tensile testing experiment

Tensile Testing Experiment

Introduction

What is Tensile TestingTensile 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 TestingTENSILE 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 ManufacturingA 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 InstallationMost 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 SpecimenConsider the typical tensile specimen shown in Figure below.

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

Holding the SpecimenThere 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 CurvesA 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 Stress2. Engineering / Normal Strain

Engineering stressEngineering stress, or normal stress, σ, is defined as

σ= FA0

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

Units of stress:

SI Units: Newton per square meter (N

m2) = Pascal (Pa)

FPS Units: pounds per square inch (psi)

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

ε= ΔLL0

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

Units of Strain

Strain is measured as mmmm

.

When force-elongation data are converted to engineering stress and strain, a stress-strain curve

that is identical in shape to the force-elongation curve can be plotted. The advantage of dealing with stress versus strain rather than load versus elongation is that the stress-strain curve is virtually independent of specimen dimensions.

Elastic versus Plastic Deformation. When a solid material is subjected to small stresses, the bonds between the atoms are stretched. When the stress is removed, the bonds relax and the material returns to its original shape. This reversible deformation is called elastic deformation. At higher stresses, planes of atoms slide over one another. This deformation, which is not recovered when the stress is removed, is termed plastic deformation.

Note The term “plastic deformation” does not mean that the deformed material 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 is called the elastic modulus or Young’s modulus:

E=σε

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

Yield StrengthIt is tempting to define an elastic limit as the stress at which plastic deformation first occurs and a proportional limit as the stress at which the stress-strain curve first deviates from linearity. The beginning of the plasticity is usually described by an offset yield strength, which can be measured with greater reproducibility. It can be found by constructing a straight line parallel to the initial linear portion of the stress-strain curve, but offset by ε = 0.002 or 0.2%. The yield strength is the stress at which this line intersects the stress-strain curve. The logic is that if the material had been loaded to this stress and then unloaded, the unloading path would have been along this offset line and would have resulted in a plastic strain of ε 0.2%.

Tensile StrengthThe tensile strength (ultimate strength) is defined as the highest value of engineering stress (shown in figure below). Up to the maximum load, the deformation should be uniform along the gage section. With ductile materials, the tensile strength corresponds to the point

at which the deformation starts to localize, forming a neck (Fig. a). Less ductile materials fracture before they neck (Fig. b). In this case, the fracture strength is the tensile strength. Indeed, very brittle materials (e.g., glass at room temperature) do not yield before fracture (Fig. c). Such materials have tensile strengths but not yield strengths.

DuctilityThere are two common measures used to describe the ductility of a material. One is the percent elongation, which is defined simply as

%El=Lf−LoLo

×100

where L0 is the initial gage length and Lf is the length of the gage section at fracture. Measurements may be made on the broken pieces or under load. For most materials, the amount of elastic elongation is so small that the two are equivalent. When this is not so (as with brittle metals or rubber), the results should state whether or not the elongation includes an elastic contribution.

The other common measure of ductility is percent reduction of area, which is defined as

%RA=A0−A fA0

×100

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

%El= %RA100−%RA

After a neck has developed, the two are no longer related. Percent elongation, as a measure of ductility, has the disadvantage that it is really composed of two parts: the uniform elongation that occurs before

necking, and the localized elongation that occurs during necking. The second part is sensitive to the specimen shape. When a gage section that is very long (relative to its diameter), the necking elongation converted to percent is very small. In contrast, with a gage section that is short (relative to its diameter), the necking elongation can account for most of the total elongation.

For round bars, this problem has been solved by standardizing the ratio of gage length to diameter to 4:1. Within a series of bars, all with the same gage-length-to-diameter ratio, the necking elongation will be the same fraction of the total elongation. However, there is no simple way to make meaningful comparisons of percent elongation from such standardized bars with that measured on sheet tensile specimens or wire. With sheet tensile specimens, a portion of the elongation occurs during diffuse necking, and this could be standardized by maintaining the same ratio of width to gage length. However, a portion of the elongation also occurs during what is called localized necking, and this depends on the sheet thickness. For tensile testing of wire, it is impractical to have a reduced section, and so the ratio of gage length to diameter is necessarily very large. Necking elongation contributes very little to the total elongation.

Percent reduction of area, as a measure of ductility, has the disadvantage that with very ductile materials it is often difficult to measure the final cross-sectional area at fracture. This is particularly true of sheet specimens.

The EquipmentThe 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 ApparatusThe 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.

SafetyThe 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 testing2. 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 15

turn increments are acceptable through the range.

c. For Plastics 15

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 Turns

Dial Indicator Reading

Elongation(mm) Force(N) Normal Stress (MPa)

Strain

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








Number of Turns



Strain

For Duralumin








Number of Turns



Strain

For Plastic








Number of Turns



Strain



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


tensile testing experiment

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