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Lecture

Fatigue cracks analysis in high-strength bainitic steel.

Thursday (27.09.2018)
09:30 - 09:45 S1/01 - A02
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Nowadays, the most modern bainitic steels are designed with much reduced carbon and other alloying element concentrations exhibiting excellent mechanical properties and are widely applied in the automobile industry as crash reinforcement bars to protect against sidewise impact and for injection lines (under pulsating loads) in common rail diesel engines [Ca09]. Although most applications of high-strength bainitic steels as structural materials are exposed to dynamic cyclic loading, only a limited amount of research has been reported concerning the fatigue induced damage in these types of steels. According to some authors [Zh15, Bh12, So14], fatigue microcracks nucleate at the interface between the bainitic ferrite and the retained austenite regardless of the strain amplitude imposed. Other authors [Zh16, Br12, Re15, Ma17] reported that microcracks initiate along slip bands and show transgranular propagation and eventually form a coalescence of short cracks. In order to understand the behavior of microcracks, it is proposed a gradual monitoring of them during low cycle fatigue at different plastic strain amplitudes by means of optical and electron microscopy along with EBSD data analysis.

The study was carried out on the bainitic steel 16CrMnV7-7 with 5 Vol. % of retained austenite. With the aim of study the initiation microcracks, low cycle fatigue tests on shallow notched cylindrical specimens were performed at room temperature under constant plastic strain amplitude. The central part of the notch was monitored and digitally recorded during the fatigue test. In order to correlate slip systems with the initiation and propagation of microcraks, Schmid and Taylor factors were analyzed in the bainitic blocks. Additionally, the dislocation structure developed during cycling was also analyzed. From main results, it is highlighted that there are two modes of initiation of microcracks according to the plastic strain applied. At low plastic strain amplitude the microcracks preferably initiate along slip systems {110} <111> with high Schmid factor and low Taylor factor, while at high plastic strain amplitude the microcracks mostly are present on high-angle block boundaries.

 

Speaker:
Prof. Dr. María Cecilia Marinelli
Universidad Nacional de Rosario - UNR
Additional Authors:
  • Dr. Marcela Balbi
    Universidad Nacional de Rosario (UNR) / The National Scientific and Technical Research Council CONICET
  • Prof. Dr. Ulrich Krupp
    Osnabrück University of Applied Sciences