Volume 59, Issue 11-12

High strength bolts with large diameters are widely used in steel structures from the infrastructure and energy conversion sectors. The brittle fracture behavior of these connections has to be taken into account, especially for structures operating at cold environment, such as the offshore wind energy plant. Generally, two groups of methods are employed for cleavage fracture assessment, i. e., global approaches developed in the fracture mechanics frame and local criteria based on the damage mechanics approach proposed by Beremin. This work aims to determine characteristics of high strength bolt steels in the strength class 10.9 for cleavage fracture assessment. Fracture mechanics tests are conducted to determine the experimental master curves. In addition, numerical approaches are employed to derive the parameters of Beremin model from the master curves.

The addition of Ni to Mn-Co films was prepared by electroplating process. Film behavior under corrosive environment was investigated by oxidation techniques. The samples were kept under stagnant air at 800 °C in heat treatment furnace at operating temperature of SOFC for 100 hours. The surface morphology and films compositions were then examined. Mn-Co films with nickel amount of 18.8 wt.-% exhibited the best oxidation resistance. Ni-doped Mn-Co films show high antioxidation behavior because of high film density and crack-free film. The film is composed of many small particles with close packing, and the average particle size is about 1 μm. Film thickness is around 11 μm with surface roughness of 82.97 nm and it has good coherence with substrates. Optimization of nickel amount also plays an important role in antioxidation performance.

Corrosion behavior of a welded joint is complicated and can be strongly dependent on its local chemical composition and microstructure of the surface. To gain a thorough insight into the grooving corrosion behavior of welded joint, it is necessary to understand the corrosion mechanism of different regions of the welded joint. In this study, the influence of the lattice constant on the electron work function (EWF) and corrosion rate of base metal and two weld metals was investigated using a constant potential polarization approach and a scanning Kelvin probe (SKP). Experimental results showed that surface EWF decreased with increasing lattice constant, whereas the corrosion rate increased with an increase in lattice constant. At the same time, it was theoretically demonstrated that the lattice constant can affect the local EWF fluctuation of a welded joint. The fluctuation further leads to the corrosion rate difference of the different regions of the welded joint. So, the lattice constant change in the surface structure is a possible reason for the average grooving susceptibility coefficient difference of two kinds of welded joint. Besides, the alloying elements distribution of two kinds of welded joint zones should be a main reason for the average grooving susceptibility coefficient difference of joints verified by the EPMA measurements and the quantitative calculatinon of the contents of Cu, Ni and Si in the joints.

The basis for the study was the necessity to design a supporting structure of the road infrastructure subjected to high strain rates occurring during vehicle accidents. Mathematical models of hardening due to the high strain rates are reviewed. Commonly used methods for determining constants for one of the models, i. e., Johnson-Cook model are characterized. The main section of the study presents a proposed method for determining the strain rate sensitivity of the material. It is a hybrid method which requires Charpy tests using unnotched specimens and a series of calculations using the finite element method to simulate the tests. Several variants of strain measurement of specimen subjected to failure in Charpy tests, as required for the proposed method are analyzed. Using the suggested method, a strain rate sensitivity factor was obtained with a value similar to the value obtained using the split Hopkinson pressure bar technique. The advantages of the suggested method compared to the existing methods are the high availability of the instruments, simple and quick processing of the results and significantly lower costs.

The performance of grating interferometers coming up now for imaging interfaces within materials depends on the efficiency (visibility) of their main component, namely the phase grating. Therefore, experiments with monochromatic synchrotron radiation and corresponding simulations are carried out. The visibility of a phase grating is optimized by different photon energies, varying detector to grating distances and continuous rotation of the phase grating about the grid lines. Such kind of rotation changes the projected grating shapes, and thereby the distribution profiles of phase shifts. This yields higher visibilities than derived from ideal rectangular shapes. By continuous grating rotation and variation of the propagation distance, we achieve 2D visibility maps. Such maps provide the visibility for a certain combination of grating orientation and detector position. Optimum visibilities occur at considerably smaller distances than in the standard setup.

