Determination of Mechanical and Fracture Properties of Silicon Single Crystal from Indentation Experiments and Finite Element Modelling

The paper aims to use experimental micro-indentation data, FE simulations with cohesive zone modelling, and an optimisation procedure to determine the cohesive energy density of silicon single crystals. While previous studies available in the literature, which use cohesive zone finite element techniques for simulation of indentation cracks in brittle solids, tried to improve methods for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, this study focuses on the evaluation of the cohesive energy density 2 from which the material toughness can be easily determined using the well-known Griffith-Irwin formula. There is no need to control the premise of the linear fracture mechanics that the cohesive zone is much shorter than the crack length. Hence, the developed approach is suitable also for short cracks for which the linear fracture mechanics premise is violate

Influence of the cell geometry on the tensile strength of open-cell ceramic foams

Nowadays used open cell foam ceramic materials are mostly of irregular structure which means that the shape of particular foam cells does not exhibit any regular pattern. On one hand, such foam structures lead to only very slight anisotropic or even isotropic behavior upon the mechanical loading, but on the other hand they do not have an optimal resistance to failure upon given loading conditions and level of porosity. The strength of the ceramic foam structure can be thus further improved by design of cells having various regular shapes. Such foams can finally exhibit an orthotropic behavior from both the elastic and strength point of view. To understand how different types of cells influence the foam characteristics in various directions, foam structures with various cell shapes were thus studied and investigated in terms of their tensile strength within this contribution. The structures were modelled by means of beam element based FE models and by utilization of the stress criterion defining failure of particular struts. Totally six different cell types were analyzed under consideration of the same porosity of the final foam structure and amount of the strength anisotropy was quantified. Relation between orientation of struts with respect to a loading direction and the foam strength was discussed in more details. Recommendations for an employment of particular cell types for specific loading conditions were given

Investigation of microstructure and mechanical properties of SLM-fabricated AlSi10Mg alloy post-processed using equal channel angular pressing (ECAP)

With the aim of improving the excellent mechanical properties of the SLM-produced AlSi10Mg alloy, this research focuses on post-processing using ECAP (Equal Channel Angular Pressing). In our article, two different post-processing strategies were investigated: (1) low-temperature annealing (LTA) and subsequent ECAP processing at 150 degrees C; (2) no heat treatment and subsequent ECAP processing at 350 degrees C, 400 degrees C and 450 degrees C. The microstructure and mechanical properties of this alloy were analyzed at each stage of post-treatment. Metallographic observations, combined with SEM and EBSD studies, showed that the alloys produced by SLM have a unique cellular microstructure consisting of Si networks surrounding the Al-based matrix phase. Low-temperature annealing (LTA), followed by ECAP treatment, facilitated the microstructural evolution of the alloy with partial breakup of the Si network and observed nucleation of beta-Si precipitates throughout the Al matrix. This resulted in a Vickers microhardness of 153 HV and a yield strength of 415 MPa. The main results show that post-processing of SLM-produced AlSi10Mg alloys using ECAP significantly affects the microstructural evolution and mechanical properties of the alloy.Web of Science1522art. no. 794

Crack path modelling in railway wheel under rolling contact fatigue

A computational model of crack path for two-dimensional p rimary crack situated in a railway wheel rim is designed. The railway wheel rim is placed on the wheel disc of railway wheel with interference fit. Crack behaviour is analysed in the case of rectilinear ride of a train under rolling cont act fatigue. Plank and Kuhn criterion is used to decide whether crack will either kink and follow mode I controlled (tensile mode) path, or it will propagate coplanar mode II controlled (shear mode). If mode I controlled crack growth is more probable then a direction of crack propagation is predicted using the maximum tensile stress range criterion. In this way a relationship between stress intensity factors and crack geometry is obtained. For comparison, crack behaviour in a solid railway wheel which is not subjected to pre-stress loading is also analysed. In the latter case the contact forces in the wheel-rail contact are considered to have i) only normal part ii) both the normal part and tangential part

Various methods of numerical estimation of generalized stress intensity factors of bi-material notches

The study of bi-material notches becomes a topical problem as they can model efficiently geometrical or material discontinuities. When assessing crack initiation conditions in the bi-material notches, the generalized stress intensity factors H have to be calculated. Contrary to the determination of the K-factor for a crack in an isotropic homogeneous medium, for the ascertainment of the H-factor there is no procedure incorporated in the calculation systems. The calculation of these fracture parameters requires experience. Direct methods of estimation of Hfactors need choosing usually length parameter entering into calculation. On the other hand the method combining the application of the reciprocal theorem (Ψ-integral) and FEM does not require entering any length parameter and is capable to extract the near-tip information directly from the far-field deformation