Key Engineering Materials Vols. 592-593

Abstract: The aim of the performed investigations was to determine the influence of two different heat treatment procedures, i.e. hardening and tempering vs. austempering, on some mechanical properties, as well as the influence of these procedures on the magnitude of some fracture mechanics parameters for example, crack tip opening displacement (CTOD) and J-integral. The experimental investigations were performed on the specimens made of spring steel 65Si7 (chemical composition: 0.69% C, 1.56% Si and 0.90% Mn).

Abstract: The kinetics of fracture was examined for a wide range of metallic materials. Using speckle photography technique, displacement vector fields were recorded for the deforming sample. The deformation localization is an autowave process, which manifests macroscopic inhomogeneities from yield point to necking. This involves a changeover in the autowave types: phase autowave stationary dissipative structure propagating autowave collapse of autowave. The loading curves obtained for all studied materials have a similar autowave pattern emerging at the pre-fracture stage, no matter what is crystalline or microstructural state of material. Using experimental data on the kinetics of autowave collapse, the space-time coordinates of failure can be predicted for the object under load long before symptoms of fracture are detected visually.

Abstract: The aim of the present work is the investigation of the influence of graphite powder addition to an unsaturated polyester type polymer (32% styrene content in mass) on the mechanical and tribological properties. For this purpose, we conducted bending tests and wear tests with a combination of four loads and three speeds for three different compositions (0, 1 and 2% graphite). The wear tests are carried out on a dry type pin on disk tribometer. The disk is made of quenched and annealed C48 steel (540 Hv hardness). Before the rubbing process, the discs are subjected to polishing in order to obtain approximately the same initial surface roughness. The results show that the addition of graphite powder improves the tribological properties; a noticeable decrease of the coefficient of friction, the mass loss and the wear rate are achieved with the increase of the graphite powder percentage for all sliding speeds and loads. A 2% graphite content causes a drop of the friction coefficient from 0.4 to 0.2. The results of bending tests showed a significant decrease of the stress and strain at failure and a slight increase in Young's modulus. In addition, for the three compositions, the results show a clear preponderance of the influence of the load on the tribological properties.

Abstract: This work is a part of an industrial waste development program. It is devoted to the study of recyclability of chips resulting from the machining of steel parts as reinforcement for concrete. We are particularly interested in this study in the rheological behavior of chips reinforced fresh concrete and its mechanical behavior at young age. The evaluation of the workability indicates that the sand over gravel optimal ratios (S/G), corresponding to the composite minimum flow time, are S/G = 0.8 and S/G = 1. The study of the chips content (W) influence on the workability of the concrete shows that the flow time and the optimal ratio S/G increase with W. Mechanical characterization tests (direct tension, compression, bending and splitting) show that mechanical properties of chips reinforced concrete are comparable to those of the two selected reference concretes (concrete reinforced with conventional fibers: EUROSTEEL fibers corrugated and DRAMIX fibers). Chips provide to control concrete a significant increase in strength and some ductility in the post-failure behavior of the composite. Recycling chips as reinforcement for concrete could be thus favorably considered.

Abstract: Conducted loading tests of plain concrete specimens are briefly introduced in the paper together with description of the performed numerical simulations of these tests in ATENA 3D software. The simulation results of concrete failure are analysed in detail and compared with the experimental results recording failure processes with the help of acoustic emission. The simulation results are in good agreement with the recorded data.

Abstract: The correlation between fracture toughness, hardness and microstructure of vacuum heat-treated high-speed steel AISI M2 was investigated. Our intention was to investigate the influence of microstructural parameters such as the volume fraction of undissolved eutectic carbides, their mean diameter, the mean distance between the carbides, as well as the volume fraction of retained austenite in the matrix, on the above mentioned mechanical and fracture properties. The experimental investigations were performed on the high-speed steel. This steel had the following chemical composition (mass content in %): 0.89% C, 0.20% Si, 0.26% Mn, 0.027% P, 0.001% S, 3.91% Cr, 4.74% Mo, 1.74% V and 6.10% W.

Abstract: In this paper the results of fatigue tests under cyclic bending of 2024 aluminium alloy with amplitude of strain energy density parameter control are presented. Based on these tests, energy fatigue characteristic of the tested material was elaborated.

Abstract: Each material contains defects and in-homogeneities in a structure volume. It has influence on the properties of material (conductivity, mass density, mechanical properties). Interaction of the ultrasonic waves with defects or in-homogeneities in the solid state is not clear. Electro-ultrasonic spectroscopy can help to clarify this phenomenon. The electro-ultrasonic spectroscopy describes defects and un-homogeneities inside the sample structure. This method is quite different from electro-acoustic effect. Ultrasonic signal is in range from 20 kHz to 150 kHz. Ultrasonic signal changes geometry of the sample in elastic range only. The sizes of cracks are changing also in the sample volume. Conductivity near the area of cracks is strongly changing due to ultrasonic vibrations. It has influence on resistance of the sample which is changing along with a frequency of ultrasonic vibrations. The amplitude of the resistance change depends on the material, number of cracks, size of cracks and Eigen frequencies of the sample excited by ultrasonic wave. We applied the electro-ultrasonic spectroscopy on two types of varistors. It can be useful for understanding the relation between microstructure and mechanical properties of these types of varistors.

Abstract: The paper contains the results of tests on the fatigue crack growth in titanium Gr.1 and Ti-6Al-4V as well as Ti-3Cu titanium alloys. The chosen materials contained titanium but their structure was different. Plane specimens with the stress concentrators in form of one-sided sharp notches were tested. Influence of the microstructure on the crack paths in plane specimens was observed.

Abstract: Youngs moduli of talc-based ceramics from the system MgO-Al₂O₃-SiO₂ are measured for temperatures up to 1000 °C via impulse excitation. It is shown that, after pressing at 50 MPa and firing at 1280 °C, MgO-rich compositions exhibit higher porosity and lower Youngs moduli (approximately 2030 % lower than predicted via micromechanical relations). The Young moduli of materials with less MgO decrease with temperature, but those of MgO-rich ceramics increase with temperature and exhibit a large hysteresis between heating and cooling. Lower absolute values are mainly due to increased porosity, but the reason for the modulus increase with temperature and the hysteresis is the higher enstatite content in the MgO-rich compositions. For a special composition the Youngs moduli are more or less temperature-independent and without significant hysteresis effects, probably due to the low content of enstatite and the high content of sapphirine.