Materials Structure & Micromechanics of Fracture

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Authors: Václav Paidar, Pavel Lejček
Abstract: Grain boundaries are decisive for many properties of materials. Due to short-range stress field their influence is primarily based on their atomic structure. Special character of grain boundary properties related to their structure, follows from the nature of atomic arrangements in the boundary cores, from the interfacial dislocation content and from the boundary mobility. All those aspects of boundary behaviour are strongly influenced by the boundary chemistry including various segregation phenomena. Approaches to the boundary classification and the interpretation of recent experimental results are discussed in the context of the complex relationship between microstructure and material properties. Such findings are essential for Grain Boundary Engineering proposed to improve the performance of polycrystalline materials.
Authors: Yannick Champion, Cyril Langlois, Sandrine Guérin, Sylvie Lartigue-Korinek, P. Langlois, Martin J. Hÿtch
Abstract: Based on the Taylor theory, a critical length scale is defined as the minimum dislocations cell size obtained at the maximum work-hardening for metals and alloys. When grain size is smaller than this length scale, corresponding also to a critical dislocation mean free path, new behaviours occur; such as ductility and strength, near perfect elasto-plasticity, high strain-rate sensitivity. Bulk samples are fabricated from Cu nanopowders (particle size 50 nm) by powder metallurgy techniques. The final grain size is comprised between the critical mean free path, evaluated at 130 nm and the size where transition to the so-called nano regime occurs (when unit dislocation no longer exists below 30 nm for Cu). Tensile tests are carried and microstructural analysis are performed before and after deformation.
Authors: Yu.V. Milman
Abstract: Aluminum-based alloys containing quasicrystalline particles of 50 – 600 nm in diameter as a reinforcing phase were produced in the form of powder or ribbons by water atomization or melt spinning techniques, respectively. Rods were compacted from powders and some ribbons by severe plastic deformation without sintering. Structure and mechanical behavior of alloys are discussed.
Authors: Vàclav Sklenička, Jiří Dvořák, Milan Svoboda, Petr Král, B. Vlach
Abstract: Aluminum-based alloys containing quasicrystalline particles of 50 – 600 nm in diameter as a reinforcing phase were produced in the form of powder or ribbons by water atomization or melt spinning techniques, respectively. Rods were compacted from powders and some ribbons by severe plastic deformation without sintering. Structure and mechanical behavior of alloys are discussed.
Authors: Yoshiyuki Kondo
Abstract: The threshold stress intensity factor (ΔKth) of small crack is affected by various factors, especially by material hardness, stress ratio and crack size. Test results showed that harder material had higher ΔKth and smaller crack had lower ΔKth. The crack closure measurement on a very small crack was done to make clear the root cause of those effects. Most of those effects could be explained by the peculiar behavior of crack closure for crack deeper than 100µm. However, everything could not be understood only by the crack closure behaviour. In addition to the difference in crack closure, (ΔKeff)th itself was also dependent on crack size when the crack depth was shallower than 100µm. Another remarkable phenomenon was experienced in this study. Unusual decrease in ΔKth was found in some case. The large decrease of ΔKth occurred under the conjunction of three factors, that is, extremely high stress ratio (R) higher than 0.8, small crack and hard material. This kind of large reduction in ΔKth in high R region is of much importance in turbo machinery that operates under high mean stress with small vibratory stress.
Authors: Masahiro Toyosada, Koji Gotoh
Abstract: Fatigue cracks remain closed at lower loading level during a part of load cycle even though a tension-to-tension loading is applied. The crack closure plays a role to obstruct the generation and growth of compressive plastic zone during unloading. Cyclic plastic work, which corresponds to an irreversible energy consumed in a cracked body is generated ahead of a crack, is required as a fatigue crack driving force. The amout of cyclic plasticity is reduced by a crack closure. The crack opening/closing model based on the Dugdale model under arbitrary stress distributions for a through thickness straight crack is proposed and the fatigue crack growth under various loadings is investigated.
Authors: Pavel Lukáč, Zuzanka Trojanová, František Chmelík
Abstract: Nondestructive methods may help to detect changes in the internal structure of a material and to explain the behaviour of the material. This paper describes a series of nondestructive tests performed on magnesium composites with a variety of matrices: commercial pure Mg and three magnesium alloys AZ91, ZC63 and ZE41. Short fibres of δ-Al2O3 (Saffil®) were used as the reinforcement. Internal friction measurements and joint dilatation and acoustic emission studies were used to demonstrate how thermal cycling influences the deformation behaviour of Mg based metal matrix composites. The values of the logarithmic decrement are influenced by the upper temperature of the cycle. The acoustic emission activity and the residual strain increase with increasing upper temperature. The results may be explained assuming that internal thermal stresses are generated.
Authors: Aleksander Karolczuk, Ewald Macha
Abstract: The paper includes a review of literature on the multiaxial fatigue failure criteria based on the critical plane concept. The criteria were divided into three groups according to the distinguished fatigue damage parameter used in the criterion, i.e. (i) stress, (ii) strain and (iii) strain energy density criteria. Each criterion was described mainly by the applied the critical plane position. The multiaxial fatigue criteria based on two critical planes seem to be the most promising. These two critical planes are determined by different fatigue damage mechanisms (shear and tensile mechanisms).
Authors: Ivo Dlouhý, Zdeněk Chlup, Aldo Roberto Boccaccini
Abstract: A number of examples exist that indicate the potential for increasing the toughness of brittle matrices by dispersing different reinforcements. For further development of these advanced materials the actual material response during mechanical loading under presence of flaws appears to be important. Theoretical and experimental knowledge acquired on different kinds of brittle matrix composites is summarised in the paper. These include glass matrix composites with metallic particles, alumina platelets, continuous SiC (Nicalon®) fibres, and both chopped fibres and ZrO2particles (hybrid composites). The composites were tested in as-received state but also after different forms of thermomechanical loading, e.g. thermal shock, thermal cycling in air, which were investigated according to the envisaged composites application. Chevron notch technique was mainly used for fracture toughness evaluation. Microstructural damage is explained based on identified fracture micromechanisms.
Authors: R. Gröger, V. Vitek
Abstract: The breakdown of the Schmid law in bcc metals has been known for a long time. The asymmetry of shearing in the slip direction 〈111〉 in the positive and negative sense, respectively, commonly identified with the twinning-antitwinning asymmetry, is undoubtedly one of the reasons. However, effect of stress components other than the shear stress in the slip direction may be important. In this paper we investigate by atomistic modeling the effect of shear stresses perpendicular to the Burgers vector on the glide of a/2〈111〉 screw dislocations. We show that these shear stresses can significantly elevate or reduce the critical resolved shear stress (CRSS) in the direction of the Burgers vector needed for the dislocation motion, i.e. the Peierls stress. This occurs owing to the changes of the core induced by these stresses. This effect may be the reason why slip systems with smaller Schmid factors may be preferred over that with the largest Schmid factor.

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