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Effects of pre-straining conditions on fatigue behavior of multiphase TRIP steels, The
Ly, Alexander L.
Ly, Alexander L.
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2015
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Ly_mines_0052N_10749.pdf
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Abstract
The combination of high strength and ductility in transformation-induced plasticity (TRIP) steels makes them promising materials for automotive applications. The steel may be stamped into sheets and serve as parts of the auto-body where low-cycle fatigue (LCF) loading conditions are prevalent. Currently, TRIP steels are not used in these fatigue-susceptible parts. Before employing the TRIP steels in such areas, their fatigue behavior must first be understood. To assess the fatigue properties of TRIP steels, a multiphase TRIP780 steel (0.2C-1.7Mn-1.7Si-0.04Al in wt pct) containing 17.3 vol pct initial retained austenite was used as the subject of study. In this investigation, TRIP steel specimens were pre-strained to 0, 5, or 15% at temperatures of -20, 20, or 80 °C. They were subsequently subjected to fully-reversed fatigue cycling at total strain amplitudes of 0.2, 0.3, 0.4, 0.5, or 0.6%. Fatigue tests continued until fracture, were stopped at 106 cycles, or were interrupted after 100 cycles. The LCF life of the TRIP780 steel was found to be relatively independent of prior strain history. However, there were slight differences in the steels pre-strained to 15%. The large loss in ductility due to pre-straining to 15% resulted in a decrease in LCF life with respect to the applied plastic strain amplitude. Decreasing the pre-strain temperature (in the range of -20 to 80 °C) did not significantly reduce fatigue life. X-ray diffraction (XRD) measurements were conducted to estimate retained austenite content after pre-straining (before fatigue testing) and after fatigue testing; the measurements were correlated to fatigue performance. Increasing level of pre-strain and decreasing pre-strain temperature resulted in lower retained austenite content or greater amounts of martensitic transformation. Although martensite content increased the strength of the steel, it did not greatly affect fatigue life; martensitic transformation had a smaller effect on fatigue life than the level of pre-strain, as cyclic plastic damage was likely concentrated in the ferrite. Very little transformation occurred during fatigue cycling. This lack of transformation could be attributed to heating during cycling (increasing austenite stability) and the small magnitudes of strain per cycle being insufficient to activate the deformation-induced transformation.
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