PSI - Issue 42

Mihaela Iordachescu et al. / Procedia Structural Integrity 42 (2022) 602–607 Mihaela Iordachescu et al. / Structural Integrity Procedia 00 (2019) 000 – 000

607

6

According to Fig. 4c, the subcritical cracking in the FIP environment is due to the failure of the lath martensite packages formed within the limits of the previous austenitic grains, and gives rise to a quasi-brittle fracture morphology (Fig. 4b) similar to the intergranular cleavage of polycrystalline materials. The sinuosity of the cross sectional profile along the rupture is due to the decohesion of the lath martensite packages because of hydrogen concentration at their borders. Fig. 4e captures the profile of the critical crack extension in air, after heat tinting, that occurs by the transversal breakage of the martensitic blocks deformed in excess ahead the crack front. The successive concatenation of these ruptures produces the cracking advance with micro-void formation and small oscillations around the growth direction. Fig 4d shows the fracture surface corresponding to the crack profile of Fig. 4e, in which the presence of micro-voids explains the moderately ductile morphology of critical cracking, comparable to the transgranular pseudo-cleavage of polycrystalline materials. The voids, nucleated at the borders of inclusions and precipitates contribute to the steel ductility (De Abreu et al., 2018, and Iordachescu and Perez et al., 2018). This failure micro mechanism converts the martensitic blocks in the weakest resistant link of the microstructure. 4. Conclusions Hydrogen-assisted cracking in high-strength steel structural bars of lath martensite microstructure is due to specific micro-mechanisms activated by the environmental aggressive action. Hence, this cracking process is different from those favored by a lower activation threshold resulting from environmental action. These micro mechanisms have been identified from interrupted slow strain rate tensile tests in the FIP environment, followed by the dehydrogenation of the specimens by thermal tinting and concluded with their final breaking in simple tension. The specific micro-mechanisms of hydrogen-assisted cracking lie in the weakening of the cohesion between the lath martensite packages, whereas the micro-mechanisms of fracture from a pre-existing crack consist of the breaking of the lath martensite blocks. This entails the environmental action shifting the failure origin to the more basic microstructural grouping level of the analyzed steel. Acknowledgements The authors gratefully acknowledge the grant RTI-2018-097221-B-I00 funded by MCIN/AEI/10.13039/ 501100011033 and by “ERDF A Way of Making Europe”, the grant PRE -2019-088263 funded by MCIN/AEI/ 10.13039/501100011033 and by “ESF Investing in Your Future” and the grant UP2021-035 funded by MCIN/AEI/ 10.13039/501100011033 and by “Next Generation EU/PRTR”. References E722: Standard Specification for Uncoated-High Strength Steel Bars for Prestressing Concrete, ASTM 2003. De Abreu M., Iordachescu M., Valiente A., 2018. On hydrogen induced damage in cold drawn lean-duplex wires. Eng Fail Anal 91, 516-526. Iordachescu M., Valiente A., Pérez-Guerrero M., Elices M., 2018. Environment-assisted failure of structural tendons for construction and building applications. Constr Build Mat 159, 499-507. Iordachescu M., Pérez-Guerrero M., Valiente A., Elices M., 2018. Environmental effects on large diameter high-strength rods for structural applications. Eng Fail Anal 83, 230-238. Morris J. W., 2011. On the Ductile-Brittle Transition in Lath Martensitic Steel, ISIJ International 51(10), 1569 – 1575. Morris J. W., Kinney C., Pytlewski K., Adachi Y., 2013. Microstructure and cleavage in lath martensitic steels. Sci. Technol. Adv. Mater. 14 014208, doi:10.1088/1468-6996/14/1/014208. Iordachescu D., Blasco M., Lopez R., Cuesta A., Iordachescu M., Ocaña J.L., 2011. Recent achievements and trends in laser welding of thin plates. J. Optoelectron Adv M, 13(8), 981-985. prEN10138-4: Prestressing steels-Part 4: Bars, CEN 2005. UNE-EN ISO 15630-3 Steel for the reinforcement and prestressing of concrete - Test methods - Part 3: Prestressing steel, ISO, 2019. Valiente A., Pérez-Guerrero M., Iordachescu M., 2016. New testing method for assessing the cracking sensitivity of stressed tendon rods in aggressive environments. Eng Fail Anal 68, 244-253.