Abstract: A nature frequency tracking system based on the crack on-line measurement is established to ensure the stability of test load in the resonance fatigue crack propagation test, and its applied method which is an improved adaptive hill climbing nature frequency tracking method for crack length on-line measuring is proposed. First, the designed crack on-line measuring system is used to acquire the crack length, the related nature frequency is calculated according to the theoretical relationship between crack length and nature frequency, and then the theoretical nature frequency value is set as an original frequency point to track the accurate nature frequency. The accurate nature frequency near the original frequency point can be obtained by applying the proposed nature frequency tracking method. A vibrating system mathematical model with two degrees of freedom and double mass and a sample stiffness model are established, and the law of nature frequency changing with crack length is theoretically obtained and verified by experiment. The experimental results show that the nature frequency tracking accuracy is 0-3 Hz, the tracking time is 3-5 s, the proposed method has the higher tracking accuracy, faster tracking speed , and more stable and reliable tracking process compared with the general hill climbing nature frequency tracking method. It can meet the resonance fatigue crack propagation test requirements.

Key words: technology of instrument and meter, electro magnetic resonance, fatigue crack propagation test, nature frequency tracking, vibrating system, fatigue crack, on-line measurment