Materials Science Forum Vols. 567-568

Abstract: In the work presented here an elastic-plastic crystalline finite element method is used to simulate the cyclic behavior of 316L austenitic stainless steel single crystals and polycrystal. The evolution of the back stress on each slip system is described using a non linear kinematics hardening law to account for the hardening induced by long range dislocation interactions. As the contribution of short range interactions is assumed to be negligible, the value of the friction stress is kept constant. Three dimensional finite element calculations are performed to simulate the cyclic stress strain curves in the case of a single crystal oriented for multiple slips, as well as for the case of the polycristal. Simulations are compared to experimental data. They seem to be satisfactory for low strain values (
εp\2 <10-3) whereas, for
εp\2 >10-3, they underestimate the hardening observed experimentally.

Abstract: High fatigue threshold values of duplex ferritic-martensitic steels are interpreted by using a unified model of roughness- and plasticity induced crack closure. Complex metallographical and fractographical analysis was performed in order to obtain characteristics of tortuous crack paths produced by crack deflection and branching mainly at austenite/ferrite interfaces. Calculated values of effective thresholds are in a good agreement with experimental data. The total level of extrinsic toughening (closure + shielding) induced by the duplex microstructure was determined to be as much as 70% of measured fatigue threshold values. This is the main reason for the high resistance to propagation of long fatigue cracks in the near-threshold region.

Abstract: This paper deals with an analysis of the possible causes of a transversal crack in a steel slab with a l300×l45 mm cross-section using results from two models. Samples were taken from and around the crack in order to analyze the concentration (i.e. chemical heterogeneity) of the relevant elements. Simultaneously, the concentration of elements at the surface of the crack was measured after the crack was opened. The chemical heterogeneity of elements was analyzed with the aid of the JEOL JXA 8600/KEVEX analytical equipment. The measurement results were processed using mathematical statistics procedures. The results proved that there was an internal crack initiating immediately below the curve of the solid-state temperature and consecutively propagating.

Abstract: An original three-dimensional (3D) model of solidification is used to describe the process of solidification and cooling of massive 500x1000x500 mm cast-iron castings in sand moulds. The calculated model of the kinetics of the temperature field of the casting is verified during casting with temperature measurements in selected points. The following dependences are later determined according to the experimental and calculated data: the average size of the graphite spheroids rg, graphite cells Rb and the average distances among the particles of graphite Lg – always as a function of the local solidification time θ [xi, yi, zi]. Furthermore, it has been found that the given basic characteristics of the structure of the cast-iron (rg, Rb and Lg) are a linear function of the logarithm of the local solidification time θ. The original spatial model of solidification can therefore be used in its first approximation for the assessment of the pouring structure of massive cast-iron castings.

Abstract: Most families of S-N curves determined at various temperatures present certain general regularities on whose basis the Basquin equation describing finite-life S-N curves can be generalized for various temperatures. This equation can be represented by straight lines with common slope if log-log fit for stress vs. temperature dependence is used. Deviations from these straight lines (anomalies) are evidence that additional degradation mechanisms are effective besides fatigue, whose temperature dependences differ from the mentioned temperature dependence of fatigue strength. In high-temperature region it is most often cyclic creep, in low-temperature region athermal processes of plastic deformation can play significant role in fatigue failure.

Abstract: Ordered alloys and intermetallides are important constructional materials, because they have a unique property – positive temperature dependence of yield stress. It is known that antiphase boundaries and washing effects of order, structures and concentrations of components near the boundaries are of great impotence in the appearance of positive temperature dependence. But superstructures have not only APB but also APB tubes, APB stripes. Their role is also important in physical and physics-mechanical properties of ordered alloys and intermetallides. The paper presents the classification of all the types of planar defects and their local properties for some alloys of different superstructure types.

Abstract: The development of the volume fraction of cumulated persistent slip bands (PSBs) in cyclically deformed nickel polycrystals was investigated in dependence on the number of cycles using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was shown that there is a large scatter of the volume fraction of PSBs from grain to grain. Three different tendencies for the development of the volume fraction with increasing number of cycles were distinguished. It was shown that there is a correlation of the orientation of the primary slip systems with the volume fraction of cumulated PSBs and the activation of PSBs during half-cycle deformation.

Abstract: Texture of a fatigue crack surface is strictly related to crack growth rate. Cracks in specimens from aluminum alloy were studied. Two types of information were used: SEM images of fracture surfaces, and 3D reconstructions of fracture surface morphologies. Sets of equidistantly focused images obtained by an optical microscope served as the basis for 3D reconstruction. Multiparametric fractal analysis was applied to characterize crack surfaces. A vector of fractal features represented each image or 3D reconstruction of selected locations of fracture surfaces. For estimating fractal characteristics, the box-counting method in 3D was used in all cases, [1]. Multilinear regression was used to express the relation between crack growth rate and feature vectors, with satisfactory results for both crack surface representations.

Abstract: A 3D time-harmonic problem for an infinite elastic matrix with an arbitrarily located interacting rigid disk-shaped inclusion and a penny-shaped crack is analyzed by the boundary integral equation method. Perfect bonding between the matrix and the moving inclusion is assumed. The crack faces are subjected to time-harmonic loading. The boundary integral equations (BIEs) obtained are solved numerically by the implementation of regularization and discretization procedures. Numerical calculations are carried out for a crack under tensile loading of constant amplitude, where an interacting inclusion is perpendicular to the crack and has the same radius. Both the normal crack-opening-displacement and the mode-I stress intensity factor are investigated for different wave numbers and distances between the crack and the inclusion.

Abstract: The evaluation of the generalized stress intensity factor (GSIF) and T-stress for the case of the surface crack terminating perpendicular to the interface between two orthotropic materials is considered. The combination of the discretization, numerical and analytic methods is used. The discretization method, such as common finite element method (FEM), is served to include the boundary condition to the GSIF solution and to describe the remote stress and displacement field region with the low influence of the singularity of the crack tip. The Lekhnickii-Eshelby-Stroh (LES) formalism is used to derive the approach solution for the near stress field of the crack tip and the singularity problem in an orthotropic 'trimaterial' using the Schwartz-Neumann's alternating technique. The problem of the stress singularity is treated as a non-linear eigenvalue problem, which leads to the characteristic equation for the stress singularities of the form rδ −1 , 0 <δ <1. Two ways of the evaluation of the GSIF are presented, using the reciprocal theorem ψ -integral) and the crack model by means of continuous distribution of dislocations. Both results are compared for a specific material. The continuous distribution of dislocations technique is also used for determination of the T-stress.