Volume 62, Issue 1

Concrete is an important engineering material whose tensile property plays an important role in structural safety. Thus, the effect of strain rate on crack evolution in concrete during tension cracking cannot be neglected. Within a strain rate range of 10 −6 to 10 −4 s −1 , an acoustic emission monitoring test for the whole process of concrete under uniaxial tension including the post-peak softening stage was conducted. Moreover, damage evolution, along with the cracking mechanism of concrete at various strain rates was discussed with respect to the effect of strain rate on acoustic emission. The results show that the acoustic emission activity of concrete is delayed due to an increase in strain rate. This indicates that the hysteresis of deformation and cracking can be observed in a uniaxial tension test of concrete. During the whole loading process, as the strain rate increased, the average level of the acoustic emission hit rate increased significantly, indicating that an increase in strain rate accelerates crack initiation and propagation in concrete. Average acoustic emission values, duration and energy also show an increasing trend, and the scatter distribution range between them and the acoustic emission amplitude changes significantly, a fact that can be used to identify the damage degree of concrete under different strain rates. A peak frequency and cd4 band wavelet energy spectrum coefficient tends to decrease, while the ca8 band wavelet energy spectrum coefficient increases. In other words, the proportion of low frequency acoustic emission signals increases, indicating that an increase in strain rate increases the proportion of macroscopic cracks in concrete. At various strain rates, the average level of acoustic emission characteristic parameters and the proportion of high frequency signals at the post-peak stage in concrete are higher than those at the pre-peak stage, indicating that the development of microcracks in concrete mainly concentrates during the post-peak softening stage.

The effect of the hybridization of basalt and carbon fabrics on physical and mechanical properties of the thick carbon and basalt hybrid woven composites manufactured by compression molding was investigated in this study. Monotype (all carbon and all basalt) and their hybrid woven fabric layer combinations were used to reinforce polyester resin. However, the effect of the Graphene Nanoplatelets (GNPs) reinforcement was also examined in polyester matrix by adding different volume fractions between 0.1 and 2 wt.-%. Afterward, the optimized GNPs at the ratio of 0.1 wt.-% was used in one of the hybrid composite combinations (10C10BGNP) in order to improve fiber matrix adhesion. The specimens were subjected to three different tests including tensile, flexural and impact tests as well as morphological characterization by SEM and XRD. Results showed that tensile strengths of thick carbon/basalt hybrid composites are higher than the tensile strengths of thick monotype carbon and basalt composites. The best tensile properties were achieved in 14C6BP hybrid composites while the highest flexural performance of 501 MPa and the highest impact energy of 14.32 J was achieved in 10C10BP hybrid composites with the addition of GNPs. Moreover, the ratio of the basalt fiber content increased the impact energy of hybrid composites to a level comparable to that of all basalt fiber composites.

Additive manufacturing allows for the production of highly complex structures due to its layer-wise local melting of powder material. For this reason, this technique has a high potential for manufacturing extremely lightweight components potential. However, laser based additive manufacturing is still restricted due to the limited amount of processable alloys, especially Fe-based materials. A main object in current research is to expand the varieties for steel that may be used. Additionally, the modification and optimization of steel powder is seen as an interesting aspect for improving the material properties of additively manufactured parts. In this work, secondary hardenable martensitic tool steel X30CrMo7-2 is investigated, starting from the raw powder which is enriched with nitrogen by gas nitriding and subsequently characterized to ensure the usability of the modified powder for laser-powder bed fusion. In a next step, the raw and nitrided powder are used to generate cylindrical specimens to allow for further analysis of the microstructure and for a mechanical characterization of compression behavior. Moreover, a variety of heat treatments is carried out. The higher content of nitrogen leads to an increase in porosity. However, the addition of nitrogen causes an increase in hardness and in the compressive yield point, especially after heat treatment. After tempering, compressive yield stress is increased from 1,111 MPa to 1990 MPa, while for conventional material it is slightly reduced from 1,316 MPa to 1225 MPa.

The strain-life approach appears to be a powerful tool to design components against fatigue failure. Given its general theoretical background, it may be applied to a wide range of cases. In the contribution at hand, its application to welded components, and especially the incorporation of size effects in the concept has been investigated. For this purpose, eight welded cruciform joints were tested for fatigue at two different load amplitudes, accompanied by supplementary investigations such as hardness measurements and tensile tests. The observed fatigue lives were compared to numerically estimated ones. Special attention was paid to the consideration of imperfections in the calculations. The weld geometries were modeled once using an idealized shape and once with a realistic representation obtained by 3D-laser scanning. Additionally, different methods to approximate cyclic material parameters were investigated. The results show an improvement in prediction accuracy if the real weld geometry is used. However, by doing so, the incorporation of size effects via a highly stressed area is open for discussion. Due to the high local stress and its steep gradients, as a result of the scanned weld surface, the calculated areas are very small, which raises the question whether this is still within the limits of its application. Regarding prediction accuracy, the fatigue lives of either one or the other load amplitudes were properly estimated. This issue may be addressed by modifying the slope of the strain-life curve for welded components, which can be achieved by a parameter fitting procedure through a large set of experiments.

