Materials Structure & Micromechanics of Fracture

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Authors: Alena Kruisová, Jiří Plešek
Abstract: Tangent moduli associated with the linear logarithmic model of hyperelasticity are derived. These relations are crucial not only to theoretical analyses but also to wave propagation and ultrasonic testing. The tangent moduli as functions of stress determine the speed of propagating acoustic waves and, therefore, indirectly point to a possible occurrence of residual stress fields in elastic solids.
Authors: Petr Haušild, Clotilde Berdin, Andreas Rossoll
Abstract: The finite element method was used in order to compute the energy balance and the stress-strain distribution in the Charpy V-notch specimen. Inertial effects were taken into account by a fully dynamic computation. It was shown that inertial oscillations are damped by viscoplasticity ahead the notch and vanish rapidly. 3D modelling is needed since large scale yielding and ductile crack growth occur. The ductile crack front is curved, which is important to account for in order to correctly describe the stress distribution in the specimen. Ductile crack growth in Charpy specimen was predicted by the GTN (Gurson-Tvergaard-Needleman) model. The GTN model allows a good failure prediction with strain rate and temperature independent damage parameters.
Authors: Vladislav Kozák, Libor Vlček
Abstract: The base of this paper is exact measurement of deformation and fracture material characteristics in laboratory, evaluation of these parameters and their application in models of finite element analysis modelling the fracture behaviour of components with defects. The base of the work is dealing with ductile fracture of forget steel 42CrMo4. R-curve is modelled by 3D FEM using WARP3D and Abaqus. Crack extension is simulated in sense of element extinction algorithms. Determination of micro-mechanical parameters is based on combination of tensile tests and microscopic observation. Input parameters for the next computation and simulation were received on the base of image analysis, namely fN and fo. The possibility of transferring these parameters to another specimen is discussed.
Authors: Gernot Trattnig, Christof Sommitsch, Reinhard Pippan
Abstract: To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.
Authors: Radek Kolman, Jiří Plešek, Michal Landa
Abstract: RUS−resonant ultrasound spectroscopy is a recent experimental−numerical method for the determination of moduli of elastic materials. Generally, all 21 elastic components of the elastic tensor can be determined by the numerical procedure based on the knowledge of a mechanical spectrum of a specimen. This involves the solution of a demanding inverse problem. Currently, the RUS technology allows the determination of material parameters of composite materials consisting of several layers with different material properties. In the present work, the fixed point iteration method in connection with the finite element method developed earlier is extended to optimize elastic moduli of layered materials. Properties of the fixed point iteration method are tested on a bicrystal specimen.
Authors: K.G. Gatos, A.A. Apostolov, J. Karger-Kocsis
Abstract: The nanocomposite formation in ethylene/propylene/diene rubber (EPDM) mixed with montmorillonite modified with octadecylamine (MMT-ODA) was investigated. The rubber formulation used proved to be critical for the final materials’ properties. The combined action of the curatives and glycidyl methacrylate (GMA), which was grafted on the EPDM, resulted in better intercalation and enhanced mechanical behavior of the rubber nanocomposites.
Authors: Michal Landa, Václav Novák, Petr Sedlák, Lluís Mañosa, Petr Šittner
Abstract: Elastic constants of solids were, until recently, evaluated predominantly by pulse-echo ultrasonic methods which are based on measuring the speed of ultrasonic waves propagation in solids. Resonant ultrasound spectroscopy (RUS) is a relatively novel method in which all components of elastic tensor are determined from measured resonance frequencies of a freely vibrating specimen. The RUS technique has been employed in this work to investigate temperature dependence of the elastic properties of the parent austenite phase in CuAlNi shape memory alloy single crystals. This phase exhibits very high elastic anisotropy (anisotropy factor A  12) and softening the shear coefficient C0 upon cooling when approaching the Ms transformation temperature. The complications (need for large number of resonant frequencies) emerging when one tries to determine all elastic constants of highly elastically anisotropic materials by the RUS technique are discussed. It is concluded that the shear coefficients C0 and C44, which are the most important for shape memory alloys, are, nevertheless, determined reliably.
Authors: S. Kúdela, H. Wendrock, L. Ptáček, S. Menzel, K. Wetzig
Abstract: Fibers fracture in tensile strained Mg and MgLi matrix composites strengthened with ~10% vol. short δ-Al2O3 fibers (Saffil) is investigated by „in-situ“ scanning electron microscopy and ex-situ“ determination of the length of fibers chemically recovered from tensile failed composites. Little interfacial reaction in Mg matrix composite results in poor interfacial bond so that composite failure proceeds via fiber pull-out with negligible fiber fragmentation. On the other hand, extensive fiber/matrix reaction in MgLi matrix composites promotes formation of strong interfaces which are linked with multiple fiber cross-breakage during tensile straining. These results are consistent with experimental tensile strengths of related composites.
Authors: Y. Kitahara, H. Shimazaki, T. Yabu, Hiroshi Noguchi, Michiru Sakamoto, Hideto Ueno
Abstract: It is common knowledge that Mg is very light and high specific strength material. But there is a problem that Mg alloy easily burns during its machining, because the ignition point of Mg is 700K.[1] The ignition point of new Mg alloy to which Ca is added is about 1000K. This new Mg alloy is called non-combustible Mg alloy. As a result, many inclusions exist in Mg alloy. Then, fatigue test was carried out and gave evidence that inclusions have an effect on the fatigue strength. Then relationships between size of inclusions and fatigue limit were drawn. Therefore, the fatigue limit can be predicted by estimating the maximum inclusion size. The method of estimating the maximum inclusion size is “Estimation for particle size distribution in materials“[2] .
Authors: Zuzanka Trojanová, Zoltán Száraz
Abstract: The high temperature behaviour of composites with the AS21 magnesium alloy matrix, reinforced by short Saffil fibres was investigated in the temperature interval from room temperature to 300 °C. The yield stress and the maximum stress decrease with increasing temperature. Two types of specimens were investigated – one with fibres plane oriented parallel to the stress axis and the other with perpendicular fibres plane orientation. Light and scanning electron microscopy were used for study of the microstructure of composites. Possible hardening and softening mechanisms are discussed. The shear stress at fibre/matrix interface was of greatest importance in this regard, though the contribution resulting from the dislocation density increase was also significant.

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