From 9f7f49839b0e6164906aa0731eb656a2cded5878 Mon Sep 17 00:00:00 2001
From: Gerard Capes <gerard.capes@manchester.ac.uk>
Date: Thu, 26 Mar 2026 12:37:14 +0000
Subject: [PATCH] Add zero_normal_stress load case

- Rename previous `null` loadcase to `zero deformation`
- Add new load case for zero normal stress
---
 matflow/param_classes/load.py | 37 +++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/matflow/param_classes/load.py b/matflow/param_classes/load.py
index 0125c017..cec5e669 100644
--- a/matflow/param_classes/load.py
+++ b/matflow/param_classes/load.py
@@ -277,7 +277,7 @@ def example_uniaxial(cls) -> Self:
         )
 
     @classmethod
-    def null(
+    def zero_deformation(
         cls,
         total_time: float | int,
         num_increments: int,
@@ -286,7 +286,7 @@ def null(
 
         target_def_grad = np.eye(3)
 
-        _method_name = "null"
+        _method_name = "zero_deformation"
         _method_args = {
             "total_time": total_time,
             "num_increments": num_increments,
@@ -298,6 +298,30 @@ def null(
             target_def_grad=target_def_grad,
         )
 
+    @classmethod
+    def zero_normal_stress(
+        cls,
+        total_time: float | int,
+        num_increments: int,
+    ) -> Self:
+        """A zero normal stress load step"""
+
+        target_def_grad = np.ma.masked_array(np.zeros((3, 3)), mask=np.eye(3))
+        stress = np.ma.masked_array(np.zeros((3, 3)), mask=np.logical_not(np.eye(3)))
+
+        _method_name = "zero_normal_stress"
+        _method_args = {
+            "total_time": total_time,
+            "num_increments": num_increments,
+        }
+
+        return cls(
+            total_time=total_time,
+            num_increments=num_increments,
+            target_def_grad=target_def_grad,
+            stress=stress,
+        )
+
     @classmethod
     def uniaxial(
         cls,
@@ -1142,9 +1166,14 @@ def create_damask_loading_plan(self) -> list[dict[str, Any]]:
         return load_steps
 
     @classmethod
-    def null(cls, **kwargs) -> Self:
+    def zero_deformation(cls, **kwargs) -> Self:
         """A zero deformation load case"""
-        return cls(steps=[LoadStep.null(**kwargs)])
+        return cls(steps=[LoadStep.zero_deformation(**kwargs)])
+
+    @classmethod
+    def zero_normal_stress(cls, **kwargs) -> Self:
+        """A zero normal stress load case"""
+        return cls(steps=[LoadStep.zero_normal_stress(**kwargs)])
 
     @classmethod
     def uniaxial(cls, **kwargs) -> Self: