ABSTRACT
Typically,theversatilityofaconstitutiveequationis“judged”byitsabilityto describetheshearstressτ12 in relatively simple rheological experiments. Sometimes, the –rst and second normal stress differences
N N1 11 22 2 22 33= − = −τ τ τ τ, (8.1)
areusedtoo,althoughtheyaremoredif–culttoobtainaccurately.Ifmeasuredin laminar¦ow,thesequantitiesonlydependonshearrate.InEquation3.90,theviscosity η expresses the relationship between shear stress and shear rate:
τ ηγ12 = , (8.2)
withtheshearrateasde–nedinEquation3.30.Similarly,the–rstandsecondnormal stresscoef–cientsareintroduced.Theyexpressthedependenceofthenormalstress differences on shear rate:
= =ψ γ ψ γ , . (8.3)
Theviscositytogetherwiththesecoef–cientsarereferredtoastheviscometric functions.
