Volume 59, Issue 9

This research investigates effects of contact pressure on multiaxial fretting fatigue behavior of Al-Zn-Mg alloy which has a wide variety of applications in automobile and rail transit engineering. Al-Zn-Mg alloy is exposed to cyclic loadings and contact pressure. The cyclic stress-strain curve and displacement amplitude curve under different contact pressure of Al-Zn-Mg alloy were investigated using point contact multiaxial fretting fatigue test equipment. Based on cyclic stress-strain curve and displacement amplitude curve under different contact pressure, fretting wear characteristics, fretting fatigue fracture properties and fretting fatigue lives as well as the effects of contact pressure on multiaxial fretting fatigue damage and fretting fatigue fracture mechanisms of Al-Zn-Mg alloy were extensively analyzed.

Graphene oxide (GO) was prepared by oxidation reaction of graphite (GF) using the Hummers’ method and modified by a cationic surfactant (cetyltrimethylammonium bromide – CTAB) followed by chemical reduction using hydrazine monohydrate (N 2 H 4 × H 2 O) to obtain MGNO powders. The structural and thermal properties of GO and MGNO powders were studied using zeta potentiometer (ZP), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results demonstrated that CTAB was successfully grafted onto the surface of the GO through non-covalent modification between CTA + ions and GO for all MGNO powders. A reasonable improvement in thermal stability of the MGNO powders was also observed. FTIR results showed the successful intercalation of CTA + ions for all MGNO powders. The optimum concentration of surfactant was found to be 120 mg × l −1 (MGNO-120) by using analysis results. The ease of synthesis of MGNO-120 with both excellent intercalation and thermal stability will greatly facilitate potential applications.

In the present study, monotonic tension tests and low cycle fatigue tests were performed on aluminum alloy EN AW-2024-T3 under various operating temperatures. The results were assessed in order to determine the Bauschinger effect in the aluminum alloy under elevated temperatures. Findings of monotonic tests served as a basis to compare the influence of temperature on mechanical properties of the material. The results of the cyclic tests were used to determine the Bauschinger effect. Finally, the changes in the presence of the effect at various testing temperatures were presented.

The combination of W and Ta is expected to be highly beneficial for many applications from aerospace, weapons, military and nuclear industry. In this paper, W and Ta alloys were successfully diffusion bonded with Ni interlayer. The process of the formation of W/Ni/Ta diffusion bonded joints was investigated by means of scanning electron microscopy, X-ray diffraction system, electron probe micro-analyzer, energy dispersive spectrometry and shear strength measurement. The results show that the shear strength increases when the bonding temperature increases and exhibits a maximum value of 244 MPa at 930 °C. The bonding of W/Ni can be attributed to the bonding of Ni to tungsten grains and the bonding of Ni to a Ni-Fe-binder mainly by elemental diffusion. The fracture takes place in the Ni/Ta interface and Ni 3 Ta and Ni 2 Ta intermetallic compounds are formed on the fracture surfaces.

Borated stainless steels are being used to control neutron criticality in reactors as control rods, shielding material, spent fuel storage racks and transportation casks. Borated stainless steels fabricated via powder metallurgy (P/M) route (grade A) exhibit superior neutron absorption, enhanced toughness and ductility compared to cast ingot metallurgy (I/M) steels (grade B), due to fine and uniform dispersion of borides. In this study, welds were made using both 304B grade A and grade B materials by gas tungsten arc welding and activated gas tungsten arc welding processes. The effect of the activated flux on the bead geometry, microstructure and mechanical properties was investigated and compared. The heat input required for P/M steel material to produce full penetration welds has been found to be significantly higher compared to I/M steel, while all welds exhibited high joint efficiencies. Metallographic examinations revealed that no porosities and shrinkage cracks have been observed either in the fusion zone or in partially melted zone of the weldments. The fusion zone structure of 304 P/M type consists of a higher amount of eutectics with less number of austenite dendrites compared to I/M type. Both P/M and I/M steel welds exhibited partially melted zones adjacent to fusion zone. P/M steel welds were found to fracture in the partially melted zone, where a significant loss in hardness was noticed due to irregular boride eutectics, while in case of I/M steel welds, failure occurred in base material. It has been concluded that the P/M steel 304B grade A exhibits good GTA weldability compared to grade B material.

