4.1. S–N Curves with Confidence Level
Based on fatigue tests at −60 °C, the test data of T- and cruciform welded joints were obtained. Then, the logarithmic S–N curve was confirmed based on data processing in
Section 3.4, as shown in
Table 5.
The logarithmic S–N curves of the two welded joints at the 50% and 95% confidence levels are shown in
Figure 10a,b.
At the logarithmic stress range of 2.50 (316 MPa), it could be observed that the logarithmic fatigue life of the different welded joints was different under the same logarithmic stress range. For example, under the logarithmic stress range of 2.50 (316 MPa), the logarithmic fatigue life at the 50% confidence level of the T-welded joints () was 5.632 (428,548 cycles), whereas that of the cruciform welded joints () was 5.623 (419,759 cycles). Under the logarithmic stress range of 2.50 (316 MPa), the logarithmic fatigue life at the 95% confidence level of the T-welded joints () was 5.587 (386,366 cycles), whereas that of the cruciform welded joints () was 5.557 (360,579 cycles). In summary, it could be observed that the fatigue life of the T-welded joints at the 50% and 90% confidence level was longer than that of the cruciform welded joints.
4.2. Structural Perspective
The fatigue properties of welded joints and the stress concentration factor, microstructure, and Vickers hardness have a close relationship. In this study, the fatigue properties of two types of welded joints at low temperature were analyzed from structural and material perspectives, and fractography.
Stress concentration phenomena are common in various engineering structures, and most of the structural damage is caused by stress concentration. Based on ABAQUS, finite element simulations of T-welded and cruciform welded joints (idealized model) were carried out to investigate the stress concentration factor (SCF) of the two welded joints (
Figure 11). Modeling was performed using a three-dimensional solid C3D8R reduced integration element, and mesh refinement was performed at the weld toe of two welded joints. The low temperature material properties were, elastic modulus 214 GPa, yield strength 434 MPa, Poisson’s ratio 0.281, and density 7.85 g/cm
3 [
39]. One end of the model was rigidly fixed and the other end was applied with a tensile force of 150 kN.
The SCF can be obtained using Equation (7):
where,
is the nominal stress (the data of and were obtained from the tests in the current study).
Then the SCF of two welded joints were calculated and put in one table, as shown in
Table 6. For the FEM, the hot-spot stress of cruciform welded joints was about 28.334 MPa higher than that of T-welded joints under the same load (150 kN). For tests at −60 °C, the hot-spot stress of cruciform welded joints was about 34.884 MPa higher than that of T-welded joints under the same load (150 kN). Both the FEM and the tests reflected that the SCF of cruciform welded joints is higher than that of T-welded joints. In addition, the error between the hot-spot stress obtained by FEM and the tests was less than 5%, which may have been caused by the measurement error in tests. Besides, the good consistency between the hot-spot stress obtained by FEM and the tests also verifies the reliability of the hot-spot stress obtained from the tests at −60 °C.
4.3. The Relationship between Fatigue Properties and Hardness Profiles
The difference in fatigue properties of T-welded and cruciform welded joints can also be analyzed from a material perspective [
40]. Research has shown that fatigue properties of a material are related to hardness [
18,
41,
42,
43]. In general, the fatigue properties increase with increasing hardness. In this study, the relationship between fatigue properties and hardness of welded Dh36 steel at −60 °C is discussed.
For the measurement at −60 °C, the specimen was placed in a low environment chamber and incubated at −60 °C for 1 h. Then, the hardness of the specimen was measured using a Vickers hardness tester. The surface temperature of the specimen was monitored throughout the measurement process. If the temperature changed by more than 5 °C, the measurement was invalid. The positions of the measurement points were determined by referring to “GB/T 2654-2008 Hardness Test Method on Welded Joints” [
44]. The distribution of the measurement points in this study is shown in
Figure 12.
Since the failure zone was located at the HAZ in this paper, the Vickers hardness in the HAZ of the T-welded and cruciform welded joints is mainly discussed here. The Vickers hardness profiles of the T-welded and cruciform welded joints at −60 °C are presented in
Figure 13. It can be seen that the Vickers hardness in the HAZ of the T-welded joints is higher (178 HV) than that of the cruciform welded joints (163.8 HV).
In general, Vickers hardness and fatigue properties show good consistency, that is, without considering inclusions and defects, plastic deformation preferentially occurs at the position with low Vickers hardness and high stress, and the specimen eventually fails in this zone. In this study, the Vickers hardness of the T-welded joints was determined to be approximately 14.2 HV higher than that of the cruciform welded joints in the failure zone. Meantime, the stress range level of T-welded joints in the failure zone was lower than that of cruciform welded joints, as shown in
Figure 14. Therefore, the difference in fatigue properties between T-welded and cruciform welded joints can be attributed to the difference in hardness and stress range level of the failure zone in this study.
