PSI - Issue 17

Maricely De Abreu et al. / Procedia Structural Integrity 17 (2019) 618–623 M. De Abreu et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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its boundary of 200 MPa fatigue limit is closer to the static collapse locus. This lower sensitivity is consistent with the differences found in the damage micromechanisms that cause the fatigue failure. The origin of the differences comes from the microstructures of the two wires steels, despite of their similitude resulting from the large axial grain elongation provided by the cold drawing process. Even in this strongly oriented microstructure the higher ductility of the austenitic of LDS is present when compared to the pearlitic matrix of ES. This microstructure feature of LDS also explains its higher damage tolerance (Valiente A. Et al., 2005, Iordachescu M. et al. 2015) and lower susceptibility to hydrogen embrittlement in aggressive environments (De Abreu M.et al., 2018) than that of ES.

Fig. 3. a) Sketch of the fatigue fracture of the wires under transverse loading; b, c) Macroscopic images of LDS wires broken in the F-QL test; d) Fatigue fracture features of LDS wire; e) Higher magnification detail showing the secondary cracking at austenite – ferrite interphase of LDS in the fatigue crack propagation stage; f, g) Macroscopic images of ES wires broken in the F-QL test; h) Fatigue fracture features of ES wire; i) Higher magnification detail of ES in the fatigue crack propagation zone. Fig. 3 presents the sketch of the macroscopic failure mode of the wires under F-QL testing (Fig. 3a) together with some representative macro and micro fractographic images of broken LDS and ES wire specimens with fatigue lives less than 2∙10 6 cycles. Figs. 2b and 2c reveal that fatigue cracking of LDS wire propagates in a plane inclined with respect to the axis of the wire, as schematically shown in Fig. 2a. The initiation of the cracking process takes place at one end of the notch generated by plastic deformation in the contact area between the tested wire and the actuator wire. The transition from the contact area to the traction free surface and the geometrical effect of the notch are superposed stress concentrators that propitiate the crack initiation. The angle formed by the fatigue cracking plane and the wire axis is approximately 40°, very similar to that found in the case of the static fracture test under transverse loading of LDS (Iordachescu M. et al., 2018). This contrasts with that of 90° which determines the wires cracking when subjected to cyclic tensile loading without transverse load. The image of Fig. 3d shows the fatigue crack features of LDS, consisting of three well-defined stages: the initiation, the inclined propagation, and the shear final failure by overloading. The transition from the initiation to propagation stages mainly contains dimples developed by plastic deformation, which differ in density and size.