A B S T R A C TIt is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R-ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R-ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R-ratio increasing. To the low R-ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R-ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse-grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack-tip stress shielding and then resulted in higher crack growth resistance.a 0 = distance between the notch root and the line of action of the externally applied load a f = crack length measured from the notch root at the termination of test f = loading frequency K max , P max = maximum stress intensity factor, maximum load K min , P min = minimum stress intensity factor, minimum load K op , P op = opening stress intensity factor, opening load K s = stress intensity factor corresponding to the stress shielding effect K eff = effective stress intensity factor range K = applied stress intensity range K tip = stress intensity factor range experienced by the crack tip K eff,tip = crack-tip effective stress intensity factor range Correspondence: Y. Xiong.

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