Constants for Neohook material

[nguyenvinhhiep]

Hi all,

I am trying to test a simple problem using hyperelastic material of Neohook. I had a look at subroutine fengy3.f that compute stored energy density related to volumetric part of strain energy function. The bulk modulus, parameter K as shown in the beginning lines of the subroutine as well as noted in many literatures, should be used in computation of U(J) and its first and second derivatives. But in this subroutine, the Lame' first parameter d(21) is used as follows:
d1 = augf*d(21)
I do not know what augf is, but equal to 1 (in my case)
d(21) is apparently 1st Lame constant. And then d1 equal to 1st Lame constant.
I am grateful for anyone pointing out where Bulk modulus is in the routine or what I have still confused.

Thank you
Hiep Nguyen

[Prof. R.L. Taylor]

There are two types of Neohookean models in FEAP.  The standard model (ELAStic NEOHook) uses the Lame parameters to define the volumetric behavior and the shear modulus the other term (which has some volumetric behavior similar to small strain elasticity).  The other modified Neohookean material (ELAStic MNEOhook) uses the bulk and shear modulus as the parameters since the volumetric and deviatoric behavior are decoupled.  Both use the same form of the J function.

The two models are described in the FEM for Solid and Structures (6th and 7th eds.)  by Zienkiewicz et al, jas well in other literature.

[nguyenvinhhiep]

Thank you very much for the quick explanation.

Best regards
Hiep Nguyen