Materials Science Forum Vols. 539-543

Abstract: The solidification models basing on the Fourier equation with the additional term controlling the kinetics of phase transition are discussed. The different approaches to this term definition lead to the different solidification models, in particular the macro and the macro/micro ones can be taken into account. In the case of macro description the equation in which the parameter called a substitute thermal capacity is considered, while in the case of macro/micro approach the linear or exponential models of crystallization can be introduced. The solution of the problem can be found using the numerical methods. In this paper the boundary element method using discretization in time is applied, the examples of numerical simulations are also shown.

Abstract: Recent studies indicate that the austenite(γ)-to-ferrite(α) transformation kinetics in low alloyed steels is solely controlled by the intrinsic mobility of the interface at least in the initial stages of ferrite growth. Then, diffusion processes in the interface significantly retard ferrite growth, so that bulk diffusion of the fast diffusing interstitial component carbon becomes relevant. Two series of dilatometer tests, one from a low to ultra-low carbon steel [1] (alloy A) and the other from an Fe-Mn steel [2] (alloy B), are considered. In case of alloy A the first stage of the transformation kinetics is apparently controlled by the intrinsic interface mobility, whereas in the second stage carbon diffusion in the interface and in the bulk material comes into play. The transition region can be modeled by an effective mobility, which depends on the interface velocity. In the second stage the interface mobility depends on the temperature only. In case of alloy B a hierarchical model allows for a direct estimation of the intrinsic mobility. The numerical results indicate that the interface mobility also changes from an intrinsic mobility at the initial stage of the transformation to an effective mobility due to solute drag during the transformation process.

Abstract: It has frequently been observed that the ductility or elongation of a material can significantly be affected by its grain size. Poor ductility with low elongation has been found in ultrafine-grained (UFG) steels when their grain size is smaller than a critical value. The so called instability of plasticity is a well known drawback for UFG steels and has greatly prevented them from wider application. Although the instability has been attributed to the lack of strain hardening capacity of ultrafine-grained materials, its mechanism is still unclear. In this paper, a newly developed model for predicting the dependence of uniform elongation on grain size is described, using which the instability of plasticity in UFG steels can be explained. The model is based on a strain hardening model for polycrystalline metals that was previously developed by the present author and was modified in the present study to take the grain size effect into account. The uniform elongation calculated using the model is in good agreement with that experimentally measured in two steels. It was found that the elongation begins to decrease rapidly when the grain size is reduced to below 2μm - a clear sign of the occurrence of the instability of plasticity.

Abstract: A physically-based multi-scale model for martensitic transformation induced plasticity is presented. At the fine scale, a model for one transforming martensitic variant is established based on the concept of a lamellae composed of a martensitic plate and an austenitic layer. Next, the behaviour of 24 potentially transforming variants is homogenized towards the behaviour of an austenitic grain. As a simple example, the model is applied to deformation and transformation of a single austenitic grain under different deformation modes.

Abstract: The apparent shear strength of rock discontinuities is lower than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities. Numerical algorithms permit to calculate contact forces at any point, and to describe the stick-slip transition. On the other hand, the critical aspects are not captured by classical theories. Multiscale simulations show that the contact domain between rough surfaces is a lacunar set. This explains the size-dependence of the apparent friction coefficient. By applying an increasing tangential force, the regime of partial-slip comes into play. However, the continuous and smooth transition to fullsliding predicted by the Cattaneo-Mindlin theory is not occurring in real situations. We implement a numerical renormalization group technique, taking into account the redistribution of stress consequent to partial-slip. This permits the critical value of the tangential force to be found. The critical force is less than the one predicted by Coulomb’s theory, and depends on the specimen size and on the topology of the interface.

Abstract: For semicrystalline materials, a stacked lamellar morphology gives rise to a strongly anisotropic mechanical response. A multiscale numerical model is used to simulate the effect of a stacked lamellar microstructure on the macroscopic behaviour. The constitutive properties of the material are identified separately for the crystallographic and amorphous domains. The averaged fields of aggregates of individual phases, having different preferential orientations are determined. The anisotropy of preferentially oriented material is investigated in different deformation modes.

Abstract: The paper presents a nonlocal elastic damage-frictional interface model. The reason to introduce nonlocal mechanical features inside the constitutive relations is justified by the fact that there are several circumstances, in which the interface displays inside an extended process zone with microstructural spatial interactions. Typically, spatial bridging mechanical effects can be effectively modeled by integral (strongly nonlocal) stress-strain relations. The paper develops an elastic nonlocal model with local isotropic damage and the relations are constructed following a thermodynamical consistent approach.

Abstract: This paper deals with the problem of decohesion in composite structures containing interfaces of similar/dissimilar materials. The analysis of the tractions transmitted between opposite faces into contact as well as of the initiation and propagation of delamination zones is studied with reference to plane structures with adhesive joints under shear stresses. A simplified monodimensional structural model is formulated and applied to develop basic understanding of the standard adhesion test. The analytical results obtained are used to optimize the performance of single-lap adhesive joints in practical applications.