The epoxy resin-bonded lead-steel structures are widely used in nuclear industry for the protection of intense radiation, and the accurate detection of the interfacial debonding is very important. This paper studies the detection of interfacial debonding in the epoxy resin-bonded lead-steel structure using laser ultrasonic method. Two-dimensional finite element method is used to simulate the propagation of laser generated ultrasonic waves in lead-steel bonded structure and the interaction with interfacial debonding. The wave reflection and the attenuation occurred at the good bonding interface and the debonding interface are clearly shown. The reflection and attenuation of laser ultrasonic induced by the interfacial debonding are analyzed theoretically and experimentally, and the feature signals for the characterization of interfacial debonding are extracted. The appropriate wave frequency and measurement position are analyzed. The C-scan image of the specimen is obtained using the laser ultrasonic method. The results show that the laser ultrasonic method is effective for the noncontact detection of interfacial debonding in epoxy resin-bonded lead-steel structure, and the interfacial debonding with a diameter larger than 4 mm can be imaged clearly with laser ultrasonic C-scan method.

This paper describes the effects of deep cryo treated high speed steel. In recent research, the cryo treatment has been acknowledged by improving the life or quality performance of tool materials. The molybdenum based high speed tool steel is frequently used in industry till date. Therefore, it is essential to observe the tool performance in machining of medium carbon steel (AISI 1045) on dry turning operation. The effect of untreated and deep cryo treated tool in turning of medium carbon steel is analyzed using Taguchi approach. The results indicated that the main cutting force obtained from cryo treated tool significantly decreased to 29 % when compared with the untreated tool. Also, it revealed that the surface roughness of machined component using deep cryo treated tool was better than of the untreated tool.

In this study, the old and well-known alloy 718 is compared with the newly developed ATI® 718Plus™ from the weldability point of view. This is done in order to gain new information that have not been documented and established yet among the high-temperature materials with high strength, oxidation resistance, thermal stability and sufficient weldability, yet. ATI® 718Plus™ shows a lower sensitivity to hot cracking than alloy 718 with approximately 10 mm total crack length (TCL) difference in Varestraint testing. In the solution-annealed condition at 982°C for 4.5 h followed by air cooling, the crack sensitivity is decreased as compared to the mill-annealed condition. Along the crack path and also ahead of the crack tip, γ-Laves eutectic is present in both alloys. The microhardness measurements showed similar hardness level of 250 HV in the weld metal of both alloys and even in the parent material of alloy 718. ATI® 718Plus™ parent metal had hardness of 380 HV and a small increase of less than 50 HV was observed for both studied alloys in the heat affected zone (HAZ). For the same grain size of ATI® 718Plus™ (8.3 μm) and alloy 718 (15.6 μm), the susceptibility to liquation cracking may increase with increasing grain size. With a small grain size, there is a possibility to accommodate more trace elements (B, S, P) due to the larger grain boundary area. The impurity elements were found in relatively small precipitates, typically borides (0.2 μm), phosphides (0.1 to 0.5 μm) and carbo-sulphides. The solidification sequence of alloy 718 and ATI® 718Plus™ is relatively similar, where the liquid starts to solidify as γ-phase followed by γ/MC reaction at about 1260 °C and then final γ/Laves eutectic reaction at around 1150 °C. Detailed knowledge about weldability of alloy 718 and ATI® 718Plus™ can be used for material selection.

The strength and mechanical responses of C/C composite open-hole and bolted plates were investigated in this paper. The tensile strength of C/C composite open-hole plates was experimentally tested and predicted by the point stress criterion. The effect of hole diameter on tensile strength of C/C composite open-hole plates was studied. The comparison of the predicted and experimental data indicated that the measured tensile strength was in reasonable agreement with the predictions. The bearing characteristic of C/C composite bolted plates was tested, and the effect of ply sequences, bolt diameter and torque on bearing properties was analyzed. The damage mechanism of C/C composite bolted plates was revealed.

Glass fiber reinforced plastic (GFRP) composite materials are widely used in novel industries. They are generally used in electrical environments due to their high thermal strength and mechanical resistance under present conditions with lower weight. In most of the applications, the GFRP is subjected to screwing for assembling. In this study, the pull-out strength performance of the screw was investigated in view of thread modification. In order to determine the optimum thread minor diameter, several pullout tests were performed. Therefore, the modified values showed significantly a better performance than standard values. This novel design was compared with numerical analyses in ANSYS and it was observed that numerical results had a good agreement with experimental results.

The ash fusion temperature determines the suitability of coal for the entrained flow gasification/combustion process. For this purpose, Indonesian KPU coal samples and their ash and slag contents were comprehensibly characterized via X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), and inductively coupled plasma optical emission spectrometry (ICP-OES) methods. To assess their environmental impacts, leachate solutions were prepared from the discharged slag for ICP-OES analyses. The experimental data were comprehensively explained in terms of ash fusion temperatures and fluidity properties of the coal ashes. More specifically, the coal slagging behavior inside of coal gasifier/combustors necessitated optimization of the operational conditions for subsequent mild slagging. The currently used coal gasification operation temperature ranges favored mild slagging at 1200 to 1300 °C (19.9 to 20.1 bar) and incomplete slagging at 1100 to 1200 °C (19.7 to 20.4 bar), which was attributed to the insufficient gasification/combustion temperature for the coal samples under the latter gasification/combustion conditions. The metallic elemental composition analysis results via ICP-OES performed on the leachate solution therefrom excluded the possibility of secondary environmental pollution from the discharged slag.

