Volume 61, Issue 4

Weight-optimized component design as well as a reliable estimation of the lifetime of metallic materials and components requires a comprehensive understanding of fatigue processes and a systematic investigation of the underlying fatigue behavior. Therefore, nondestructive testing methods, digitalization of measurement techniques as well as signal processing can be combined with a short-term procedure in order to acquire potentially more information about fatigue processes, while experimental effort and costs are reduced significantly. This leads not only to considerable advantages over conventional methods for determining S-N curves, but also over established short-term procedures, due to the possibility of applying this information from just a few specimens to attain fatigue life calculations. The StressLife tc approach is a new short-term calculation method which considers the nonlinear relation between the elastic, elastic-plastic and plastic portion of the material response in the deformation process. Within the scope of the present work, the change in temperature of SAE 1045 (C45E) specimens was measured during fatigue tests via an infrared camera in order to feed the thermal response back into the new StressLife tc approach for a reliable fatigue life calculation.

This article researches the formation of ceramic coatings of zirconium dioxide (ZrO 2 ) on a X 12 Cr Ni 18 10 Ti steel surface after mechanical alloying. The study of the coating surfaces shows a coalescence of defected coating particles with subgrains created. This process is more representative due to the difference in toughness between zirconium oxide and steel. It is suggested that an adhesive bond between the coating and support structure is provided by the strain welding of the powder on the support structure coating. When using X-phase analysis, neither the diffusive mixing of coating/support structure components nor the formation of new compounds and phase changes are revealed. Surface contamination of the coating with carbon was found. It decreased in the direction of the border between the coating and the support structure. This may be due to the mechanical sorption of carbon dioxide from the environment.

The near-threshold crack extension mechanisms of long cracks in a commercial aluminum alloy in peak-aged and overaged condition were evaluated at a stress ratio of R = −1. Tests were performed both in a conventional resonant testing machine, using flat dog-bone specimens with through-wall cracks and on an ultrasonic fatigue testing system using hourglass-shaped samples with surface cracks. After introducing the initial crack, the crack was propagated at a nominally constant K max with values in the range of the initially determined threshold. The primary precipitates were found to act as microstructural barriers causing a local pinning of the crack front independent of the testing procedure and hence keeping the crack from extending continuously. This effect depends on the orientation of the initial crack with respect to the rolling texture which, in turn, determines the spatial distribution of the primary precipitates and the grain morphology. Overaging had a tendency to enhance the pinning potential of the primary precipitates.

Ultra-high strength steel 22MnB5 with 1.5 mm thickness was laser beam butt welded under varied welding parameters such as focal diameter, welding speed and beam power. The ultra-high strength material is softened by the laser beam welding process due to tempering in the heat-affected zone, which causes a loss in tensile strength and hardness compared to the base material. The influence of softening on the mechanical-technological properties was investigated. Hardness profiles of welded specimens were examined and tensile tests with digital image correlation were carried out. The digital image correlation showed the area in which the strain was concentrated as well as the location of fraction initiation. The microstructure was examined using micrographs and scanning electron microscopy images; the composition of the structure was subjected to X-ray diffraction. Dilatometry was used to map the individual areas of the heat-affected zone with respect to temperature and hardness. An increase in the energy per unit length influenced the width of the heat-affected zone and thus, the width of the softened zone. It was not possible to exert influence on the depth of hardness drop. The tensile strength, however, increased due to the narrower width of the softened zone.

Two types of reactive powder concrete beams, one with normal mild steel and another with unbonded prestressing tendons, were studied experimentally. Specifically, static load and constant amplitude fatigue tests were performed. The beam cracks, midspan deflections, and variations of extreme concrete and steel strains with fatigue loading cycles were compared. The fatigue test results show that fatigue life is more than 2 million cycles for both types of RPC beams within the usual low fatigue stress range (98 % probability of appearance). The crack width, maximum compressive strain, midspan deflection, and maximum tensile strain are 0.08 mm, 814 μξ, 7.6 mm, and 484 μξ, respectively for unbonded prestressed reactive powder concrete beams. They are 0.12 mm, 1147 μξ, 10.3 mm, and 1226 μξ, respectively for ordinary reinforced reactive powder concrete beams. All of these values obtained from the fatigue test are significantly below the corresponding failure values under the static test. The study results show that an unbonded prestressed reactive powder concrete beam has better fatigue performance than an ordinary reinforced reactive powder concrete beam.