Numerous FEM calculation models for load introduction onto rolling mill rolls have been developed. The stress that can arise during heat treatment as well as during roller use was calculated. This has made it possible to optimize roll use conditions and to develop new roll qualities. The following results are described: stress in the heat treatment of rolls, the calculation of large work rolls of a heavy plate mill, the counter-bending of rolls whose body length is greater than 6,000 mm, von Mises stress in roll necks, contact stress at the edge region of working and back-up rolls, stress distribution in pre-stressed composite profile rolls in use

Ramor 500 armor steel is classified as a high-strength ballistic protection steel with thicknesses of 505–590 HV with a thickness of 2–30 mm. ANSYS is a simulation software program used in engineering fields. This program is used after the modeling phase and allows testing in a virtual environment before the prototype is produced. In this study, 3 mm thick and 65 × 55 mm Ramor 500 steel which is used as armor steel and basic combat vehicle in the armed forces in the market has been selected. These pairs of steel groups were combined by submerged arc welding at 250 constant amps, 20, 25, 30 volt voltages and 20, 25, 30 cm × min welding feed rates. Radiographic examination tests, metallographic microstructure and microhardness investigations of the structures formed in the intersection after the joints were performed. ANSYS package software has analyzed the mechanical distortions that occur after the merge process. As a result of the analysis, it was examined that all weldings had full penetration in radiographic examination test. In the microstructure examination, a coarse-grained structure from the weld seam to the base material and a grain structure that changed from the smaller grained structure were observed. In the microhardness test and ANSYS software, it was observed that the hardness of the welded seams increased compared to the hardness of the base material, elastic tensions were very high during the welding process and it was replaced by permanent stresses.

A large number of strip-like structures were found distributed along the axial direction of inconel 783 alloy bolts. Large-sized Nb were concentrated and precipitated in the interior, which makes the material structure uneven, resulting in a reduction of toughness and plasticity. Adopting a heat treatment method for increasing the solution temperature and at the same time prolonging the aging time improved the material properties to a certain extent. The results show that the large Nb easily develops inclusions, which reduces the impact properties of the material. After heat treatment, the strip-like structures disappeared, the size of the Nb decreased and nano-scale carbides were precipitated out, thus increasing plasticity by 11.5 %, toughness by nearly two times and improving the comprehensive mechanical properties in general.

Due to their excellent mechanical properties, fiber-reinforced plastics are increasingly being used in technical lightweight products. The multi-material design of fiber-reinforced plastic and metal leads to great lightweight constructions because the potential of the materials can be efficiently used for the specific field of application. This restricts conventional thermal joining technologies and shows the demand for cost-effective and efficient mechanical and adhesive joining technologies. This paper depicts the development of a new type of auxiliary joining element with integrated pin structures whose purpose is to increase the load-bearing capacity of mechanically joined fiber-reinforced plastic/metal combinations. In addition, the hole area of the fiber-reinforced plastic can be relieved in this way by transferring the operating loads into the laminate via the pin structures. In addition to experimental studies of the application methodology, the quasi-static and dynamic load-bearing capacity will be investigated. This paper presents detailed information about the development of the new auxiliary joining element and the characteristics of the joints, including corrosion effects generated by a corrosion camber.

The present paper reports the chemical, mechanical and morphological characterization of Coccinia Indica (CI) fiber. The CI fiber composite was fabricated via compression molding. The result of the chemical analysis of CI fiber shows that the fiber contains more cellulose and skimpy lignin, ash and wax. Scanning electron microscopy (SEM) analysis reveals that the fiber possesses a multi-cellular structure with parenchyma and chlorenchyma tissues in the cell walls. Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) studies divulge that the CI fiber contains alkali bonds and a crystalline index of 29.99 %. A thermogravimetric analysis (TGA) reveals that the major degradation of the fiber occurs at a temperature range of 204.16 °C to 376.3 °C. According to the various mechanical test results, tensile, flexural and impact strength grows with an increase in the percentage of fiber weight and fiber length. The maximum values were found to be 30 mm in fiber length and 35 wt.-% for fiber loading. The SEM fractography result shows that the predominant mechanism for mechanical failure is due to fiber breakage, fiber pull out, matrix fracture and fiber fracture. This confirms the use of fiber reinforced composites for various industrial applications.