In this paper, friction and wear properties of nano-carbon reinforced Cu/Ti 3 SiC 2 /C nanocomposites are evaluated by orthogonal experiment method. The primary and secondary order of factors affecting wear characteristics of the nanocomposites and the best working conditions of specimens are obtained by means of range analysis and variance analysis. Results demonstrate that change of graphene nanoplatelets (GNPs) content has a very small effect on friction and wear properties of the nanocomposites under the premise of total content of GNPs and multi-walled carbon nanotubes (MWCNTs) is 1 wt.-%. Under these experimental conditions, the optimal level is that content is GNPs 0.8 wt.-% and MWCNTs 0.2 wt.-%, load is 350 N and rotational speed is 50 r × min −1 . Microscopic analysis shows that the additions of graphene nanoplatelets and multi-walled carbon nanotubes help to improve wear resistance of the nanocomposites. Main wear mechanisms of the nanocomposites are abrasive wear, delamination wear and oxidation wear.

The effects of austenization temperature on the microstructures and mechanical properties of a newly designed ultra-high strength low alloy martensitic steel were systematically studied. The microstructures of the martensitic steels which were quenched from different temperatures between 860 and 980 °C were investigated by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) and discussed. The results showed that the martensite laths were found to coarsen slowly and the carbide precipitates dissolved gradually with increasing austenization temperature. As the austenization temperature increased from 860 to 980 °C, the volume of retained austenite and the numerical ratio of high angle grain boundaries (HAGBs) were observed to increase while the numerical ratio of low angle grain boundaries (LAGBs) decreased. Rockwell C hardness (HRC), tensile strength and yield strength increased at first and then decreased, while impact toughness was greatly improved with increasing austenization temperature. The fracture mechanism was brittle fracture when austenitized at low temperatures, while it was ductile fracture when austenitized at high temperatures. The mechanical properties were significantly influenced by the formation of retained austenite, the dissolution of carbides, and the numerical ratio of HAGBs and LAGBs.

Findings about the microstructural features of, spring steels are necessary for the producers to enhance their mechanical properties. There are several reports revealing the basic relation between microstructure and fatigue performance. However, the results are commonly obtained from universal test procedures and have limited use due to the lack of real service conditions. In this study, the microstructural features of 51CrV4 alloy, used as spring steel component, were investigated by metallographic examinations starting from raw material to the final product. Its fatigue behavior was investigated using a self-designed test machine and a test procedure approved by the automotive industry to simulate the service conditions. Fractographic examination of fatigue failed surface was carried out to specify the effect of microstructural features on the fracture. It was concluded that (i) both oxide and decarburization layers were minimized by shot peening and (ii) although tested samples had superior fatigue resistance and failed above 10 5 cycles limit, oxide layer played a major role for crack initiation.

Being one of the most commonly applied sheet metal forming processes in automotive industry, deep drawing technologies are challenged by the concerns of global warming for higher fuel economy requirements in the recent years. To reduce the weight of the vehicles in order to obtain fuel economy, lighter and safer materials are used in automotive industry. Aluminum alloys, due to their low density compared to steels, are an important group of materials, in particular for lightweight construction of vehicles. In this study, two different aluminum alloys, namely AA5754-H22 and AA6061-T6, are evaluated for their potential use in nonload bearing applications in commercial vehicles by comparing their deep drawability characteristics. For this purpose, they are characterized by uniaxial tensile and Nakajima tests. In the next step, the deep drawing experiments are performed under different levels of blankholder force. The distance of the cup center to the edge of the die is also evaluated to investigate the formability of the designed cup for commercial use and to obtain the required product quality.

For austenitization, 100Cr6 bearing steel is generally heat treated in the two-phase area after the softening annealing process, and it is found that the austenitization conditions have crucial influences on the final microstructures and its properties. A mathematical model modified by the JMAK equation was introduced and used to investigate the influence of the heating rate on the kinetic parameters. Also, the mathematical model of transforming ferrite into austenite and the carbide dissolution was established based on the additive rule. The kinetic parameters have been fitted by combining them with the thermal expansion behavior, and their values are in good agreement with the experimental ones. The model was validated by these results.

In the present study, commercially pure micrometer-sized magnesium powder was reinforced with different volume percentages of TiO 2 nanoparticles using powder metallurgy process followed by hot extrusion. The powder metallurgy processing of the materials includes blending the powders together followed by compaction and high temperature sintering in inert atmospheric condition. Microstructural analysis of the extruded reinforced magnesium has revealed that the TiO 2 nanoparticles enhanced the recrystallization process in the magnesium matrix and also significantly refined the grain morphology. Analysis of X-ray diffraction spectrum of the extruded powder processed magnesium has revealed significant pyramidal plane texturing parallel to the extrusion direction due to the incorporation of TiO 2 nanoparticles. Texture coefficient calculated from the X-ray diffraction spectrum using Harris texture index formula revealed that the presence of increasing volume percentage of TiO 2 nanoparticles reinforcement gradually relaxed the strong alignment of basal plane and enhanced the pyramidal plane texturing parallel to the extrusion direction.

