This paper attempts to predict how the microstructural features and mechanical properties of the individual constituents affect the deformation behavior and formability of ferrite-pearlite steels under quasi-static loading at room temperature. For this purpose, finite element simulations using representative volume elements (RVEs) based on the real microstructures were implemented to model the flow behavior of the ferrite-pearlite steels with various microstructural morphologies (non-banded and banded). The homogenized flow curves obtained from the RVEs subjected to periodic boundary conditions together with displacement boundary conditions were validated with the experimental results of the uniaxial tensile tests. Then, the initial microstructural inhomogeneity and Johnson-Cook damage criteria were employed for both non-banded and banded RVEs to estimate the onset of plastic instability under different loading paths ranging from uniaxial tension to equi-biaxial tension. Finally, the forming limit diagrams of both ferritic-pearlitic microstructures were predicted, which show a good agreement with the experimental results of the Nakazima stretch-forming tests (less than 13 pct error). It implies that the initial microstructural inhomogeneity criterion adequately enables to predict the plastic instability in the ferritic-pearlitic steel sheets without using any damage or failure criterion. The most commonly observed damage mechanism is the severe plastic deformation of the ferrite grains near the pearlite colonies due to the strength contrast between ferrite and pearlite. Another significant finding is that the microstructural morphology has a crucial influence on the strain partitioning, strain localization, and formability of the ferritic-pearlitic steels.
In this paper, the dynamic response of functionally gradient steel (FGS) composite cylindrical panels in steady-state thermal environments subjected to impulsive loads is investigated for the first time. FGSs composed of graded ferritic and austenitic regions together with bainite and martensite intermediate layers are analyzed. Thermo-mechanical material properties of FGS composites are predicted according to the microhardness profile of FGS composites and approximated with appropriate functions. Based on the three-dimensional theory of thermo-elasticity, the governing equations of motionare derived in spatial and time domains. These equations are solved using the hybrid Fourier series expansion-Galerkin finite element method-Newmark approach for simply supported boundary conditions. The present solution is then applied to the thermo-elastic dynamic analysis of cylindrical panels with three different arrangements of material compositions of FGSs including αβγMγ, αβγβα and γβαβγ composites. Benchmark results on the displacement and stress time-histories of FGS cylindrical panels in thermal environments under various pulse loads are presented and discussed in detail. Due to the absence of similar results in the specialized literature, this paper is likely to fill a gap in the state of the art of this problem, and provide pertinent results that are instrumental in the design of FGS structures under time-dependent mechanical loadings.
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