Observing nonlinear effects in vibrating strings
  Nonlinear effects were observed in a forced vibrating string. The motion of the string becomes elliptic as the amplitude of the vibration increases. The fundamental resonance frequency depends on the amplitude of the vibration. At sufficient high amplitudes the system exhibits hysterisis: the resonance frequency is different when measured increasing the driving frequency or decreasing it. The experimental results are well understood in the framework of a 2-dimensional model assuming that the string tension is not constant due to the varying length of the string throughout the oscillation. 
Miguel C. Brito, Jorge Maia Alves, Joao M. Serra, Antonio Vallera