change/correction in fiber structure tensor implementation?

[fethio]

I noticed a difference in the definition of the fiber structure tensor between versions 8.4 and 8.5 exists. In 8.4 it was defined as:

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      tau(1) = a(1)**2
      tau(2) = a(2)**2
      tau(3) = a(3)**2
      tau(4) = a(1)*a(2)*2.d0
      tau(5) = a(2)*a(3)*2.d0
      tau(6) = a(3)*a(1)*2.d0
while in version 8.5 it reads (apart from the name change tau -> mm)

Code: [Select]

      mm(1) = a(1)**2
      mm(2) = a(2)**2
      mm(3) = a(3)**2
      mm(4) = a(1)*a(2)
      mm(5) = a(2)*a(3)
      mm(6) = a(3)*a(1)
(Note the missing *2 in the last three items.)

Is this correction of a previous error, or is it due to the way things are handled differently in 8.5?

[K.Li]

I think mm(6) is the Voigt notation of a(3) \otimes a(3),  there is no 2 here. Check  the formulation of this model in Prof Holzapfel's paper in 2000

[Prof. S. Govindjee]

I believe that the factors of two in the expression for tau(4:6) are errors in version 8.4.  The Cauchy stresses are stored in Voigt notation so, for example, sig(4) = sigma_{12}  NOT = 2*sigma_{12} [note, the Berkeley ordering for entries 4:6].  The expressions in version 8.4 are giving incorrect factors of two in the shear stresses. 

If you are using 8.4, you should delete the *2.d0 on tau(4:6).

[fethio]

Thank you for the clarification Prof Govindjee.
My user element was based on the fiber in ver8.4, and had to be corrected based on the discussion in this post.