3.3. FCG behavior
The FCG rate of BM, AS-HAZ and WNZ are shown in Fig. 11. Generally, the curve of FCG consists of three regions, namely, slow crack growth region (I) in low stress intensity factor (ΔK) range, stable propagation region or Paris regime (II) in medium level range of ΔK and rapid propagation region (III) in high range of ΔK. In present work, the investigated range of ΔK was about 7 to 30 MPa m1/2. From 7 to 20 MPa m1/2, all samples showed approximate linear dependency of ΔK on the FCG rate, indicated characteristic of Paris regime (II). In this region, BM sample presented a consistently higher FCG rate as compared with AS-HAZ and WNZ. The FCG rate of AS-HAZ rose faster than WNZ with increasing of ΔK. When ΔK exceeded 20 MPa m1/2, a noticeable change in the slope of the curve was observed, indicating that crack growth was entering into the rapid propagation region (III). In this region, the FCG rate of BM and AS-HAZ was close, much higher than that of WNZ.