage section is reducedrelative to that of the remainder of thespecimen
so that deformation and failure will be localized in this region. The gage length is theregion over which
measurements are made andis centered within the reduced section.

Holding the Specimen
There are various ways of gripping the specimen, some of which are illustrated in Figure below

The end may be screwed into a threaded grip, or it may be pinned; butt ends may be used, or the grip
section may be held between wedges.

Stress-Strain Curves
A tensile test involves mounting the specimen in a machine,
such as those described in the previous section, and
subjecting it to tension. The tensile force is recorded as a
function of the increase in gage length. The Figure shows a
typical curve for a ductile material. Such plots of tensile force
versus tensile elongation would be of little value if they were
not normalized with respect to specimen
dimensions.Therefore, engineers commonly use two
normalized parameters.

1. Engineering / Normal Stress
2. Engineering / Normal Strain

Engineering stress
Engineering stress, or normal stress, σ, is definedas

where F is the tensile force and A0 is the initialcross-sectional area of the gage section.

Units of stress:
SI Units: Newton per square meter ( ) = Pascal (Pa)

FPS Units: pounds per square inch (psi)

Engineering strain
Engineering strain, or normal strain,ε, is defined as

where L0 is the initial gage length and ΔL is
thechange in gage length (L -L0).

Units of Strain
Strain is measured as .

When force-elongation data are converted
toengineering stress and strain, a stress-strain curve
that is identical in shape to theforce-elongation
curve can be plotted. The advantageof dealing with
stress versus strainrather than load versus
elongation is that thestress-strain curve is virtually
independent ofspecimen dimensions.

Elastic versus Plastic Deformation.
Whena solid material is subjected to small stresses, thebonds between the atoms are stretched.
Whenthe stress is removed, the bonds relax and thematerial returns to its original shape. This reversible
deformation is called elastic deformation. At higher stresses, planes of atomsslide over one another.
This deformation, whichis not recovered when the stress is removed, istermed plastic deformation.

NoteThe term“plastic deformation” does not mean that the deformedmaterial is a plastic (a polymeric
material).

For most materials, the initial portion of the Stress strain curve is linear. The slope of this linear region
iscalled the elastic modulus or Young’s modulus:

When the stress rises high enough, the stress-strainbehavior will cease to be linear and thestrain will not
disappear completely on unloading.The strain that remains is called plasticstrain. The first plastic strain
usually correspondsto the first deviation from linearity.

Yield Strength
It is tempting to define an elastic limit as thestress at
which plastic deformation first occursand a proportional
limit as the stress at whichthe stress-strain curve first
deviates from linearity. The beginning of the plasticityis
usually described by an offset yieldstrength, which can be
measured with greaterreproducibility. It can be found by
constructinga straight line parallel to the initial linear
portionof the stress-strain curve, but offset byε = 0.002
or 0.2%. The yield strength is the stress atwhich this line
intersects the stress-strain curve. The logic is that if the
material hadbeen loaded to this stress and then
unloaded, theunloading path would have been along this
offsetline and would have resulted in a plastic strain of ε
0.2%.

Tensile Strength
The tensile strength (ultimate strength) is definedas the
highest value of engineering stress (shown in figure
below). Up to the maximum load, the deformationshould
be uniform along the gage section.With ductile materials, the tensile strengthcorresponds to the point
at which the deformationstarts to localize, forming a neck (Fig. a).Less ductile materials fracture before
they neck(Fig. b). In this case, the fracture strength is thetensile strength. Indeed, very brittle
materials(e.g., glass at room temperature) do not yieldbefore fracture (Fig. c). Such materials
havetensile strengths but not yield strengths.

Ductility
There are two common measuresused to describe the ductility of a material. One is the percent
elongation, which is defined simplyas

where L0 is the initial gage length and Lf is thelength of the gage section at fracture. Measurementsmay
be made on the broken pieces or underload. For most materials, the amount of elasticelongation is so
small that the two areequivalent. When this is not so (as with brittlemetals or rubber), the results should
statewhether or not the elongation includes an elasticcontribution.

The other common measure ofductility is percent reduction of area, which isdefined as

where A0 and Af are the initial cross-sectionalarea and the cross-sectional area at fracture, respectively.If
failure occurs without necking, onecan be calculated from the other:

After a neck has developed, the two are nolonger related. Percent elongation, as a measureof ductility,
has the disadvantage that it is reallycomposed of two parts: the uniform elongationthat occurs before
necking, and the localizedelongation that occurs during necking. The secondpart is sensitive to the
specimen shape.When a gage section that is very long (relativeto its diameter), the necking elongation
convertedto percent is very small. In contrast, witha gage section that is short (relative to its diameter),
the necking elongation can account formost of the total elongation.

