An axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice is used to study deformation mechanisms of ferrite-pearlite structural steels. Microstructural characteristics of the steels were parameterized by the pearlite volume fraction, the aspect ratio of the pearlite particles, and the neighboring factor, which represents the ratio of interparticle spacing in the longitudinal direction to that in the transverse direction. FE analyses were carried out to investigate the macroscopic and microscopic response of unit cells with morphological features based on idealizations of the microstructures of the actual steels. Tensile properties of each constituent phase were obtained experimentally and used in the analyses. As compared to traditional axisymmetric models, the BCC cell model appears to be able to capture more realistically the behavior of the materials, and it accurately estimates the tensile behavior of the ferrite-pearlite steels even with a relatively large volume fraction of the pearlite phase. The effects of volume fraction and morphology of the second-phase particles on deformation behavior were also investigated.KEY WORDS: structural steel; ferrite-pearlite steel; plastic deformation; unit cell model; FEM analysis.and two-phase materials with spheroidal inhomogeneities. The distribution of voids or second-phase particles is idealized by an array of hexagonal cylinders, each containing a spherical void or second-phase particle. This model is quite accurate if the volume fraction of voids or second-phase particles is small, or when there is relatively small contrast in deformation resistance between the matrix and particle phases, however, it does not provide an adequate prediction for the macroscopic material behavior when the interaction between adjacent voids or second-phase particles is not negligible. Recently, Socrate and Boyce 13) proposed a simplified axisymmetric finite element unit cell model to investigate macroscopic and microscopic stress-strain behavior of toughened polycarbonate. This axisymmetric unit cell model (V-BCC model) is based on a Voronoi tessellation of the BCC lattice, which better represents the spatial distribution of voids or second-phase particles, and can accommodate a local deformation mode consisting of shear localization of matrix materials between voids. Socrate and Boyce investigated porous materials, but this new micromechanical model can be valuable to investigate many cases of twophase materials.In this study, the axisymmetric Voronoi cell model was used to estimate the deformation behavior of two-phase structural steels, and to investigate the effects of volume fraction and the morphology of the second phase on macroscopic and microscopic response. The applicability of this new micromechanical model was verified by comparisons with experimental results. Figure 1 shows microphotographs of the steels used for the test. Ferrite-pearlite structural steels were selected for an investigation of the ability of the microme...

The effect of the specimen geometry and the inclusion content on the critical condition for ductile crack initiation was determined using notched round bar tensile specimens, and the critical void volume fraction and secondary void nucleation effect were investigated using the Gurson-Tvergaard constitutive model. It is shown that the secondary void nucleation from pearlite nodules plays a dominant role in ductile crack initiation, and void nucleation and void growth behavior are strongly affected by the stress triaxiality and the plastic strain. In the large plastic strain and low stress triaxiality region, a large number of secondary voids were nucleated in the early stage of deformation and void volume fraction grew to a critical value in spite of the relatively low void growth rate at low stress triaxiality. On the other hand, in the low plastic strain and high triaxial stress region, void volume can grow rapidly even though the volume of secondary nucleated void was smaller than that of high plastic strain region. Critical void volume fraction decreases largely with increasing in MnS content in the high triaxial stress region; however, the effect of MnS content is small in the low stress triaxiality and large plastic strain region.

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The heavy-gaugehigh strength steel plates with low yield ratio (YR)

In the present study, a series of fatigue crack growth tests were carried out in order to examine the effects of stress ratio R upon crack growth rates, together with the crack closure behaviors. Fatigue tests were conducted with center-notched specimens of two kinds of pressure vessel steels (500 MPa class and 800 MPa class) under cyclic axial loading in various stress ratios R ranging from −5 to +0.8. Crack opening stress levels were determined by the unloading elastic compliance method. An expression of fatigue crack growth rates under a wide range of stress ratios was proposed, taking into account the relationship between stress ratio R and crack opening stress ratio U. The crack growth behaviors near the threshold conditions were also discussed.