Abstract: A new model is presented for characterizing the propagation of long fatigue crack based on the analysis of test data of LZ50 carbon steel. The cracking threshold, fracture roughness and cyclic ratio are incorporated in the model. The behaviour of fatigue crack propagation nearby the cracking threshold is well reflected. And in the high stress intensity factor range the growth rate growing up to the critical fracturing point is well also revealed. The present model is derived from a comparing analysis of the existent models, I.e. Paris-Erdogan equation, Forman-Kearney-Engle model, and Elber equation. Paris-Erdogan equation is invalid to the regimes of nearby the cracking threshold and the stress intensity factor range. Forman-Kearney-Engle model is also invalid to the regime nearby the cracking threshold. And the Elber equation has a big deviation to the critical fracturing point in the high stress intensity factor range. All of the disadvantages in the existent models have been well overcome by the present model.

Key words: Cyclic ratio, Fracture roughness, Growth model, Long fatigue crack, Threshold