10.7 Properties of Materials 335Fig.10.2
Standard cylindrical test piece.
overthegivengaugelength.Incrementsofloadareappliedandthecorrespondingextensionsrecorded.
Thisprocedurecontinuesuntilyieldoccurs,whentheextensometerisremovedasaprecautionagainst
thedamagewhichwouldbecausedifthetestpiecefracturedunexpectedly.Subsequentextensionsare
measured by dividers placed in the gauge points until, ultimately, the test piece fractures. The final
gaugelengthandthediameterofthetestpieceintheregionofthefracturearemeasuredsothatthe
percentageelongationandpercentagereductioninareamaybecalculated.Thesetwoparametersgive
ameasureoftheductilityofthematerial.
Astress–straincurveisdrawn(seeFigs.10.9and10.13),thestressnormallybeingcalculatedon
thebasisoftheoriginalcross-sectionalareaofthetestpiece—thatis,anominalstressasopposedto
anactualstress(whichisbasedontheactualareaofcrosssection).
Forductilematerialsthereisamarkeddifferenceinthelatterstagesofthetest,asaconsiderable
reductionincross-sectionalareaoccursbetweenyieldandfracture.Fromthestress–straincurve,the
ultimatestress,theyieldstress,andYoung’smodulus,E,areobtained.
There are a number of variations on the basic tensile test just described. Some of these depend
on the amount of additional information required and some on the choice of equipment. There is a
widerangeofstrainmeasuringdevicestochoosefrom,extendingfromdifferentmakesofmechanical
extensometer,suchasHuggenberger,Lindley,Cambridge,totheelectricalresistancestraingauge.The
last would normally be used on flat test pieces, one on each face to eliminate the effects of possible
bending.Atthesametime,astraingaugecouldbeattachedinadirectionperpendiculartothedirection
ofloadingsothatlateralstrainsaremeasured.Theratiolateralstrain/longitudinalstrainisPoisson’s
ratio,ν.
Testing machines are usually driven hydraulically. More sophisticated versions use load cells to
recordloadandautomaticallyplotloadagainstextensionorstressagainststrainonapenrecorderas
thetestproceeds,anadvantagewheninvestigatingthedistinctivebehaviorofmildsteelatyield.
Compression Tests
A compression test is similar in operation to a tensile test, with the obvious difference that the load
transmitted to the test piece is compressive rather than tensile. This is achieved by placing the test
piecebetweentheplatensofthetestingmachineandreversingthedirectionofloading.Testpiecesare