Correcting lamé parameters for plane stress model

Author: luedkej

skfem/models/elasticity.py

@@ -28,7 +28,6 @@ def lame_parameters(E, nu):
 
 def plane_stress(E, nu):
     """Map Young's modulus and Poisson ratio to plane stress lame parameters.
-    
     Parameters
     ----------
     E
@@ -38,14 +37,14 @@ def plane_stress(E, nu):
 
     Returns
     -------
-    float
+    lam: float
         The first Lamé parameter for plane stress scenario (lambda)
-    float
+    mu: float
         The second Lamé parameter for plane stress scenario (mu)
-
     """
-    return (E * nu / (1. - nu**2),
-            E / (2. * (1. + nu)))
+    lam = E * nu / (1. - nu**2)
+    mu = E / (2. * (1. + nu))
+    return (lam, mu)
 
 
 def linear_stress(Lambda=1., Mu=1.):