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fix: correct the sign of calculated strain, stress resultant, and modulus #310

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Merged
mortenengen merged 1 commit into from
Jan 13, 2026
Merged

fix: correct the sign of calculated strain, stress resultant, and modulus #310

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18 changes: 12 additions & 6 deletions structuralcodes/sections/section_integrators/_shell_integrator.py
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Original file line number Diff line number Diff line change
Expand Up @@ -65,8 +65,11 @@ def prepare_input(
prepared_input = []
IA = []
z_list = []

# A positive in-plane strain gives tension
# A positive curvature gives a negative strain for a positive z-value
for z in z_coords:
fiber_strain = strain[:3] + z * strain[3:]
fiber_strain = strain[:3] - z * strain[3:]
if integrate == 'stress':
integrand = material.constitutive_law.get_stress(fiber_strain)
elif integrate == 'modulus':
Expand All @@ -81,7 +84,7 @@ def prepare_input(
z_r = r.z
material = r.material

fiber_strain = strain[:3] + z_r * strain[3:]
fiber_strain = strain[:3] - z_r * strain[3:]
eps_sj = r.T @ fiber_strain

if integrate == 'stress':
Expand Down Expand Up @@ -118,9 +121,12 @@ def integrate_stress(
Nx = np.sum(fiber_stress[:, 0])
Ny = np.sum(fiber_stress[:, 1])
Nxy = np.sum(fiber_stress[:, 2])
Mx = np.sum(fiber_stress[:, 0] * z)
My = np.sum(fiber_stress[:, 1] * z)
Mxy = np.sum(fiber_stress[:, 2] * z)

# A positive stress at a positive z-value gives a negative moment
# contribution
Mx = -np.sum(fiber_stress[:, 0] * z)
My = -np.sum(fiber_stress[:, 1] * z)
Mxy = -np.sum(fiber_stress[:, 2] * z)
return Nx, Ny, Nxy, Mx, My, Mxy

def integrate_modulus(
Expand All @@ -142,7 +148,7 @@ def integrate_modulus(
D = np.zeros((3, 3))
for C_layer, z_i in zip(MA, z):
A += C_layer
B += z_i * C_layer
B -= z_i * C_layer
D += z_i**2 * C_layer

return np.block([[A, B], [B, D]])
Expand Down