Volume 61, Issue 10

Structured materials can be a progressive alternative to commonly used flat sheets. There is an enormous range of possibilities for using structured sheet metals due to their better stability. However, the effective use of these materials as well as their implementation in production requires precise knowledge about specific properties of structured sheet metals. The aim of this work is to obtain new knowledge about stiffness and stability during the deformation of this material. The structured sheet metals were investigated by means of buckling tests. The steel type investigated is a commonly used deep-drawing steel DC04. Six types of structure arrangements were compared to non-structured (flat) specimens as a reference. The influence of the type of structure arrangement as well as the deformation speed on the mechanical properties was investigated.

This work investigates the isothermal oxidation behavior of as casted 29Cr-8Ni ferritic stainless steel at 1173 and 1273 K for 126 ks at an air flow rate of 0.67 cm 3 × s −1 using thermogravimetric analysis (TGA). The oxidation resistance at both temperatures was similar. At 1173 and 1273 K, the kinetic mechanisms were cubic: between parabolic and logarithmic, and parabolic, respectively. The oxidation kinetic constants k at 1173 and 1273 K were 1.648 × 10 −4 and 8.152 × 10 −4 g × cm −2 × s −1 , respectively. The exponential growth rates n at 1173 and 1273 K were 0.3093 and 0.5692, respectively. The oxidation behavior of 29Cr-8Ni ferritic stainless steel was discussed and compared with that of as casted 26Cr-16Ni and as casted AISI 309 austenitic stainless steel.

Aluminum alloys exhibit good strength, excellent corrosion resistance and good forming properties along with low specific density. Due to these superior properties, they are widely used in the automotive, chemical, and food industries, especially in shipbuilding and the yacht industry. It is important to join these alloys used in the mentioned industrial areas. However, it is very difficult to join Al and its alloys via traditional welding methods despite its widespread use. In this study, 5754-H111 and 6013-T6 aluminum alloys were joined by friction stir welding. Welding operations were carried out at 1250 rpm using welding feed rates of 12.5, 25, and 32 mm × min −1 . Metallographic investigations were carried out to determine the microstructural and macrostructural properties of the weld zones. Tensile, bending, and hardness tests were also performed to determine the mechanical properties of the welded samples. The results obtained as a result of the tests were evaluated and interpreted. Micro-void defects were found to occur in the weld metal–HAZ transition zones. It was established that the welding speed affected the tensile strength. The highest tensile strength was obtained at a feed rate of 25 mm × min −1 . It was determined that no cracking or fraction occurred in the welded samples as a result of the bending test.

Uniaxial tensile and ratcheting tests were performed with a 3D DIC system to investigate the tensile strain evolution and heterogeneous ratcheting of each area in a gas tungsten arc welding (GTAW) welded joint. The results indicated that with increasing applied stress, the strain was initially concentrated in the base metal (BM) owing to its low yield stress, and then extended gradually to the heat affected zones (HAZ) and subsequently to the weld metal (WM) until the occurrence of quasi-shakedown. Furthermore, the most remarkable ratcheting strain and ratcheting rate were obtained in the BM but it manifested lower ratcheting deformation in the BM area of the GTAW welded joint as compared with the pure base metal.

Dental prostheses are manufactured by synthesizing poly-methyl methacrylate (PMMA) in a cavitation model with incorporated acrylic teeth. For the analysis of stress-strain state in the acrylic base and the artificial acrylic teeth, it is necessary to know the mechanical properties of each material. Today it is possible to find the data on the physical and chemical properties of the acrylic base in literature, but data on the modulus of linearity of acrylic teeth is not available. The paper presents the determination procedure of the linearity of the acrylic base and teeth, by performing separate tensile tests of the acrylic base and compression of a cylindrical sample, which is partly made of an acrylic base and dental acryl. Tests were performed at room temperature under the stroke control of a servo-hydraulic testing machine, while the elongations were measured by digital image correlation (DIC) measurement on the surface of the samples. The tensile mechanical properties of the acrylic base were determined by tensile testing of round bar specimens. The linearity modulus of acrylic teeth was determined by cyclic pressure loading of the cylindrical sample with successive increase in pressure force. The modulus of linearity of the acryl base and acryl dental is determined by partial analysis of the obtained cyclic pressure stress-strain curve. Analysis shows that the stress-strain curves deviated and modulus depended on stress level in the tested specimens.

