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--- doc:user:elements:volumes:user_defined_materials [2024/06/06 15:28] – radermecker
+++ doc:user:elements:volumes:user_defined_materials [2024/10/22 14:15] (current) – radermecker
@@ Line 1: / Line 1: @@
-====== Implementing a Python-Derived HyperMaterial in C++ ======
+====== User defined materials ======
-The goal is to create a Python class (e.g., in the input file) that derives from the C++ class `PythonHyperMaterial` and overwrites the following functions:
+This section explains how any user can define its own constitutive equation in Metafor (usermat or user-defined feature).
-  * `def PythonHyperMaterial.__init__(self, _no):`
+The idea is to create a Python class (e.g., in the input file) that derives from the C++ class `PythonHyperMaterial` and overwrites the following functions:
-    * `_no` being the material number.
-  * `def computeStressPython(self, FTotal):`
+  * ''def PythonHyperMaterial.<nowiki>__init__</nowiki>(self, _no):'' with ''_no'' being the material number chosen.
-    * **Input:** The total deformation gradient.
+  * ''def computeStressPython(self, FTotal):''
-      * Sent from C++ to Python through a `std::vector` [F11, F12, F13, F21, F22, F23, F31, F32, F33]
+    * **Input:** The total deformation gradient ''FTotal'', sent from C++ to Python through a ''std::vector'' [F11, F12, F13, F21, F22, F23, F31, F32, F33], hence ''FTotal[0] = F11'', etc.
-    * **Output:** Cauchy stress, in the current configuration.
+    * **Output:** The Cauchy stress, in the current configuration, returned from Python to C++ through a Python list [Sig11, Sig22, Sig33, Sig12, Sig13, Sig23]
-      * Sent from Python to C++ through a Python list [Sig11, Sig22, Sig33, Sig12, Sig13, Sig23]
 The relation between stress and strain is defined by the user.
+<WRAP center round important 60%>
+Selective integration with pressure report ''SRIPR'' for the integration will not work.
+</WRAP>
+<WRAP center round info 60%>
+The material tangent moduli is automatically approximated using finite central difference.
+</WRAP>
-===== Remarks =====
-  * **Why does `PythonHyperMaterial` derive from `HyperMaterial`?**
-    Hyper (Cauchy) Materials are based on the total deformation gradient. In the next merge, I will (try to) add Material perturbation to evaluate (finite difference) the Material Tangent Operator for all Cauchy Hypermaterial, including `PythonHyperMaterial`.
-    :!: Therefore, so far, global perturbation (of the internal forces) must be used :!:
-    ```python
-    elementsProps.put(STIFFMETHOD, STIFF_NUMERIC)
-    ```
-  * **Why doesn't `ComputeStressPython`** (i.e., the user-defined material) use Matr3/... etc?
-    This user-defined feature should be kept **as simple as possible** (KISS). Therefore, the user can rely on simple data types which they can easily transform, for instance, to `np.array` (see example). A student has already used this feature without any questions after receiving the example below. If the application requires an efficient implementation, adding the material directly in Metafor remains the preferred procedure (especially with its tangent moduli).
 ===== Example: A NeoHookean Model =====
+<code python>
+from wrap import *
+import numpy as np
+metafor = None
-```python
 class UserMat(PythonHyperMaterial):
     def __init__(self, _no):
         PythonHyperMaterial.__init__(self, _no)
@@ Line 51: / Line 44: @@
 def getMetafor(_p):
     # ... some code ... #
@@ Line 61: / Line 53: @@
     elementsProps = ElementProperties(Volume2DElement)
     elementsProps.put(MATERIAL, 1)
-    elementsProps.put(STIFFMETHOD, STIFF_NUMERIC)
     # ... some code ... #
     return metafor
+</code>