In times of industrialization, much low temperature waste heat is released and no viable technology exists which can produce electricity from this low energy density heat. So the long sought-after class thermoelectric (TE) material which directly achieves conversion between thermal and electrical energy obtains much attention. However, the traditional TE materials are alloys of inorganic materials and expensive, and most of them have some level of toxicity, so the research on organic TE materials is very important. The polyaniline (PANI, i. e., a conducting polymer) and PANI matrix Ge 0.94 Pb 0.01 Bi 0.05 Te composite material were prepared. The chemical structure, microstructure and thermoelectric properties were investigated by FTIR, XRD, SEM and ZEM. Results showed that the molecule chains in the PANI were not ranged very neatly, and Ge 0.94 Pb 0.01 Bi 0.05 Te in the composite material formed interconnected network as 20 wt.-% Ge 0.94 Pb 0.01 Bi 0.05 Te was added. Power factor of the composite material increased greatly while its ZT was almost two times of PANI. The addition of Ge 0.94 Pb 0.01 Bi 0.05 Te was an effective method to increase the thermoelectric properties of PANI.

Literally, hard machining describes machining of parts having hardness over 45 HRC. Besides its advantages like high metal removal rate, easiness of adapting to complex part geometries, possibility of dry cutting; this operation, which can substitute grinding in most cases, has some disadvantages. One of them is the significant increase of surface roughness due to tool wear even when the tool life limit is not exceeded. In this study, considering hard turning praxes, higher depths of cut (0.5 to 1.0 mm) were examined when dry turning AISI D2 cold work tool steel, through-hardened to 62 HRC. TiN coated mixed ceramic inserts (Al 2 O 3 + TiCN) were employed in the operations. Relationship between surface roughness and cutting parameters (cutting speed, feed and depth of cut) was modeled and analyzed using a Box-Behnken response surface methodology (RSM) design. A linear model best described this relationship. Despite the higher depths of cuts, the surface roughness values achieved were comparable to those in grinding operations. Finally, the optimal values of cutting parameters for minimum surface roughness were predicted.

Extensive demands in petroleum products and the presence of these substances in large areas of industrial and nonindustrial sectors cause contamination of some soil layers by different means. It is not always possible or the only way to remedy these layers prior to using them for engineering purposes. Instead, considering the changes happening in their mechanical properties may result in a more economical and optimal utilization. Among all petroleum products, gas oil due to the high volume of product and wide use in industry, transport and agriculture, has the largest share in the pollution of contaminated sites. The important point is that the overflow and spills of many diesel engines and gas oil tanks penetrate the bedding above the foundations of mentioned machines and facilities. These foundations which are designed based on the geotechnical parameters of the bed soils, must have a satisfactory performance through the years under various loadings. In this study, extensive physical and mechanical tests on remolded samples of pure and sandy clays contaminated with different gas oil contents have been performed to examine the influences of gas oil on the mechanical properties of sites consisting of fine soils. Results indicate a decrease in Atterberg limits and an increase in maximum dry density with increasing gas oil content. The shear strength parameters (cohesion and friction angle) of fine soils both exhibit a turning change point at 8 % gas oil content, while their variations trends are quite opposed.

The initiation and evolution of the cracks around the hole in coal specimens with a central hole under different uniform loads are studied by the rock failure process analysis software RFPA2D. The results show that homogeneity not only has an influence on the peak strength of coal specimens, but also on the crack propagation directly. The lower homogeneity, the more extensive the stress concentration areas, the rougher the surface crack, and the more irregular the crack propagation path. However, homogeneity has a certain impact on acoustic emission (AE) number and instantaneous effective coalescence rate of cracks, but the influence degree is significantly lower than that under nonuniform load gradient and original cracks. Nonuniform load can cause the obvious stress concentration on the higher load area of the coal specimens (right side), which mainly reflects the micro-crack initiation in the area, and the greater the nonuniform load gradient, the more significant is this trend. The peak stress of models does not change with the change of load forms (the change from uniform load to nonuniform load), but nonuniform load will accelerate the effective coalescence of the surface crack, and the larger the nonuniform load, the shorter the time to achieve the effective coalescence of the surface crack. The original cracks with a certain inclination weaken the stress concentration around the hole. When the dip angle of original crack reaches a certain range (such as 45°), this trend will be more obvious. But it is not closely related to the homogeneity of the coal and the size of the load gradient, and has a certain universal applicability. Finally, the stress around circular tunnel is analyzed based on the theory of elastic mechanics, and a criterion for judging the crack initiation around the laneway is proposed.