Add checks for generalized alpha

perpendicular-flap/solid-fenics/solid.py

@@ -89,10 +89,18 @@ def neumann_boundary(x, on_boundary):
 # alpha method parameters
 alpha_m = Constant(0)
 alpha_f = Constant(0)
+
+"""
+Check requirements for alpha_m and alpha_f from
+    Chung, J., and Hulbert, G. M. (June 1, 1993). "A Time Integration Algorithm for Structural Dynamics With Improved Numerical Dissipation:
+    The Generalized-α Method." ASME. J. Appl. Mech. June 1993; 60(2): 371–375. https://doi.org/10.1115/1.2900803
+"""
+assert(alpha_m <= alpha_f)
+assert(alpha_f <= 0.5)
+
 gamma = Constant(0.5 + alpha_f - alpha_m)
 beta = Constant((gamma + 0.5)**2 / 4.)
 
-
 # Define strain
 def epsilon(u):
     return 0.5 * (nabla_grad(u) + nabla_grad(u).T)