Correlation of the high and very high cycle fatigue response of ferrite based steels with strain rate-temperature conditions

Abstract

The discrepancies observed between conventional and ultrasonic fatigue testing are assessed through the mechanisms of dislocation mobility in BCC metals. The existence of a transition condition between thermally-activated and athermal regimes for screw dislocation mobility is studied under fatigue loading based on infrared thermography and microstructural characterization, here in the case of DP600 dualphase steel. Evidence is obtained regarding the microstructural sources of crack initiation, which is found to be consistent with the existence of a transition in the modes of deformation. From the analysis of the experimental data gathered in this work, guidelines are given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. The inevitable temperature increases under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a BCC structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue.

In Situ Determination of Phase Stress States in an Unstable Medium Manganese Duplex Steel Studied by High-Energy X-ray Diffraction

“…This result is consistent with the fact that PLC mechanisms are commonly observed in carbon-rich austenite at room temperature, as in FeMnC TWIP (Twinning-Induce Plasticity) steels [50][51][52]. On the contrary, PLC is reported in ferritic phases only at high temperatures (typically 300-400 • C) [53,54]. The observed instabilities seem also to evolve as a function of the deformation, switching from a A-type PLC to a B or C-type PLC (after a displacement of 2.5 mm, i.e., 10% of elongation).…”

The formation of discontinuous gradient regimes during crack initiation in high strength steels under very high cycle fatigue

“…The cyclic plastic deformation in the materials is highly localized in the VHCF regime or the elastic deformation regime [36]. This high strain localization leads to the accumulation of irreversible plastic deformation followed by small crack nuclei that initiates at the interface of inclusion and matrix, the inclusion, interior of grain, the phase or grain boundaries [37,38]. This indicates that the micro crack may form at first in some local regions before the formation of grain refinement during the cyclic loadings, which is supported by the observation of FGA formation surrounding the internal grain boundary for high strength steels in VHCF regime [39].…”

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

“…In VHCF regime, the high strain rate and the induced rise of temperature during tests could conduct to dynamic strain aging phenomena in sensitive steels. Indeed, Torabian et al showed that for a ferritic‐martensitic steel, strain aging, which resulted from the high temperature during ultrasonic tests, strengthens the surrounding ferrite matrix. Therefore, the plastic zone at the crack tip is smaller and requires higher stresses to propagate leading to an increase in the fatigue life for a given stress amplitude.…”