Fig. 9. Phase portraits and Poincare maps below and above the critical amplitude at X0 ¼ 0:8 for regular ((a), A ¼ 0:31 m and initial
conditions ½xin; vin ¼ ½0:15; 0:1) and chaotic ((b), A ¼ 0:41 m and the initial conditions ½xin; vin ¼ ½0:15; 0:1) solutions. The dominant
Lyapunov exponents are k1 ¼ 0:1625 and k1 ¼ 0:0354, respectively. (c) and (d) give the corresponding time responses. Here vx is given in
m/s while y is in m.