Materials Structure & Micromechanics of Fracture

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Authors: Miroslava Ernestová
Abstract: The paper summarizes results of tensile tests in low alloy steel (LAS) specimens (steels 15Kh2MFA and 15Kh2NMFA). Slow Strain Rate Tensile tests (SSRT) were performed in air at temperatures from 22 to 325°C over a wide range of strain rates from 2.5×10-6 to 1.67×10-3 s-1. The possible effect of strain rate and temperature to mechanical properties of tested LAS is searched for. The dynamic strain ageing (DSA) was observed within certain temperature ranges at lower strain rates tested and its hardening effect in terms of the maximum strengthening stress decreased linearly with the increase of log strain rate. It has been found that the occurrence of susceptibility to environmentally assisted cracking (EAC) of tested steels in high temperature water (HTW) is corelated to the DSA behavior. The result suggest that DSA reduces ductility of reactor pressure vessel (RPV) steel and its role in enhancing the EAC of RPV steels should not be neglected, in view of the coincidence with susceptibility zones for DSA and EAC in terms of strain rate and temperature. A reasonable coincidence was observed between the susceptibility to DSA exhibited by SSRT in air and with the EAC behavior observed in laboratory experiments.
Authors: Stanislav Vĕchet, Jan Kohout, Klára Hanzlíková
Abstract: Two ADI heats transformed at temperatures of 400 and 380 °C during temporal range from 2 minutes to 9 hours were studied in details, with emphasis on structure composition and mechanical properties (tensile and fatigue properties were determined). In the case of the shortest dwells the level of mechanical properties is influenced by martensite, which occurs in the structure as a result of subsequent cooling. UTS and yield stress increase slightly with the dwell of isothermal transformation while the values of elongation to fracture as well as of fatigue limit are very closely dependent on the amount of the retained austenite in the microstructure.
Authors: Viera Gärtnerová, M. Chalupová, Aleš Jäger, Zuzanka Trojanová
Abstract: Magnesium alloy AZ91 (9 % Al, 1 % Zn, 0.2 % Mn in wt.%) with different reinforcements has been used to study fracture mechanisms and crack development. SiC particles and /or Saffil fibres were used as the reinforcement. Fracture surfaces of specimens prepared by impact tests in the temperature range from room temperature to 300 °C were investigated by scanning electron microscope (SEM). Possible mechanisms of fracture are discussed in the relation to the test temperature.
Authors: T.T. Lamark, Miloš Janeček, Yuri Estrin
Abstract: The influence of the surface condition and elevated temperature on the fatigue properties of high pressure die cast magnesium alloy AS21X was investigated. Separately cast specimens were tested under tensile cyclic loading, at room temperature and 150°C. With the as-cast surface layer intact, no significant temperature influence on fatigue life was found for large stress amplitudes, whereas the fatigue strength at 107 cycles was found to drop from 50 MPa at 20°C to 40 MPa at 150°C. Fracture surface analysis revealed that the predominant fatigue cracks originated from pre-existing surface defects.
Authors: Jaroslav Balík, Miloš Janeček, Josef Pešička
Abstract: The growth of cracks in base steel P91 of 9Cr-1Mo class and in intercritical layer of HAZ is measured under creep coditions. For long term tests, a material degradation was detected consisting in an increase of crack growth rate and in a decrease of crack initiation time. An attempt is made to connect these effects with drop in ductility during thermomechanicalexposition.
Authors: J. Göken, J. Swiostek, Dietmar Letzig, Karl Ulrich Kainer
Abstract: Magnesium cast parts as well as indirectly and hydrostatically extruded profiles were used in order to investigate the influence of different microstructures on the damping behaviour of magnesium AZ-alloys. Especially, hydrostatic extrusion leads to a significant grain refinement in the extruded profile compared to indirect extrusion. Strain dependent damping measurements were carried out at room temperature using alloys AZ31, AZ61 and AZ80. Damping was determined as the logarithmic decrement of free bending beam vibrations. The results of these measurements after indirect and hydrostatic extrusion are shown and discussed.
Authors: T. Podgrabinski, Petr Slepička, V. Rybka, Václav Švorčík
Abstract: Permittivity of about 1 µm thin films prepared from polymethylmetactrylate (PMMA) solution doped with 20 % of diphenyl sulfoxide was studied. Permittivity of the films was measured as a function of the temperature. The measurement of the dependence of polarization on electrical field was performed using a standard Sawyer-Tower circuit. The presence of the dopant increases the composite permittivity namely above the PMMA glass transition temperature. Hysteresis loops observed on the measured polarization vs. electrical field dependence indicate easier and more pronounced polarizability of the composite comparing to pristine PMMA.
Authors: Jacek Bomba, Jerzy Kaleta, Paweł Sawa
Abstract: The paper presents an application of giant magnetostriction phenomenon into particular vibration generator. It was assumed that the generator should enable tests of construction under local loads applied with broad spectrum of frequencies. Mass of a device must be minimal, it should exert large forces with large amplitudes and high efficiency. The applied material is Terfenol-D - an alloy of rare earth elements and iron which exhibits giant magnetostriction. It is a compound of Laves phases with cubic MgCu2 structure. Examinations ofmagnetomechanical properties of Terfenol preceded construction. The efficiency of transformation of magnetic energy into the mechanical one was tested. The key role in efficiency is played by initial compressing prestresses involved and stimulation by constant magnetic field. The output quantities of constructed device as force, displacement and frequency are completely controlled. The control system includes modern power DC amplifier. It was shown some experimental results of examination of the magnetostrictive actuator. Examples of generations of mechanical vibrations in various constructions enter the new area of applications of Smart Magnetic Materials.
Authors: L. Berka, N.V. Murafa
Abstract: Hardness testing is a method frequently used for evaluating the resistance of body surfaces to the influence of contact loading. The Vickers hardness test is applied for this purpose in a case when the material is sufficiently ductile and no cracks occur in the corners of the indents. Testing of the surface hardness of such materials as glass and ceramics on the basis of the Vickers testing method must take into account the energy that is spent on crack spreading. This paper describes a more exact and accurate method for evaluating resistance to microcrack formation on the surface of a material. To evaluate the conditions for crack spreading, it is necessary to test a specimen under loading in bulk. The suggested procedure involves a bent strip. As bending stresses are known, from them and from the differences in crack length along and across the strip, the resistance of the material surface to crack propagation is determined.
Authors: Jerzy Kaleta, Daniel Lewandowski, Piotr Zając
Abstract: The paper presents measurement and results of mathematical modelling of damping in cyclic loaded magnetorheological composites - MRC. The original experimental set-up for cyclic shearing mode has been created. Examples of possible applications, measurement and processing mechanical signals (shear stress - T, deformation - g) and magnetic signals (strength of magnetic field - H) were provided. For the modelling four-parameters viscoelastic-viscoplastic model was chosen. Result of identification confirmed an appropriate choice of the model.

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