where a is the beam's curvature angle; and the debonded region is
located between two angles of a1 and a2. The Rayleigh-Ritz method
is applied to descritize the motion equations. The response of the
beam is assumed to be harmonic with frequency u. Trigonometric
functions are used as trial functions such that satisfy geometry
boundary conditions. For instance, the following series can be used
for a simply-supported curved beam whose boundaries can move
in circumferential direction