Key Engineering Materials Vols. 592-593

Abstract: Several possible routes are available for estimation of fracture behaviour from the results of small punch tests performed at constant rate of deflection. The routes include: (i) measurement of relevant dimensions directly on ruptured specimens, (ii) determination of critical deflections on the load vs. deflection curves and (iii) integration of these curves up to specific points. Equivalent fracture strain, fracture energy or fracture toughness are then evaluated from the obtained quantities. The mutual relations among the quantities are demonstrated by the results of small punch tests performed on a Fe-Al-based alloy in the temperature range extending from brittle up to ductile fracture appearance.

Abstract: Carbon bonded alumina (Al₂O₃-C) in various compositions are developed for the production of open cell filters, which are used for melt metal filtration processes [. The Small Punch Test (SPT) is used to determine the mechanical properties of such materials. Previous investigations showed two different types of fracture behavior [2,, which can be distinguished by typical features of the load deflection curves of the SPT. This paper clarifies this behavior by examining the fracture surfaces using scanning electron microscopy (SEM).

Abstract: Present paper studied the improvement of the fracture toughness under mixed mode loading obtained by using carbon nanotubes reinforcement in fiber glass mats/ epoxy laminates. Mixed-mode bending tests were performed considering different loading ratios G_II/G_I. Laminates were manufactured using the epoxy resin Biresin® CR120 reinforced with fiber glass triaxial mats ETXT 450 and multiwalled carbon nanotubes with 98% of carbon. It was observed that the total fracture toughness increases linearly with the mode II loading component and that linear mixed-mode fracture criteria reproduces the GI versus GII relationship. The incorporation of small quantity, up to 0.5%, of carbon nanotubes into matrix improves significantly mixed-mode fracture toughness.

Abstract: For the description of the behavior of anisotropic materials within the framework of the phenomenological approach the model which is taking into account distinctions in strength on compression and a stretching is offered. The model as allows carrying out researches with any orientation of properties of a material concerning system of coordinates. Numerical modeling is carried out in three-dimensional statement. The range of velocities of interaction up to 3000 m/sec is considered.

Abstract: The present work focuses on the experimental multi-scale characterization of fracture of an AISI 4135 steel by using the Disk Pressure Test (DPT). In order to precise the specific features of hydrogen embrittlement, comparison was made between disks burst under helium and hydrogen gas. SEM - EBSD analysis of disks samples before and after the test allowed to analyze and to compare the main microstructural mechanisms of the failure process. The location of the main crack initiation was consistent with Finite Element (FE) simulations of the DPT.

Abstract: Residual elastic strains were measured by neutron diffraction using POLDI materials science diffractometer at PSI-Switzerland on pre-loaded notched flat tensile specimens made of the high-chromium tempered martensitic steel F82H-mod steel. To calculate the residual stresses using Hooke's equation, three perpendicular components of the residual strain field were determined. The measured residual strains and stresses were compared with those deduced from finite element simulation calculations. A very good agreement was found for the strains in the loading plane of the specimen while a somewhat larger discrepancy was observed for the out-of-plane residual strain, which was tentatively attributed to an uncertainty in the initial lattice spacing in that direction.

Abstract: A concept of atomic mechanisms governing strength of nanosized defect-free crystals is presented. It is exhibited that these mechanisms consist in local instability of the lattice. Two main reasons for localization of instability in three-dimension (3D) crystals are analyzed, namely, (i) fluctuation of local stresses induced by thermal vibrations of atoms, and (ii) non-uniform distribution of local stresses caused by a surface tension. Based on this conception, explanations of both the temperature dependence of strength of 3D nanocrystals and scale effect are given. Ideas on the reasons for and regularities of change in strength at transition from 3D to 2D (graphene) and 1D (monatomic chain) crystals are represented. It is shown that dimensionality of crystal is one of the main factors governing strength of defect-free crystals. Experimental values of the strength of carbon monatomic chains are given, which times exceeds the strength of graphene and is the highest attainable level of strength in the world.

Abstract: The work presents the results on grains refinement of steel containing 0,45 wt pct carbon resulted from severe plastic deformation (SPD). Different steel structures from prior solutioning and/or thermomechanical treatment were prepared for deformation experimental. A coarse grain ferrite-pearlite structure was achieved applying solutioning. By application of thermomechanical (TM) controlled forging process, performing multistep open die forging, the refined ferrite-pearlite mixture was prepared. Final structure refinement of steel, having different initial structure, was then accomplished applying warm Equal Channel Angular Pressing (ECAP) at 400°C. Employment of this processing route resulted in extensive deformation of ferrite grains and cementite lamellae fragmentation. Applying the highest shear stress (ε_ef - 4) the mixed structure of subgrains and ultrafine grains was present within the ferrite phase. In pearlite grains, modification of cementite lamellae due to shearing, bending, twisting and breaking was found efficient. The coarse cementite lamellae spheroidization was more efficient in prior TM treated steel. The tensile deformation records confirmed strength increase and diversity in strain hardening behaviour.

Abstract: The deformation mechanisms in ultrafine-grained hexagonal close packed Zn were investigated at different strain rates and temperatures. The influence of grain size on the deformation mechanisms was revealed by comparing the results obtained on ultrafine-grained and coarse-grained Zn. It was found that for coarse-grained Zn at room temperature and strain rates lower than 10^-2 s^-1 twinning contributed to plasticity besides dislocation activity. For strain rates higher than 10³ s^-1 the plasticity in coarse-grained Zn was controlled by dislocation drag. In ultrafine-grained Zn the relatively large dislocation density (~10¹⁴ m^-2) and the small grain size (~250 nm) limit the dislocation velocity yielding the lack of dislocation drag effects up to 10⁴ s^-1. For ultrafine-grained Zn, twinning was not observed in the entire strain rate range due to its very small grain size. During room temperature compression at strain rates higher than 0.35 s^-1 and in high temperature creep deformation of ultrafine-grained Zn besides prismatic and pyramidal <c+a> dislocations observed in the initial state, <a>-type basal and pyramidal dislocations as well as other <c+a>-type pyramidal dislocations were formed.

Abstract: The molecular dynamics simulation of nanoparticle synthesis under pulse electric dispersion of metal nanowires with interfaces was carried out. Atomic interactions were described using the potentials calculated within the framework of embedded method atom. Surface properties, defect structure energy, elastic characteristics and a number of other features, which are critical for the simulation of the electric pulse metal wire dispersion, can be described by these interatomic potentials with high accuracy. The nanowires simulated had a cylindrical shape. Along the cylinder axis use was made of periodic boundary conditions, while in the other directions, a free surface was simulated. Heating of the nanowires was performed by scaling of the atomic velocities following a linear law while maintaining a Maxwell distribution. In the course of high-rate heating the specimen experienced fracture followed by the formation of clusters. Influence of interfaces, heat rate, temperature distribution along sample profile on structure of generated nanoparticles was investigated. The atoms were assumed to belong to one cluster, given that the spacing between the nearest atoms was less than the threshold distance. The latter was assumed to be equal to the radius of the second coordination sphere in a perfect metal lattice.