Volume 58, Issue 2

The continuing trend to increase the performance and durability of machines requires the use of materials, whose structural integrity must be ensured far beyond the classical fatigue limit. In this so-called „very high cycle fatigue“ regime (VHCF), the materials show a strong scatter in fatigue life, which makes life service assessment difficult. Present studies on austenitic-ferritic duplex steel 1.4462 (X2CrNiMoN22-5-3) have shown that the reason of this scatter in fatigue life can be found in the microstructural length scale mainly due to the barrier effect of grain and phase boundaries. The integration of an ultrasonic fatigue testing system in a high-resolution scanning electron microscope allows observing and evaluating the active microstructural mechanisms of fatigue crack initiation and early crack propagation, such as the formation and propagation of slip bands, during the majority of VHCF life. Additional micro texture measurements by electron backscatter diffraction (EBSD) provide data for the development of a mechanism-oriented numerical short crack simulation. In the context of crack initiation and propagation in the VHCF regime in duplex stainless steel, the focus of the present paper is put on the technical realization of the in-situ ultrasonic fatigue testing in the scanning electron microscope.

In this study, P295GH (17Mn4) pressure vessel steel and AISI 304 stainless steel were joined by using ER 309L electrodes. Welded specimens were fatigued by four point bending experiments. Cracks occurred on the stainless steel side of the weldment during the fatigue tests. Finite element (FE) analyses were conducted by fixing 2-D precracked model for welded 17Mn4 pressure vessel steel and AISI 304 stainless steel by using ER 309L electrodes. True stress and true strain values of the AISI 304, 17Mn4 steels and ER 309L electrodes were determined by tensile tests. Stress analyses were conducted after entering the linear and nonlinear properties of the materials into software manually. Eventually, fracture mechanics analyses were conducted for the model, and J integral fracture toughness values of the crack on the weldment were calculated.

The topic of fatigue strength of joints of flat and honeycomb-structured thin sheets has not yet been adequately studied. This concerns, for example, the development of an appropriate test method. In many cases, there is still a lack of information, how effective joints of flat and structured sheet metals are in comparison with reference joints of flat sheets. Combinations of resistance spot-welded specimens of flat and honeycomb-structured thin sheets of deep-drawing steel DC04 with a thickness of 0.5 mm were examined. Adopted clamping plates were applied for the testing. Joined smooth specimens were used as a reference material. The analysis of the frequency [1] was applied for comparison. The fatigue limit of a joint of a flat and a structured sheet metal is not significantly affected by spot welding. The fatigue limit of these spot-welded joints is comparable to that of an unjoined honeycomb-structured sheet. By means of frequency or stiffness analysis, stability and damage mechanisms of differently shaped joints were explained. The joining of bridges of the structure shows a better stability than the combination of honeycombs. The stiffness analysis is a favored method for the clarification of the damage mechanisms in a spot-welded joint. The reasons for the low fatigue strength values of the joined flat-structured specimens compared to flat-flat joints are strong notch effects in the bridge area and momentum effects by the structure height of the studied honeycomb structure. The spot weld with a pronounced indentation also affected the fatigue strength of both flat and structured joined sheet metals.

In this paper, the effect of nickel content on the mechanical properties of aluminum were investigated. High purity Al and Ni were melted in an induction furnace and cast into a metal mold. Microstructural characteristic and mechanical properties of the alloys were studied in detail. The addition of nickel to pure aluminum increased the tensile strength and decreased the elongation at break value of the specimens. Maximum wear resistance was obtained with the addition of 3 wt.-% Ni. Further addition of nickel caused a decrease in wear resistance.

Kurzfassung Computertomografie (CT) ist eine bewährte Methode in den Bereichen Fehleranalyse und Qualitätskontrolle. Die Energie der hierbei benutzten Röntgenstrahlung bestimmt die Eindringtiefe der Strahlung und schränkt damit die Grösse und das Material des zu untersuchenden Objektes ein. Für grosse, dichte und schwere Objekte sind Photonenenergien von mehr als einem Megaelektronenvolt (MeV) nötig, um nutzbare Transmissionswerte zu erreichen. Ein wichtiger Faktor für die Qualität hochenergetischer Röntgentomografien ist der Anteil der Streustrahlung auf den CT-Bildern. Röntgenstrahlung kann sowohl im zu untersuchenden Objekt, als auch vom Messinstrument und -umfeld streuen und führt zu einem verfälschten Transmissionsbild. Neben Streueffekten kommt für MeV-Photonen zusätzlich der physikalische Prozess der Paarbildung ins Spiel. Die Auswirkung der einzelnen Streuprozesse auf das Transmissionsbild soll hier untersucht werden. Dafür wird mit Hilfe von Monte-Carlo-Simulationen der Einfluss der verschiedenen Streubeiträge untersucht. Im Gegensatz zu früheren Studien, wird hier nicht nur ein Satz von einfachen geometrischen Objekten aus unterschiedlichen Materialien untersucht, sondern es werden auch vereinfachte Modelle der Komponenten eines CT-Scanners benutzt, um den Beitrag der Streuung verschiedener Systemkomponenten abzuschätzen.

