PSI - Issue 5

Mihaela Iordachescu et al. / Procedia Structural Integrity 5 (2017) 1304–1309 M, Iordachescu et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 4b consists of trans and intergranular decohesion, as detailed in Fig. 4c and, it results from the hydrogen embrittlement of the austenitic - ferritic interface and the anodic dissolution of the ferritic grains. The remaining area is the result of the ductile rupture of the resistant ligament by plastic collapse. The influence of hydrogen embrittlement on the damage tolerance of LDS steel was assessed regarding two plastic collapse models of cracked wires, which provide the upper bounds of damage tolerance and are in good agreement with the respective failure behavior of the ES and DSS wires (Iordachescu et al. 2014). The predictions of the models are given in Fig. 5 in terms of the quotient between the failure loads in the cracked and smooth condition (P m /P 0 ) versus the relative crack size area (A f /A 0 ). The plot also contains the LDS experimental data obtained from the fracture tests made with fatigue-precracked hydrogen free specimens and as well as from the FIP tests. Then, as resulting from Fig. 5 the theoretical model of plastic collapse in tension accurately approximates the damage tolerance of LDS wires, hydrogen free. Hydrogen uptake slightly decreases the damage tolerance of LDS wires. Heavily cold-drawn wires, of lean duplex steel fail by plastic collapse when hydrogen embrittled even containing fatigue cracks. A subcritical cracking process occurs until the applied load equalizes the plastic bearing capacity of the resistant ligament. Then, the wires of this type are highly damage tolerant materials. In the standard FIP test, the lifetime of this wires is not unlimited, but it exceeds that of cold-drawn eutectoid wires used for prestressing by more than 10 times. The hydrogen-induced damage changes the micromechanisms of failure from void coalescence to trans and intergranular decohesion. The environment-assisted damage progressively extends as a subcritical transversal crack from the wire surface until it reaches the critical size for the plastic tensile collapse of the resistant ligament. However, some previous cracking in axial direction seams to occur, so that collapse can take place in pure tension, with no bending contribution. This feature, which is quite similar to that found in cold-drawn, high alloyed duplex steel wires increases the damage tolerance up to its upper bound behavior.. 4. Conclusions

Acknowledgements

The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation through the project BIA 2014-53314 – R and the collaboration with INOXFIL S.A. who kindly provided the high-strength, duplex steel wires.

References

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