The composite parts bonded with embedded single lap joint (SLJ) subjected to tensile load were analyzed previously. The embedded adhesive was applied between two composite parts and the mechanical behavior of the joint was examined. In this study, composite parts bonded with single-lap joint (SLJ) and double-lap joint (DLJ) subjected to tensile load were analyzed by using 3D finite element method (FEM). The advantages of two different joint methods were investigated. In the analysis, the composite parts were carbon/epoxy (AS4/3501-6) with different fiber orientation angles and the adhesive was DP 410. The models for the numerical analyses were generated by using the ANSYS software package. The finite element analyses (FEA) were carried out to predict the failure loads. Stress in x-, y-, z-directions, shear stresses and the von-Mises stresses on adhesive were obtained at time of failure for predetermined parameters. Consequently, the effects of orientation angles, overlap dimensions and joining methods were examined. The most effective parameters and joining process were determined for composite parts with single-lap and double-lap joints.

In this study, bending response of circular aluminum tube and factors affecting collapse behaviour were investigated by FE simulations. Geometrical parameters such as wall tube thickness, tube diameter, support distances, punch diameter and material properties were considered in analyses systematically. The results of analyses revealed that local buckling plays an important role on collapse load and buckling displacement is directly proportional to tube diameter, regardless of D/t and yield strength of tube material. The ratio of buckling displacement to tube diameter corresponds to 40 % of tube diameter. Moreover, an attempt was made to construct an equation as a function of D, t and yield strength for estimating collapse load. Finally, ANOVA test was used to analyze the influence of some parameters such as diameter of tube, wall thickness and yield strength. It is found from the test that the most significant factor on maximum load is wall thickness (79.6 %).

In this work, effect of spark plasma sintering (SPS) temperature on structure and phase composition (SPC) of Ti-Al-Nb-based alloys produced by SPS technique at 1000 to 1550 °C has been investigated. X-ray phase analysis, raster electron microscopy and energy dispersive X-ray spectroscopy with determination of element distribution were applied to study sintered alloys. It was found that microstructure of alloys sintered at 1000 °C and 1200 °C is heterogeneous. These alloys consist of Ti 3 Al grains and Nb 2 Al phase as well as dispersed separations of Ti 2 AlNb phase and unreacted particles of niobium and titanium. Results of structure and phase analyses showed that these alloys become homogeneous and laminar with increasing sintering temperature.

Heat dissipation plays an important role in improving luminous efficiency and reliability for light emitting diode (LED) devices. In this paper, graphene/epoxy composite is synthesized and used as thermal interface material for LED heat dissipation enhancement. Experiment is conducted to test the interface resistances of the novel composite materials with different graphene mixing ratios. Moreover, the thermal performances and junction temperatures of LED device with and without such thermal interface materials are analyzed and compared to show the heat dissipation effect of the proposed composite thermal interface material. The results show that thermal conductivity of graphene/epoxy composite increases with growing graphene mixing ratio and the interface resistance can be decreased by more than 70 % with the composite TIM. The case study indicates that junction temperature decreases from 72 °C to 67 °C for a 10 W power LED after using TIM, and such temperature difference becomes larger for higher LED power. Furthermore, with graphene/epoxy composite thermal interface material, temperature is distributed more uniformly in the heat sink, also benefitting LED heat dissipation. This work is important for practical heat sink design and optimization for LED equipment.