4.4. Fractography
First, the fracture morphology was examined using a magnifying glass, and an obvious crack initiation zone, crack propagation zone, and instantaneous fracture zone [
14] were observed. In
Figure 15a,b, I is the crack initiation zone, II is the crack propagation zone, and the size of the crack propagation zone is related to the material toughness and the load level. III is the instantaneous fracture zone, which is the fresh section formed by the instantaneous fracture after the crack expands to a certain size [
45]. Most cracks are generated in zones with defects or high stress [
46]. The crack propagation zone was smooth and flat and appeared as a “beach mark” in the zone. The instantaneous fracture zone was rough and white.
For the fracture of the T-welded joints, stress concentration was present in one side, and there was no stress concentration in the other side; therefore, the crack was generated in the side (I) with stress concentration and expanded downward in the form of an ellipse until it was broken, as shown in
Figure 15a. For the fracture of the cruciform welded joints, since there was a strong stress concentration in both sides, the crack was preferentially generated on the side with defects. As shown in
Figure 15b, cracks were generated in one side (I) and then expanded in the form of an ellipse until failure.
The fractography of T-welded and cruciform welded joints at −60 °C was also analyzed using SEM images. The morphology of the crack initiation zone, the crack propagation zone, and the instantaneous fracture zone clearly differed [
47]. Some radiation stripes were observed in the crack initiation zone, as shown in zone I of
Figure 15a,b. These stripes are steps formed by many tear planes that were parallel to each other but at different heights. In addition, the crack propagation zone contained obvious transgranular fatigue cracks (zone II of
Figure 15a,b). The instantaneous fracture zone shows signs of ductile fracture (zone III of
Figure 15a,b) [
48].
(I) Crack initiation zone
The crack initiation zone consisted of a relatively large planar defect, and a number of microcracks were observed in the crack initiation zone [
49]. These cracks were generated at the surface with stress concentration. There were more fatigue sources at the crack initiation zones of cruciform welded joints than those of T-welded joints, and the stress concentration factors of cruciform welded joints were higher than those of T-welded joints, as shown in zone I of
Figure 15a,b. This phenomenon is consistent with the conclusion of
Section 4.1.
(II) Crack propagation zone
At least 10 fatigue striation spacings were taken as the population from the crack propagation zone at and the mean of the striation spacing was obtained by averaging the population. The results showed that the striation spacings of cruciform weld joints were 0.84 μm, while the striation spacings of T-welded joints were 0.79 μm.
In the SEM images of the crack propagation zone, clear fatigue striations were observed in the crack propagation zone of the two welded joints, as shown in zone II of
Figure 15a,b. According to the “plastic passivation model” proposed by Laird [
46], the spacing of the fatigue striations represents the crack propagation of the stress cycle, which can reflect the fatigue crack growth rate
[
50]. From the zone II of
Figure 15a,b, the spacing of fatigue striations of T- welded joints was less than that of cruciform welded joints. This means that the crack propagation rate of the T-welded joints was lower than that of the cruciform welded joints.
(III) Instantaneous fracture zone
The instantaneous fracture zone was in the stage of crack instability. The instantaneous fracture zones of the two welded joints contained obvious dimples. However, the dimples of the cruciform welded joints were significantly smaller, as observed in
Figure 15a,b. Combined with
Figure 3c and
Figure 4c, it can be observed that more lath-like martensite and less residual ferrite were observed in the HAZ of the T-welded joints than in the HAZ of the cruciform welded joints. The presence of lath-like martensite reduces the toughness of the material and makes the dimples shallow and small.
4.5. Effect of Low Temperature on Fatigue Properties of DH36 Welded Joints
Low temperature has a great influence on fatigue [
51]. Here is an example of cruciform welded joints to discuss the effect of low temperature on the fatigue properties of welded DH36. The fatigue test results of cruciform welded joints of DH36 steel studied by Ph. P. Darcis [
47] were cited and compared with the results at −60 °C in this paper, as shown in
Figure 16.
At the same logarithmic stress range, the logarithmic fatigue life of cruciform welded joints at room temperature (RT) and −60 °C were different. For example, at the logarithmic stress range of 2.50 (316 MPa), the logarithmic fatigue life () of cruciform welded joints at −60 °C was 5.55 (354,813 cycles), while the logarithmic fatigue life of cruciform welded joints () at RT was 5.181 (151,808 cycles) which was almost three times that of room temperature. This means the fatigue properties of cruciform welded joints at −60 °C were better than at room temperature.
The effect of low temperature on fatigue properties can still be analyzed from the aspect of Vickers hardness. The Vickers hardness of cruciform welded joints at room temperature and −60 °C was put in one figure, as shown in
Figure 17. It could be found that Vickers hardness profiles at −60 °C was higher than that of room temperature. In particular, the Vickers hardness at −60 °C was 12.17 HV higher than that of room temperature in the failure zone.
The low temperature raises the Vickers hardness of the welded joints, and the increase of hardness enhances the fatigue properties of the welded joints of DH36 steel. In this paper, the fatigue properties at low temperature were investigated from the aspect of hardness, and a more comprehensive and detailed study on the mechanism of fatigue at low temperature still needs to be carried out.