Due to their advantages of high porosity, large specific surface area, and abundant surface topography, porous fiber-sintered plates are widely used in microreactors. The fiber surface in the cutting process generates a rich micro-surface morphology which can effectively improve the catalyst load and promote reaction efficiency. There is currently a lack of quantitative analysis of the microchannel surface topography features. In this study, a new image segmentation technique based on level set algorithms was developed to study the microscopic features of the metal fibers processed under different processing parameters, allowing for the establishment of a matching relationship between the microscopic roughness of the fiber surface and the equivalent diameter of the fibers. The experimental results show that the surface topography characteristic coefficient (STCC) was approximately inversely proportional to the cutting speed (r × min −1 ), which was approximately proportional to the feed (mm × r −1 ). The cutting depth (mm) effect on the STCC value of the fibers was not obvious. At low speed, with a cutting speed n = 20 r × min −1 , feed rate f = 0.08 mm × r −1 , and cutting depth a p = 0.11 mm, the STCC of the metal fiber reached a maximum value of 115.927. At high speed, with a cutting speed of n = 400 r × min −1 , feed rate f = 0.06 mm × r −1 , and cutting depth a p = 0.17 mm, the metal fiber STCC reached a maximum of 128.605. The parameters corresponding to the maximum STCC value can be used as the optimal turning parameters of metal fiber material for making a porous fiber microchannel reactor.

Electrofusion welding and butt fusion welding are common methods for joining polyethylene pipes used for water and gas distribution. Because of the wide use of these two methods in infrastructure engineering, welded pipe joints and pipe materials should be examined with the intent of reducing errors in projects. The purpose of this study is to increase knowledge of electrofusion and butt fusion welding methods and examine them in order to establish which offers the best performance for a specific engineering service project. Tensile and hydrostatic pressure tests are performed on un-welded and welded pipes for high-density polyethylene pipes. Additionally, a numerical analysis of the experimental results is conducted by employing the ANSYS program for tensile test samples. In tensile tests, according to the maximum average load, electrofusion welding values are larger than those of butt fusion welding and the elongations of the un-welded pipe at the fracture in butt fusion welding are greater than in electrofusion welding. For short-term hydrostatic pressure tests, the resistance in butt fusion welding for defects of roundness and alignment is better than that of electrofusion welding.

Today, ultrasonic welding is used in many different areas due to ease of application compared to other welding methods. Computer, electric, automotive and aerospace industries are the leading areas for these applications. With its ability to easily join materials like Al, Cu, Ni, ultrasonic welding has become a preferred method for use in the welding of solar collectors which are components in solar energy systems. The system parameters for welding solar collectors must of course be adjusted according to the properties of the materials used so that the heat transfer between specimens is not affected. In this study, the selective coated plate and the heat carrier pipes of solar collectors were welded using ultrasonic seam welding, and the optimum welding parameters were determined according to the Cu-DHP copper alloy used. For this purpose, copper specimens were welded using welding speeds of 4, 8, 12 and 16 m × min −1 and amplitudes of 4, 5 and 6 μm, while pressure and frequency values were fixed. The shear strength values of these welded specimens were calculated, and images of the weld areas were obtained with the help of a scanning electron microscope. Through hardness measurements made on the interfacial region of the welds, it was concluded that optimal welding performance was obtained at an amplitude of 4 μm and a welding speed of 12 m × min −1 . At these parameters, a mean shear strength value of 0.1386 MPa and a mean hardness value of 112 HV were measured.

Nanocomposites have attracted much attention from scientific and industrial areas in recent decades because of their mechanical, physical, thermal and barrier properties and unique design possibilities. These materials have been commercially used in the automotive, electronic, biomedical, packaging etc. industries and their usage areas have been increasing day by day. In this study, nanocomposite adhesives were produced by adding nano graphene and multi-walled carbon nanotube (MWCNT) particles to a single component epoxy adhesive at the ratios of 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1, 3 and 5 wt.-%. Then, these nanocomposite adhesives were used to prepare single-lap joints. Shear tests were performed to specify their adhesive shear strengths and joint tensile strengths. It was observed that mechanical strength increased to the reinforcement amount of 0.5 wt.-%. After that, the strength values decreased due to a tendency towards agglomeration, viscosity increment and due to a difficulty in homogeneously applying the adhesive. Furthermore, it was found that nano graphene reinforcement improved the mechanical strength of the epoxy-based adhesive slightly more than MWCNT reinforcement.

