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Tensile testing experiment

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Tensile testing experiment

  1. 1. Tensile Testing ExperimentIntroductionWhat is Tensile TestingTensile testing is a way of determining how something willreact when it is pulled apart –i.e. when a force is applied to itin tension. In Tensile Testing, a specimen is subjected to anincreasing axial load whilst measuring the correspondingelongation until it fractures. The test is designed to give theyield stress, ultimate tensile stress and the percentageelongation (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 bethoroughly tested before it can beflown. Hydraulic stress testing is acommon technique for checkingthe strength and flexibility ofwings, fuselage, propellers, andentire airframes. Tens or evenhundreds of hydraulic actuators
  2. 2. push and pull at an airframe to test for failures and materials fatigue that might occur under flightconditions. 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. Inbolts testing, Tensile strength is the maximum tension-applied load the fastener can support prior tofracture. Usually, if strength is only requirement carbon steel bolts are sufficient. For specialapplications, non-ferrous metal bolts can be considered.
  3. 3. TheoryTensile SpecimensThe Shape of the SpecimenConsider 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 thespecimenso that deformation and failure will be localized in this region. The gage length is theregion over whichmeasurements are made andis centered within the reduced section.Holding the SpecimenThere are various ways of gripping the specimen, some of which are illustrated in Figure belowThe end may be screwed into a threaded grip, or it may be pinned; butt ends may be used, or the gripsection may be held between wedges.
  4. 4. Stress-Strain CurvesA tensile test involves mounting the specimen in a machine,such as those described in the previous section, andsubjecting it to tension. The tensile force is recorded as afunction of the increase in gage length. The Figure shows atypical curve for a ductile material. Such plots of tensile forceversus tensile elongation would be of little value if they werenot normalized with respect to specimendimensions.Therefore, engineers commonly use twonormalized parameters. 1. Engineering / Normal Stress 2. Engineering / Normal StrainEngineering stressEngineering stress, or normal stress, σ, is definedaswhere 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 strainEngineering strain, or normal strain,ε, is defined aswhere L0 is the initial gage length and ΔL isthechange in gage length (L -L0).Units of StrainStrain is measured as .When force-elongation data are convertedtoengineering stress and strain, a stress-strain curvethat is identical in shape to theforce-elongationcurve can be plotted. The advantageof dealing withstress versus strainrather than load versuselongation is that thestress-strain curve is virtuallyindependent ofspecimen dimensions.
  5. 5. 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 reversibledeformation 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 polymericmaterial).For most materials, the initial portion of the Stress strain curve is linear. The slope of this linear regioniscalled the elastic modulus or Young’s modulus:When the stress rises high enough, the stress-strainbehavior will cease to be linear and thestrain will notdisappear completely on unloading.The strain that remains is called plasticstrain. The first plastic strainusually correspondsto the first deviation from linearity.Yield StrengthIt is tempting to define an elastic limit as thestress atwhich plastic deformation first occursand a proportionallimit as the stress at whichthe stress-strain curve firstdeviates from linearity. The beginning of the plasticityisusually described by an offset yieldstrength, which can bemeasured with greaterreproducibility. It can be found byconstructinga straight line parallel to the initial linearportionof the stress-strain curve, but offset byε = 0.002or 0.2%. The yield strength is the stress atwhich this lineintersects the stress-strain curve. The logic is that if thematerial hadbeen loaded to this stress and thenunloaded, theunloading path would have been along thisoffsetline and would have resulted in a plastic strain of ε0.2%.Tensile StrengthThe tensile strength (ultimate strength) is definedas thehighest value of engineering stress (shown in figurebelow). Up to the maximum load, the deformationshouldbe uniform along the gage section.With ductile materials, the tensile strengthcorresponds to the pointat which the deformationstarts to localize, forming a neck (Fig. a).Less ductile materials fracture beforethey neck(Fig. b). In this case, the fracture strength is thetensile strength. Indeed, very brittlematerials(e.g., glass at room temperature) do not yieldbefore fracture (Fig. c). Such materialshavetensile strengths but not yield strengths.
  6. 6. DuctilityThere are two common measuresused to describe the ductility of a material. One is the percentelongation, which is defined simplyaswhere L0 is the initial gage length and Lf is thelength of the gage section at fracture. Measurementsmaybe made on the broken pieces or underload. For most materials, the amount of elasticelongation is sosmall that the two areequivalent. When this is not so (as with brittlemetals or rubber), the results shouldstatewhether or not the elongation includes an elasticcontribution.The other common measure ofductility is percent reduction of area, which isdefined aswhere A0 and Af are the initial cross-sectionalarea and the cross-sectional area at fracture, respectively.Iffailure 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 beforenecking, and the localizedelongation that occurs during necking. The secondpart is sensitive to thespecimen shape.When a gage section that is very long (relativeto its diameter), the necking elongationconvertedto 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.
  7. 7. For round bars, this problem has been solved by standardizing the ratio of gage length todiameter to4:1. Within a series of bars, all withthe same gage-length-to-diameter ratio, thenecking elongation willbe the same fraction ofthe total elongation. However, there is no simpleway to make meaningfulcomparisons of percentelongation from such standardized bars with thatmeasured on sheet tensilespecimens or wire.With sheet tensile specimens, a portion of theelongation occurs during diffusenecking, 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 thesheet thickness. For tensile testingof wire, it is impractical to have a reduced section,and so the ratio ofgage length to diameteris necessarily very large. Necking elongationcontributes very little to the totalelongation.Percent reduction of area, as a measure ofductility, has the disadvantage that with veryductile materialsit is often difficult to measurethe final cross-sectional area at fracture. This isparticularly true of sheetspecimens.
  8. 8. 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 formetallic materials will not be found. This is because the movement in the chucks and the mechanismscan 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. Thebottom of the specimen is connected to the large springs, the deflection of which is measured on dialindicator. Thus, the specimen elongation is given by subtracting the Dial indicator reading from thenumber of turns. For example if the hand-wheel has turned three times the dial indicator reading fromthe number of turns is 2.83 the elongation = 3.00 – 2.83 = 0.17mm. The dial indicator also provides anindication of the force being applied to the specimen; since the springs have a combined rate of100N/mm each dial indicator division is equal to one Newton. So if the dial indicator reads 2.83mm thenthe force is 283N.SafetyThe guards provided with this machine should be fitted at all times. On specimen failure themechanism snaps back rapidly and students should be aware of this. NEVER operate this machineunless 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. 9. 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)
  10. 10. Observation ChartsFor 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) ANumber of Dial Indicator Normal Stress Elongation(mm) Force(N) StrainTurns Reading (MPa)
  11. 11. Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimatetensile Strength σUSUse 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.
  12. 12. For Alumnium 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) ANumber of Dial Indicator Normal Stress Elongation(mm) Force(N) StrainTurns Reading (MPa)
  13. 13. Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimatetensile Strength σUSUse 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.
  14. 14. For Duralumin 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) ANumber of Dial Indicator Normal Stress Elongation(mm) Force(N) StrainTurns Reading (MPa)
  15. 15. Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimatetensile Strength σUSUse 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.
  16. 16. For Plastic 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) ANumber of Dial Indicator Normal Stress Elongation(mm) Force(N) StrainTurns Reading (MPa)
  17. 17. Plot a graph of Stress-Strain Graph and Indicate on the graph Yield Strength σY and Ultimatetensile Strength σUSUse 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.

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