For round bars, this problem has been solved by standardizing the ratio of gage length todiameter to
4:1. Within a series of bars, all withthe same gage-length-to-diameter ratio, thenecking elongation will
be the same fraction ofthe total elongation. However, there is no simpleway to make meaningful
comparisons of percentelongation from such standardized bars with thatmeasured on sheet tensile
specimens or wire.With sheet tensile specimens, a portion of theelongation occurs during diffuse
necking, andthis could be standardized by maintaining thesame ratio of width to gage length. However,
aportion of the elongation also occurs duringwhat is called localized necking, and this dependson the
sheet thickness. For tensile testingof wire, it is impractical to have a reduced section,and so the ratio of
gage length to diameteris necessarily very large. Necking elongationcontributes very little to the total
elongation.

Percent reduction of area, as a measure ofductility, has the disadvantage that with veryductile materials
it is often difficult to measurethe final cross-sectional area at fracture. This isparticularly true of sheet
specimens.

The Equipment
The Mini Tensile Tester provides means of stretching a specimen to destruction to produce a force-
elongation graph from which the yield stress and ultimate stress can be extracted.

Note:Due to the simplistic nature of the equipment and chucks, an accurate answer for the modulus for
metallic materials will not be found. This is because the movement in the chucks and the mechanisms
can be of a greater magnitude than the extension in the elastic region.

Operating the Apparatus
The hand-wheel at the top of the machine pulls the top of the specimen up by 1mm per turn. The
bottom of the specimen is connected to the large springs, the deflection of which is measured on dial
indicator. Thus, the specimen elongation is given by subtracting the Dial indicator reading from the
number of turns. For example if the hand-wheel has turned three times the dial indicator reading from
the number of turns is 2.83 the elongation = 3.00 – 2.83 = 0.17mm. The dial indicator also provides an
indication of the force being applied to the specimen; since the springs have a combined rate of
100N/mm each dial indicator division is equal to one Newton. So if the dial indicator reads 2.83mm then
the force is 283N.

Safety
The guards provided with this machine should be fitted at all times. On specimen failure the
mechanism snaps back rapidly and students should be aware of this. NEVER operate this machine
unless the guards are in place.

Procedure:
1. Select an appropriate specimen for testing
2. Measure length (L) of specimen’s thin part as show in the diagram below.
3. Measure the cross sectional breadth (B) and height (H) of the specimen rod using micrometer
screw gauge.
4. Take an initial reading against the scale at the back. Let this be R1.
5. Take up the slack in the mechanism by tuning the hand-wheel until the dial indicator begins to
move.
6. Align the grove on the hand-wheel to the nearest mark on the scale, and zero the dial indicator
using the outer bezel.
7. Turn the hand-wheel and take dial indicator readings in the increments as shown below
a. For Steel and Duralumin ½ turn increments are acceptable through the range.
b. For Aluminium turn increments are acceptable through the range.
c. For Plastics turn increments for the first three turns are acceptable, then increments
of 2 turns until destruction
8. Towards the end of the test the material will yield rapidly and an accurate dial indicator reading
may not be easily seen. If the reading does not stabilize after 20-30 seconds, then take the
specimen to fracture by continually turning the hand-wheel until it snaps (with a bang!).

9. Wind the hand-wheel back until the ends of the specimen touch and read off the new length o
the scale at the back (R2).

Specimen Test Section Dimensions (Nominal)

Observation Charts

For Steel

Quantity Symbol Observation

Length of Specimen (mm) L

Breadth of Specimen (mm) B

Height of Specimen (mm) H

Initial Scale Reading (mm) R1

Final Scale Reading (mm) R2

Cross Sectional Area of Specimen (mm2) A

Number of Dial Indicator Normal Stress
Elongation(mm) Force(N) Strain
Turns Reading (MPa)

Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimate
tensile Strength σUS

Use the graph to calculate the Young Modulus of Elasticity (E)

What is the Percentage Elongation (Ductility) of Specimen?

Calculate Percentage Deviation of Young Modulus from the Reference Book.

For Alumnium








Turns Reading (MPa)

For Duralumin








Turns Reading (MPa)

For Plastic








Turns Reading (MPa)

Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimate
tensile Strength σUS

Use the graph to calculate the Young Modulus of Elasticity (E)

What is the Percentage Elongation (Ductility) of Steel Specimen?

Calculate Percentage Deviation of Young Modulus from the Reference Book.

Tensile testing experiment

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