The blank holder force (BHF) plays an important role in the control of material flow. Low BHF leads to wrinkling, while high BHF leads to tearing at the part in the deep drawing process. Therefore, BHF control along the blank periphery is very crucial for the success of the deep drawing process. In this paper, spatial variable blank holder forces in the deep drawing of a double bowl sink were predicted by using a method based on a sequential response surface and hybrid algorithm. Predicted blank holder forces from the method were verified on a multipoint control hydraulic press and a 26 % improvement in the minimum thickness of the part formed as compared to the BHF constant was obtained.

Through a coal mass sample with a rich microstructure and a self-designed experiment using lump coal to determine the characteristics of gas emission, this paper analyzes gas emission in a coal mass with rich microstructure. The results show: The differences between raw coal and a briquette are mainly found in the scale and number of their microstructures. Nevertheless, the two samples share a high degree of similarity. The gas emission speed of a coal particle is higher than that of coal mass with a rich microstructure. The decay rate of gas emission speed also is confirmed as the same process. It takes longer for coal mass with a rich microstructure to reach the limit of its gas emission volume than for coal particles. Gas emission in a coal mass is a continuous process of gas desorption, migration and seepage. During the process, the coal mass reacts in four possible ways: coal mass immediately participating in gas emission, gradually participating in gas emission, providing a gas source for gas emission and as a non-affected coal mass. The relation between the gas emission speed of the coal mass with a rich microstructure and test time shows a monotone decrease in the power function, and the relation between the cumulative volume of the gas emission and the test time reveals a monotonically increasing logarithmic function. The gas emission for coal mass with a rich microstructure can be well expressed by modified logarithmics.

The main objective of this paper is to study the effect of water absorption on the flexural strength of new green sandwich composite material made of flax as skin reinforcement and agglomerated cork as core. In this work, three different types of skin reinforcements and three different densities of agglomerated cork core have been used for the manufacturing of sandwich composites. The manufactured sandwich composites were subjected to three-point bending tests under both dry and wet conditions. The three-point bending results confirmed a decrease in the specific flexural strength by 14-42 % in wet samples due to water absorption. Also, the increase in core density leads to an increase in flexural strength both in dry and wet conditions. During testing under dry conditions, the specific flexural strength of flax reinforced sandwich composite was found to be superior because of the higher strength to weight ratio factor of flax fiber when compared to a glass reinforced sandwich composite and glass/flax reinforced hybrid sandwich composite. Due to the hydrophilic nature of flax fibers, flax reinforced sandwich composite has a reduced specific flexural strength in wet conditions when compared to glass reinforced sandwich composite and glass/flax reinforced hybrid sandwich composite.

Application of polymer matrix composites in mechanical systems, where relative motions are involved, requires an investigation on the tribological performances of the composites. The prime objective of this work is to identify the optimum combination of input parameters such as speed N (rpm), load L (N), distance D (m) and material for minimum weight loss and minimum coefficient of friction. The performances are measured after performing experiments (full factorial) using pin on disc setup. Wavelet based pseudo color scaling and wavelet fusion methods are adopted to identify the optimum conditions. Accordingly, high speed, low load, moderate distance applied on epoxy with glass fiber and carbon particles is found to be optimum for minimum weight loss and minimum coefficient of friction.

Basalt fiber is a new type of high quality reinforced material used in composite materials. The performance of cement-based materials can be improved by adding basalt fiber, but its damage mechanism is more complicated. Acoustic emission technique can monitor the internal damage of materials. In this paper, the influence of the basalt fiber on the compressive failure process of mortar and the effect of the basalt fiber volume fraction variation on the acoustic emission characteristics in the damage process were investigated. The characteristic parameters of the acoustic emission signals of basalt fiber reinforced mortar with 4 different volume fractions under axial load were analyzed. Through analysis of the acoustic emission cumulative hit number, the acoustic emission cumulative energy, the RA value, the AF value and the b value, it could be found that the acoustic emission parameters of different damage stages were different, and the failure mode of the specimen could be identified according to the acoustic emission parameters. Besides, the addition of basalt fiber improved the rigidity and the ductility of the specimen.