The effect of processing parameters related to temperature on the interface bonding strength of automotive overmolding injection parts was investigated using moldflow simulation. In this study, the average depth of the melting layer, the melting time of the melting layer and the temperature of the melting layer on the basal body in the interface zone are used as a qualitative index, and the interface bonding strength of the tensile test and the results observed by scanning electron microscopy are used as a basis for verification. The results show that processing parameters related to temperature have a great influence on the interface bonding strength of parts. With an increase in melt and mold temperature, interface bonding strength is also strengthened. The most influential factor is the melt temperature of the second material.

Head expanders provide an important interface connection between the armature of a vibration shaker and test objects. The most important parameters are the mass of the test specimen and the resonance frequency of the head expander. For that reason determining the natural frequency of the head expander is essential. Moreover, the mass of the head expander had to be lightweight. A head expander for an LDS V450 model electrodynamic vibration shaker was designed and manufactured using AZ91D magnesium alloy to be as light and rigid as possible. The natural frequencies, mode shapes and vibration characteristics of the head expander were determined by performing structural, FE modal and harmonic analyses as well as vibration measurements. Additionally, an experimental modal test was performed and the equipment was subjected to a white noise vibration test to verify all the results. Analyses indicated that the head expander manufactured was light, rigid, durable and suitable for vibration tests.

Studying the micromechanical properties of gas-bearing shale is very important for understanding it. In the nanoindentation measurement of gas-bearing shale, the load-displacement curve often shows pop-in at the loading stage, namely the change of load is small but the change of penetration depth is large. Pop-in can introduce a large error in the calculation of the mechanical parameter. To investigate the influence factors of pop-in, the maximum load, load rate and sample characteristics have been addressed. The results show that pop-in is related to cracks and pores in the shale. Due to the heterogeneity of shale, pop-in cannot be avoided, but can be reduced by choosing reliable load modes and test parameters. Pop-in can increase the maximum indentation depth and decrease the elastic modulus and hardness. The influence of pop-in on hardness is larger than on the elastic modulus. At a high load rate it is easy to induce pop-in. The influence of the load rate on pop-in is larger than that of the maximum load.

In this study, a test setup was designed and manufactured in order to study vibration based self-loosening characteristics of preloaded bolted joints, especially used in the automobile industry. Details of the designed test setup and its operating principles are presented in addition to the test setups available in the literature. Using the test system developed and implementing a design of experiments procedure, first trials were conducted by using the test setup. Under predefined conditions, it is possible to identify when theloosening of bolts occurs and what tightening moment value is required to prevent self-loosening.

Graphene oxide (GO) is a carbon-based filler which possesses superior properties for the preparation of polypropylene (PP) nanocomposites. The homogeneous dispersion of nanofiller in a polymer matrix involves a compatibility between base and matrix. This research work analyzes the behavior of GO and its distribution in PP as well as the morphological, physical and mechanical properties of prepared nanocomposites. Since PP is non-polar and GO is polar, there may be a compatibility mismatch between filler and matrix, which may not lead to homogeneous distribution due to poor chemical interaction. Maleic-anhydride grafted polypropylene (MA-g-PP) was selected as a suitable compatibilizer in order to disperse filler into the matrix. X-ray diffraction (XRD) reveals an increase in intergallery spacing which confirms the formation of GO. Scanning electron microscopy (SEM) shows the improved distribution of GO sheets in the PP matrix for the compatibilized system. Tensile strength and modulus of elasticity are improved for the compatibilized PP nanocomposites when compared to the uncompatibilized and to the pure system. Moreover, an enhancement of storage modulus was evidenced for compatibilized nanocomposites. Polar-polar interaction between GO and compatibilizer leads to the homogeneous distribution of filler into the matrix which is clearly responsible for an improvement in the technical properties of polymer.

