Introduction to Aircraft Structural Analysis (Elsevier Aerospace Engineering)

10.7 Properties of Materials 339

Fig.10.6

Brinellhardness test.

inmm^2 iscalledtheBrinellhardnessnumber(BHN).InFig.10.6,ifDisthediameteroftheball,Fis
theloadinkg,histhedepthoftheindentation,anddisthediameteroftheindentation,then

BHN=

F

πDh

=

2 F

πD[D−

√

D^2 −d^2 ]

Inpractice,thehardnessnumberofagivenmaterialisfoundtovarywithFandDsothatforuniformity
thetestisstandardized.ForsteelandhardmaterialsF=3000kgandD=10mm,whileforsoftmaterials,
F=500kgandD=10mm;inaddition,theloadisusuallyappliedfor15s.
IntheBrinelltest,thedimensionsoftheindentationaremeasuredbymeansofamicroscope.To
avoidthisrathertediousprocedure,directreadingmachineshavebeendevised,ofwhichtheRockwell
istypical.Theindentingtool,againahardenedsphere,isfirstappliedunderadefinitelightload.This
indentingtoolisthenreplacedbyadiamondconewitharoundedpointwhichisthenappliedundera
specifiedindentationload.Thedifferencebetweenthedepthoftheindentationunderthetwoloadsis
takenasameasureofthehardnessofthematerialandisreaddirectlyfromthescale.
AtypicaldynamichardnesstestisperformedbytheShoreScleroscope,whichconsistsofasmall
hammerapproximately20mmlongand6mmindiameterfittedwithablunt,rounded,diamondpoint.
Thehammerisguidedbyaverticalglasstubeandallowedtofallfreelyfromaheightof25cmonto
thespecimen,whichitindentsbeforerebounding.Acertainproportionoftheenergyofthehammer
isexpendedinformingtheindentationsothattheheightoftherebound,whichdependsontheenergy
stillpossessedbythehammer,istakenasameasureofthehardnessofthematerial.
Anumberoftestshavebeendevisedtomeasurethe“scratchhardness”ofmaterials.Inonetest,the
smallestloadingramswhich,whenappliedtoadiamondpoint,producesascratchvisibletothenaked
eyeonapolishedspecimenofmaterialiscalleditshardnessnumber.Inothertests,themagnitudeof
theloadrequiredtoproduceadefinitewidthofscratchistakenasthemeasureofhardness.Abrasion
tests,involvingtheshakingoveraperiodoftimeofseveralspecimensplacedinacontainer,measure
theresistancetowearofsomematerials.Insomecases,thereappearstobeaconnectionbetweenwear
andhardnessnumber,althoughtheresultsshownolevelofconsistency.

Impact Tests

Ithasbeenfoundthatcertainmaterials,particularlyheat-treatedsteels,aresusceptibletofailureunder
shock loading, whereas an ordinary tensile test on the same material would show no abnormality.