Fe-B alloys exhibit continuous networks of eutectic borides, which destroy the continuity of matrix and result in the brittleness of the material. In order to improve its toughness, Fe-B alloy was treated by hot forging process in this study. Three-body abrasion wear behaviors of forged and unforged Fe-B alloy under three different abrasive (SiC, garnet and quartz sand) and applied load conditions were studied. Wear mechanisms were investigated by using scanning electron microscope (SEM) and 3D laser scanning microscope (LSM). The result shows that under three-body abrasion condition, Fe-B alloy exhibits excellent wear resistance. The wear resistance of forged samples is higher than that of unforged samples on the three different kinds of abrasive. A correlation between the hardness of the abrasive, movement patterns of abrasive particles and applied load was found. The movement patterns of quartz sand are sliding between the contacting surfaces under the load of 3 kg. In the wear process of Fe-B alloy, sliding/cutting is the dominant removal mechanism in the case of quartz sand abrasive and rolling is the dominant removal mechanism in the case of garnet abrasive during three-body abrasion under the load of 3 kg. When Fe-B alloys are tested under the load of 1, 3 and 5 kg, it is found that load can change the movement patterns of abrasive particles. With increasing load, the movement patterns of abrasive particles are changed from rolling to sliding mechanism. Our investigations reveal that the three-body abrasive wear mechanism of forged and unforged Fe-B alloy depends on material/abrasive hardness ratio and applied load.

In this study, the effects of particle addition on the microstructure, mechanical properties and fracture behaviors of Al/B 4 C particle reinforced composite produced by hot pressing method were investigated. After the production of 5, 10 and 15 wt.-% of B 4 C reinforced composites, T6 heat treatment was applied. It was specified that a strong interface bond occurred between the matrix and particles, and during the fracturing of composites, this effect increased the particle fracture. It was observed that particles were fractured parallel to the fracture surface and to each other fracture surface of MMCp. In general, cavity of particle separated from the matrix was not seen. In the production of MMCp, it was also observed that hot pressing provided a density higher than 99 %. After the heat treatment the maximum hardness value of 173 HB was reached with 15 wt.-% B 4 C reinforced sample. The transverse rupture strength (TRS) decreased with the increase of reinforcement ratio. The highest TRS was 510 MPa with the 5 wt.-% reinforced material, whereas the lowest value was obtained to be 387 MPa with the 15 wt.-% B 4 C reinforced material.

This study aims to examine the fracture and surface adhesion behavior of hydroxyapatite (HA) coatings created in acitric acid – sodium citrate buffer system. Coatings were implemented on the surface of Ti6Al4V. The modulus of elasticity, hardness and adhesion strength values of the presented coatings were determined, the fracture toughness values were calculated and scanning electron microscope (SEM) images were taken as well. Finally, the mechanical properties found by the tests implemented to hydroxyapatite coatings applied on Ti6Al4V, which yielded successful outcomes were compared to the mechanical properties of bone. The fracture toughness achieved in this study was found to be more compatible with the mechanical properties of the original bone itself when compared to the literature findings. Besides, the adhesion strength was found to be remarkably high.

Studies have been carried out to investigate the performance of hot dip galvanized coating on reinforcement corrosion. The coated and uncoated concrete specimens were subjected to accelerated corrosion to determine the time to corrosion initiation. The accelerated corrosion test results clearly showed that the specimens with hot dip galvanized coatings performed very well against reinforcement corrosion and were better than uncoated specimens.

In the current study, hard facing nickel alloy powders were coated in situ on pure iron by uniaxial hot pressing under vacuum atmosphere to improve its hardness and wear properties. The nickel alloy powders were placed onto the iron powders within a graphite die and then sintered at 1000 °C under 40 MPa for 10 minutes. Hardness and wear properties were studied by using a Vickers hardness tester and a ball-on-disk tribometer, respectively. The wear rates of the samples were evaluated using a sliding velocity of 0.13 m × s −1 under normal loads of 20, 30, 40 and 50 N at room temperature. The interface layer between the nickel layer and pure iron substrate was also evaluated by scanning electron microscopy and energy dispersive spectroscopy. The results showed that sintered samples exhibited strong bonding at the interface. The hardness and wear properties of the materials increased effectively after nickel coating.

Industrial dyes disposal represents one of the major environmental problems. Although the different techniques were used for its removal, the adsorption technique proved to be the most effective. In the current paper, removal of the disperse dye using activated carbon was investigated. The controlling factors of the removal process were studied by experimental design. Initially, the parameters; namely: dye concentration, pH value, etching time, agitation speed, carbon concentration and organic to aqueous solution ratio were screened using Placket Burman design. Then the significant factors resulting from screening design were investigated using full factorial design to find the optimum removal conditions. The results indicated that dye concentration, carbon concentration and organic to aqueous solution ratio are the most significant factors among the screened ones. The complete dye removal can be achieved for low dye concentration, while for high dye concentration, the dye removal depends on the activated carbon concentration where the removal percent ranges from 51.7 up to 97 %.

A novel process for fabrication of nano-sized copper powders in organic oleic acid medium was developed employing a high-energy electrical explosion method. Formation of copper oxides was inhibited by the oleic acid medium during the electrical explosion of copper wires recovered from waste jelly-filled cable as a raw material for synthesis of nano-colloids and final dried nano-sized copper powders. High voltage of 320 V was repeatedly applied between the two horizontal electrodes for 3000 electrical explosions across the horizontal electrodes assembly. The copper powders were finally recovered via dispersion of the colloid by ultra-sonification and subsequent drying in an oven at 60 °C for 1 h and 5 h, respectively. The copper powders thus fabricated were analyzed using FESEM, BET surface area analyzer, TEM and elemental analyzer for determination of morphology, porosity and elemental purity. All of them turned out to be superior compared to similarly produced copper nano-sized powders in deionized water or ethyl alcohol (EtOH) media.