The effect of microstructures on fatigue crack growth in Q345 steel welded joint

Abstract

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.

Fatigue crack propagation for Q345qD bridge steel and its butt welds at low temperatures

“…Similarly, the concept of dislocation activation energy may also explain the increased fatigue crack propagation thresholds. There is no obvious increase in the crack propagation rate observed, as the stress ratio rises from 0.1 to 0.5, which is also similar to the data given by Xiong 23 and de Jesus, 41 who explained the possible weak crack closure effect in the base material at positive stress ratios may lead to the marginal effect of the stress ratio. 16 According to the results in Section 3 and the above discussions, the base material and weld metal exhibit distinctly opposite fatigue crack propagation behavior within the testing temperature range from the RT to −60°C.…”

“…Figure 9A to C shows the effect of stress ratio on the fatigue crack propagation rate in the base material at different temperatures. There is no obvious increase in the crack propagation rate observed, as the stress ratio rises from 0.1 to 0.5, which is also similar to the data given by Xiong 23 and de Jesus, 41 who explained the possible weak crack closure effect in the base material at positive stress ratios may lead to the marginal effect of the stress ratio.…”

“…Especially at the RT, the fatigue crack propagation rate at the stress ratio of R = 0.5 is approximately 3 times the propagation rate at stress ratios of R = 0.1 and 0.2. 23 As the stress ratio increases, the influence of crack closure effect becomes weaker. More specifically, Figure 6 demonstrates a more significant reduction in the fatigue resistance at stress ratios of R = 0.1 and 0.2 than that at R = 0.5 at low temperatures.…”

“…23 As the stress ratio increases, the influence of crack closure effect becomes weaker. 23 The different crack closure effects in the welded C(T) specimens explain the different propagation rates at stress ratios of R = 0.1 and 0.5, while the base material with weaker crack closure 41 presents no apparent differences in the fatigue resistance at these 2 stress ratios. Figure 10A to C compares the results of fatigue crack propagation rates in the base material and weld metal at stress ratios of R = 0.1 and 0.5 under different temperatures.…”

Effects of stress ratio on crack growth rate and fatigue strength for high cycle and very-high-cycle fatigue of metallic materials

“…Elber, 1970;Huang and Moan, 2007;Jones et al, 2008;Ostash et al, 2011;Baptista et al, 2012;Kim et al, 2012;Xiong and Hu, 2012;Zambrano et al, 2012). Among these, Elber (1970) introduced the concept of crack closure and used the effective stress intensity factor range DK eff instead of stress intensity factor range DK as the driving force for fatigue crack growth, namely da/dN = C(DK eff ) m , where DK eff = K max À K op , with K max being the maximum stress intensity factor and K op the value corresponding to crack opening stress.…”