The model of a string on an elastic foundation is applied to study adhesive zone delamination induced by combined vibrational - pulsed actions. The incubation time criterion is utilized in order to predict a critical delamination condition. The influence of the background ultrasonic field onto the value of threshold stress-pulse amplitudes is demonstrated using the example of several particular cases of external actions. A significant reduction of critical amplitudes is obtained with certain frequencies of background vibration field.