The design approaches are not rated for gear tooth volume, the designers are not aware of the loss or gain on the cost, and failure or success of the design. This study provides a novel method to rate and convert textbook gear design approaches into ANSI/AGMA standard. The steel pinions with lowest and highest strengths covering minimum and maximum strengths available for the gear design meshing with cast iron gear were used in order to ensure that the method presented is generic. The method relies on obtaining design data (module m (mm) and face width F (mm)) for the most common design approaches considering bending fatigue. The gear design approaches are rated for the loss or gain using dimensionless geometric rating numbers, and then dimensionless conversion factors are obtained. Mean values of conversion factors independent of input power for the desired speed ratio are used to derive correlation equations for converting the design results of the selected approach to ANSI/AGMA standard. The method verified by case studies is generic and applicable to any gear design approach. Now, the most commonly used spur gear design approaches are rated by considering the gear volume. Furthermore, m and F obtained from the easiest textbook design approaches can be easily converted into the most accurate but more complicated, challenging and time-consuming ANSI/AGMA standard with minimum effort and error. It does not only allow saving time and resources but also provides safer and reliable designs for designers who are not bound to use a specific gear standard.

Carbon black is the typical reinforcing agent used to enhance the physico-mechanical properties of rubber compounds. 40 to 60 wt.-% of carbon black is required to achieve the desired properties for tubeless tire liners and tire tread. Fly loss of carbon black particles during fabrication directly affects the respiratory tract of the human. Carbon is a type of carcinogen that induces lungs cancer, when exposed for a certain period of time. The research work aims in minimizing the quantity of carbon black in the rubber compound and compensating the physico-mechanical properties through minimal loading of nanoclay. Butyl rubber (IIR) nanocomposites are prepared using organically modified Cloisite 30B type nanoclay and carbon black. In order to overcome the compatibility mismatch between nonpolar IIR and polar nanoclay, chlorobutyl rubber (CIIR) is used as a compatibilizer. Morphology, cure characteristics, mechanical and gas barrier properties of the prepared rubber compounds are analyzed. Faster cure time, increase in torque, mechanical and gas barrier properties were achieved for dual filler (nanoclay and carbon black) containing rubber compound. The results prove that minimal quantity of nanoclay helps to minimize bulk quantity of carbon black by retaining the desired physico-mechanical properties.

The dry sliding wear behavior of polymer composite with hallosyite nanotube has been investigated. According to the response surface methodology, the experiments were conducted at ambient conditions using a pin-on-disc machine with a steel counterpart. Sliding velocity, normal applied load and volume fraction of nano fillers were considered as influencing factors. For the different responses, statistical models were developed. An analysis of variance (ANOVA) was performed to ascertain the significance of the individual factors. The results indicated that at higher normal applied load and sliding speed, wear rate is higher. An increase in volume fraction of nano fillers decreases the wear rate. The various wear characteristics of the worn composite surfaces were analyzed using scanning electron microscope (SEM).

Polymer matrix composites (PMCs) play an increasingly important role in everyday life from energy conservation to national security. Recently, PMCs have been found to be used for sanitaryware due to their low density, giving a sense of warmth, which allows application for broad aesthetic features, high impact resistance, the ease of shaping and the possibility to produce thin sections. It is an alternative to ceramic sanitaryware products. While thin section PMC products range in the forefront with lightness and aesthetic features, thick section PMC products are aroused interest because of the impact resistance and ease of production. Little attention is devoted to the characterization of PMC sanitarywares up to date. Therefore, in order to identify manufacturing processes and determination of the chemical and mineralogical composition, optical microscopy, scanning electron microscopy (SEM) with SE, BSE and EDX detector, X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF) and thermogravimetric (TG/DTA) analysis of commercial PMC sanitarywares with two different structures (thick and thin sections) were carried out. Mechanical properties such as three-point flexural strength, flexural modulus and impact resistance were identified. As a result of these studies, matrix to filler ratio, filler shape, size and composition and manufacturing technique of PMC sanitarywares were determined. Such understanding is necessary in order to satisfy the market request and to improve the properties of the sanitarywares.

The 3D digital image correlation technique has been employed to measure the 3D profile and normal deformation of the marble free surface of Jinping hydropower station under biaxial compression to ensure the safety of the engineering structure. The 3D profile is consistent with the pattern of rock cutting, which shows that the digital image correlation method shows a high precision of engineering measurement. From the diagram of the displacement gradient and the load, it is concluded that the displacement gradient increases sharply before the destruction of the specimen, which proves that the destruction and the displacement gradient has close correlation. Due to the fact that the displacement contour is a relatively dense area which is more prone to produce tension effects than the maximum normal displacement, it is easier to destroy the cracks. Based on the above conclusions, the distribution of free surface contours of the displacement field can determine the area which is more likely to show free surface damage cracks.