Hi, Dear all, Facing problem to understand strain energy function invariant terms A typical strain energy function consist of strain invariant can be defined as followed W(I1,I4)=C0+C1(I1-3)(I4-1)+C2(I1-3)^2+C3(I1-4)^2+C4(I1-3)+C5(I4-1), I1 and I4 are so called invariants of Green's strain tensor. (large deformation) I1=trC=λ1^2+λ2^2+λ3^2 I4=N1*λ1^2*N1+N2*λ2^2*N2+N3*λ3^2*N3. here is the complete link taken http://www.engin.umich.edu/class/bme456/ch6fitelasticmodelconstant/bme456fitmodel.htm [Broken] 1. I read from article that N is a unit vector along the stretch direction, so can i conclude that I4 consist of unit vector multiply with principal stretch? 2. the lamda in the formula is stretch ratio or principal stretch?