A method for the non-linear ultrasonic online detection of early
fatigue damage to a
metal material belongs to the field of nondestructive detection. The method comprises the following steps: determining an
excitation signal parameter according to the thickness of a tested piece and inputting the parameter to an arbitrary
function generator to generate a sound
signal; determining a threshold value of an
acoustic emission instrument according to the amplitude of a no-load
noise signal; performing
fatigue loading on the tested piece, continuously detecting an
acoustic emission signal in real time with an
acoustic emission sensor, amplifying the acoustic emission signal, inputting the acoustic emission signal into the acoustic emission instrument, and judging ring with the acoustic emission instrument when the amplitude of the acoustic emission signal exceeds the preset the threshold value of the acoustic emission instrument; detecting a non-linear ultrasonic signal at
equal time interval if the acoustic emission instrument does not display the ring or the times of the continuous ring is not more than an empirical value; and stopping detection if the displayed ring times is more than the empirical value, because fatigue cracks are generated and develop. On the basis of non-linear ultrasonic nondestructive detection, the method of the invention introduces acoustic emission technique, so the method does not make incorrect judgment when detecting the early
fatigue damage to the
metal material and realizes continuous online detection.