The impermeability of recycled fine aggregate concrete (RFAC) with different contents of recycled fine aggregate (RFA) was tested according to the standard test method of ASTM C1202. The moisture states of RFA were controlled at air-dried (AD), oven-dried (OD) and saturated surface-dried (SSD) states. Both the interfacial structure and the pore structure of RFAC were analyzed. Research results show that, for the same total water-cement (TWC) ratio and the same content of RFA, the interfacial transition region of RFAC became denser by increasing the moisture amount of RFA from oven-dried, air-dried to a saturated surface-dried state; meanwhile, the density and porosity of hardened cement paste (HCP) improved as well, which resulted in the higher impermeability of RFAC. Whereas by contrast, for the same moisture state and by increasing the RFA content, both the interfacial transition region and the property of the interface structure of RFAC became weaker. Also, the decreasing density and porosity of HCP resulted in lower impermeability of RFAC.

Combined with the increased significance of additive manufacturing technologies in recent years, the FDM-process in particular became more and more important and widespread in private and industrial applications. In the course of the development of a variety of material types, a validation for technical utilization is of great interest. For that reason, standardized samples in three different layer orientations, made of 16 different filament materials, were FDM-printed and tensile tested at room temperature in order to determine their mechanical behavior. Besides the usual plastic types for FDM-printing, such as PLA, ABS or PETG, compound filaments from the four categories metal, carbon, wood, and stone were examined. Carbon showed for any technical applications the most practical results, since the particles increase overall strength and simultaneously contribute to reduced weight. The other composite materials too, for environmental and eco-friendly reasons, are still of interest, although tests have shown that no significant change in resilience has occurred. Moreover, it is found that a crosswise printing direction leads to the best results.

In this study, a simple and green chemistry approach is described for the preparation of reduced graphene oxide (RGO). To this aim, we used a novel reducing agent (vitamin C) for the synthesis of RGO. The GO was reduced at different weight ratios of vitamin C:GO (1:1, 1.5:1 and 2:1). The XRD, FTIR and EDS results showed the deoxygenation of GO due to the loss of hydroxyl, carbonyl, and epoxy groups. The thermal stability of the GO was lower than those of all the RGO powders, and the BET surface area of the GO was much lower than those of all the RGO powders. The results showed that the structural and thermal properties of RGO powders depend on the ratio of vitamin C:GO. When the ratio is 1:1, the RGO powder has the best thermal stability, and the highest BET surface area (m 2 g −1 ) was found to be a 2:1 ratio.

Strain in pavement caused by traffic loading is a critical driver for various distress mechanisms. In this study, strain foils (SFs) are utilized to measure longitudinal strains at the bottom of a hot mixed asphalt (HMA) layer in an indoor inverted pavement. The validity and reliability of SF-measured strain are investigated through three tests using a heavy vehicle simulator (HVS). The three tests focus on the consistency and stability of strain measured by individual SFs and various SFs installed at similar positions, as well as the consistency of strain measured by SF and an asphalt strain gauge (ASG) at different test speeds, load levels, and loading cycles in an accelerated pavement test (APT). Fast Fourier transform is utilized to filter out interference in the raw SF strain data. Filtered SF-measured strains were converted into ASG-measured strain based on effective gauge lengths. Research results show that converted SF-measured strains did not reveal a significant statistical difference compared with strains simultaneously collected by ASG under different test loads and speeds throughout the APT at 300,000 repetitions. Moreover, individual SF can provide consistent results with the coefficient of variation (CV) less than 3.3 % under different speeds and load levels, and SFs installed at similar positions showed similar average maximum strain variations (MSVs) with a maximum difference of 5.8 %. Moreover, the linear correlations between the average MSV and the test load, as well as the speed further verified the validity of SF-measured strain. It is believed that SF could be a viable alternative for strain monitoring in asphalt pavement.