10.7 Properties of Materials 345
Fig.10.16
Stress–strain curve for a fiber composite.
andaluminumalloy.However,thefibersthemselvescanhavemuchhighervaluesofstrengthandmod-
ulus of elasticity than the composite. For example, carbon fibers have a tensile strength of the order
2400N/mm^2 andamodulusofelasticityof400000N/mm^2.
Fibercompositesarehighlydurable,requirenomaintenance,andcanbeusedinhostilechemical
andatmosphericenvironments;vinylsandepoxyresinsprovidethebestresistance.
Allthestress–straincurvesdescribedintheprecedingdiscussionarethoseproducedintensileor
compressiontestsinwhichthestrainisappliedatanegligiblerate.Arapidstrainapplicationwould
result in significant changes in the apparent properties of the materials, giving possible variations in
yieldstressofupto100percent.
10.7.3 Strain Hardening
Thestress–straincurveforamaterialisinfluencedbythestrainhistory,ortheloadingandunloading
of the material, within the plastic range. For example, in Fig. 10.17, a test piece is initially stressed
in tension beyond the yield stress at “a” to a value at “b.” The material is then unloaded to “c” and
reloadedto“f,”producinganincreaseinyieldstressfromthevalueat“a”tothevalueat“d.”Subsequent
unloadingto“g”andloadingto“j”increasestheyieldstressstillfurthertothevalueat“h.”Thisincrease
instrengthresultingfromtheloadingandunloadingisknownasstrainhardening.Itcanbeseenfrom
Fig.10.17thatthestress–straincurveduringtheunloadingandloadingcyclesformsloops(theshaded
areasinFig.10.17).Theseindicatethatstrainenergyislostduringthecycle,theenergybeingdissipated
intheformofheatproducedbyinternalfriction.Thisenergylossisknownasmechanicalhysteresis
andtheloopsashysteresisloops.Althoughtheultimatestressisincreasedbystrainhardening,itisnot
influencedtothesameextentasyieldstress.Theincreaseinstrengthproducedbystrainhardeningis
accompaniedbydecreasesintoughnessandductility.