Learning Forum

[kshi84]

Hello all,

I am trying to write a UMAT that requires the total strain from the previous cycle as well as needing to store internal stress modes for the material model I am trying to implement. Setting IHYPER = 1 and LHV = (some number) in the keyword seems to wipe hsv(1) - hsv(9) at the beginning of every cycle.

How do I make use of the deformation gradient and save internal stress modes at the same time?

Many thanks,

K

[Armin Abedini]

Hello,

In Ansys LS-DYNA, when IHYPER is set to a nonzero value, the deformation gradient is stored in the HSV array, specifically in nine positions after the user-specified number of history variables (LHV/NHV). This means HSV is extended by 9 elements for the deformation gradient, but these elements (HSV(1)-HSV(9)) are reserved for the deformation gradient and are overwritten at the start of each cycle. To preserve your internal stress modes or other state variables across cycles, you must use the additional HSV entries beyond these first 9 reserved slots. 

• When IHYPER = 1, the first 9 HSV slots are reserved for the deformation gradient and are not persistent across cycles.

• To store internal stress modes, increase LHV (the number of history variables) so that your custom state variables are stored in HSV(10) and higher. These positions are persistent and will not be wiped at the beginning of each cycle.

• Access the deformation gradient from HSV(1)-HSV(9) and your internal variables from HSV(10) onward in your UMAT implementation