Some doubts about material LAW38

Altair Forum User

Hi all,

From the manual of /mat/law38, the curves are always nominal stresses versus engineering strains. However, from the manual of /prop/solid, It is recommended to use IHKT = 2 with HEPH and Lagrange type total strain, Ismstr = 10 for foam or rubber materials like laws 38, 42, 69, and 82. I think Ismstr=10 is not small strain formulation, so true stress vs true strain curve should be used. This two comments seem incompatible. How to do this? How to model package foam with LAW38?

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Thank you

Roy

 

Altair Forum User

Hi Roy,

This is a misunderstanding.  The Ismsr flag controls the strain calculation method in the solver. And Ismstr = 10 is fully okay for LAW38, where you input engineering stress-strain curve. 

The strain calculation is based on the total strain formulation. But the stress inside the material law is calculated using the engineering stress-strain curve.

Please go through comment 5 for Type 14.

Regarding the material for package foam, If it is an energy absorbing foam, you can try with LAW33. If it is pure elastic foam, try with LAW70. 

Altair Forum User

Altair Forum User said:

Hi Roy,

This is a misunderstanding.  The Ismsr flag controls the strain calculation method in the solver. And Ismstr = 10 is fully okay for LAW38, where you input engineering stress-strain curve. 

The strain calculation is based on the total strain formulation. But the stress inside the material law is calculated using the engineering stress-strain curve.

Please go through comment 5 for Type 14.

Regarding the material for package foam, If it is an energy absorbing foam, you can try with LAW33. If it is pure elastic foam, try with LAW70. 

Hi George,

1. From the help of /mat/law38, it seems the engineering stress-strain curve should be defined.

2. However, from the help of /prop/solid, comment 3, the stress-strain curve in material law is true stress-strain for Ismstr=10.

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3. But from comment 5 for /prop/solid, it seems that Ismstr=10 for law 38 is OK. 

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I think there are some conflicts among above comments.

 

Roy

Altair Forum User

Hi Roy,

As stated above the statement 3 of /PROP/SOLID is misunderstood.

Using Ismstr flag the strain calculation method is controlled in RADIOSS solver and for that context true stresses and strains are used.

The statement here is bit confusing and we will try to modify the same.