Heat treatable aluminium alloys of the 6xxx series are widely used in the hood, trunk and roof sections of a car body as they offer better corrosion resistance, high strength to weight ratio and excellent weldability characteristics. Gas metal arc welding (GMAW), a high productivity process is extensively employed in the automobile sector due to its ease of automation. HAZ softening is one of the most common problems that occurs during the welding of aluminium alloys by GMAW because of its high heat input. Cold metal transfer (CMT) is the most suitable welding process for welding aluminium alloys especially for controlling HAZ softening issues that occur due to the low heat input involved in this process. However, softening continues to exist in the welded joints. In this investigation, Post -weld heat treatment (PWHT) was employed to increase joint efficiency and to minimize the HAZ softening of AA6061-T6 aluminum alloy joints welded by GMAW and CMT. The results show that the tensile strength of a post-weld heat treated CMT joint is 10 % higher i. e., 242 MPa (76 % of joint efficiency), than that of the as welded CMT joint. This is due to the nucleation of strengthening precipitates in the weld metal and heat affected zone regions.

The wear and electrochemical corrosion behavior of Al6061-CSA composites with varying weight percentages of CSA reinforcement have been investigated. A pin on disc apparatus was used to characterize the wear resistance of the prepared composite specimens. Open-circuit corrosion potential (OCP) and potentiodynamic polarization measurements (PDP) were used to study the corrosion behavior of prepared Al6061-CSA composites in 3.5 % NaCl solution. The characterization was performed through EDS and SEM. The wear rate of the composites decreased with increase in wt.-% of the CSA. It was observed that with an increase in CSA, the coefficient of friction and friction force of the composites increased. The composites produced have better corrosion resistance than the base matrix aluminum alloy Al6061. The corrosion resistance of the composites starts to increase with an increase in CSA content up to 6 wt.-%, and then it starts decreasing. FSEM and XRD analysis were performed on the morphology of composites in 3.5 % NaCl solution.

Today, energy consumption and environmental issues are important topics in all industries around the globe. However, quality is in direct proportion with energy consumption, since better surface finish means more energy consumption. The main objective of this work is minimizing both surface roughness and power consumption by estimating the optimum machining parameters. In this study, turning tests were carried out on three different hardened AISI 1040 steels (10, 15, 20 HRC) at three different depths of cuts (1.2, 2.4, 3.6 mm), feed rates (0.15, 0.25, 0.35 mm × rev −1 ) and cutting speeds (120, 140, 160 m × min −1 ) without coolant. The effects of cutting parameters and workpieces hardness on surface roughness, sound level and power consumption were examined. These analyses were conducted using a full factorial experimental design method. The response surface methodology and analysis of variance were also used to determine the effects of input parameters on the response variables. Experimental results showed that an increase in the feed rate value causes an increase in the surface roughness, the sound level, and the power consumption values. The results of the presented work show that feed rate is the most effective machining parameter that affects surface roughness and power consumption. Following feed rate, depth of cut and cutting speed also have an important impact. Thus, decreasing the value of feed rate and depth of cut will reduce the amount of power consumption.

In this study, the effects of spot welding parameters on the weld quality of DP600 steel were experimentally investigated. The welding parameters such as welding current and time were determined based on microstructural characterization, microhardness tests and tensile-shear tests. The joints were welded with two parallel welds perpendicular to the tensile-shear test direction. The shunting effect was evaluated based on microhardness and nugget size. A microstructural examination was performed using optical microscopy. The results show that parallel welding increased current diversion along the shunt path, reducing hardness and nugget size in the second weld. Additionally, higher welding current and welding time caused increased hardness and a higher tensile load due to the formation of lath martensite.

In this study, the weldability of P460 and S355 steel was investigated by application of the submerged arc welding (SAW) process. In the analyses, the effects of various parameters were handled at different welding speeds, the two levels of the current values of 475 A to 525 A and 525 A to 575 A, commercial submerged additive metals, and commercial aglomera welding powders. A notch impact test was carried on to determine the strength of the joints. Microhardness measurements were made at a Vickers load of 500 g. Optical microscopy and EDS analyses were utilized in the weld zone to investigate the microstructure changes which depend on the welding parameters. The results of this study indicate that P460 steel maintained sufficiently good weldability by application of the submerged welding process.