The cyclic stable response in terms of true stress vs. strain for the DP980 steel sheet and welded joint is presented in
Figure 10a,b. These hysteresis loops were obtained towards the middle of the LCF life, where the cyclic stress relaxation was overpassed. Comparing
Figure 10a,b, the DP980 steel sheet exhibited a slender hysteresis loop for 0.006 and 0.008 strain amplitudes. Thus, the plastic strain was higher for the welded joint subjected to high strain amplitudes under cyclic stable response. In the case of strain amplitudes equal to 0.004 or smaller, both materials presented narrow hysteresis loops, with just a small difference between the DP980 and welded joint. The welded joint exhibited the slender hysteresis loops, which apparently indicated a smaller plastic strain than in the DP980 steel. However, the hysteresis loops correspond to the cyclic stable response of the materials and the welded joint could not present the smaller plastic strains.
The stable cyclic and tensile stress-strain curves for both materials (DP980 steel sheet and welded joints) are presented in
Figure 11a,b. Cyclic mechanical properties are summarized in
Table 4. Both materials presented a cyclic softening behavior with respect to the tensile one, as observed in the stress-strain curves by the experimental applied strain amplitudes. However, the softening effect severity is larger in the DP980 steel. The softening behavior for both materials can also be verified by comparison of cyclic and tensile yield strength. On average, for both materials, the cyclic yield strength reduced by 30% with respect to the tensile yield strength. Moreover, the cyclic stress-strain curves fitted to the Ramberg-Osgood equation indicated that for strain amplitudes larger than the applied experimentally, there is a mixed cyclic behavior with softening and hardening of both materials DP980 and welded joint. However, this mixed cyclic behavior must be verified by performing additional LCF tests with larger strain amplitudes.
The strain-life curves are presented in
Figure 12a for DP980 steel sheet and
Figure 12b for the welded joint. Total, elastic, and plastic strain amplitudes are presented with the Coffin-Manson relationship and the determined coefficients for the elastic and plastic equation terms. Overall, a larger fatigue life was presented in the DP980 steel sheet than in the welded joint. The welded joint exhibited low scatter, and in some cases, the number of cycles to failure was almost identical at the corresponding strain amplitude, such as for
εa = 0.006 where two samples presented the complete fracture at almost the same
Nf (~1900 cycles). Thus, the number of LFC test repetitions was regarded as adequate to analyze the fatigue response of the welded joint.
Figure 12a,b showed that for the applied strain amplitudes, the welded joint presented the larger plastic strains. This result was not clear from the previously presented hysteresis loops in
Figure 10a,b, but it can be explained based on the yield strength of the materials. The welded joint presented a lower yield strength, which can be associated with the tempered martensite in the LT-HAZ of the welded joint (
Figure 5d) and is a result of the weld thermal cycles. A lower yield strength resulted in the plastic strain arising early for the welded joint. The tempered martensite also induced a softening mechanism, which consists of the coarsening of martensite laths and a decrease of the dislocation density that reduces the movement of the dislocations. These phenomena have been previously reported in the literature [
27]. To further analyze the hardening and softening behavior of the DP980 steel sheet and welded joints,
Figure 13 presents the materials’ cycle response in terms of the variable stress amplitudes during the LCF tests. The response for the DP980 steel sheet (
Figure 13a) showed an initial hardening, which increased up to 760 MPa for
εa = 0.008, followed by a minor softening until the final failure fracture. This mixed behavior can be observed in fatigue specimens evaluated at
εa = 0.006 and
εa = 0.004. This behavior has been reported elsewhere [
21,
28]. Furthermore, the damage in dual-phase steels is typically divided by hardening of the ferrite phase (increasing the hardening rate with increasing the strain amplitude) due to the dislocation structure, which blocks the movement and forms barriers for plastic deformation. As the cyclic strain continues, substructures form due to dislocation rearrangement (such as veins and cells), leading to softening [
28,
29]. In the case of the welded joint (
Figure 13b), for
εa = 0.008 and
εa = 0.006 strain amplitudes, the welded joints show continuous cyclic softening. At lower strain amplitudes, the welded joint presented hardening followed by softening, without a definitive stable behavior. The hardening behavior is associated with lower cyclic stresses closer to the yield strength; thus, some hardening of the limited ferrite phase could be present. The diminution in the capacity of the weld joint hardening is attributed to the reduction of ferrite content. It is well known that the martensite phase is hard and brittle. For this reason, its capability to withstand cyclic strain is reduced compared with the ferrite phase. In fact, these results are similar to the obtained for quenched and tempered carbon steel [
30]. This can be explained by the formation of unblocked dislocations and the creation of a fatigue substructure, inducing a reduction of the internal stress, as suggested by Sankaran et al. [
30], who assessed a medium carbon microalloyed forging grade steel 38MnSiVS5 quenched and tempered steel under LCF conditions.
For completeness,
S-
N curves for the DP980 steel sheet and welded joint deduced from the LCF tests are presented in
Figure 14a,b.
Table 5 presented the fatigue properties for the DP980 steel sheet and welded joint. Failure of fatigue samples took place at the LT-HAZ (
Figure 14), since at this area, the tempered martensite requires the lowest stress to be deformed in the welded joint compared to the other martensite types located in the FZ and HT-HAZ. Therefore, the deformation distribution was not homogeneous, and it was concentrated at the position of the softened zone in the LT-HAZ (
Figure 15).