Volume 59, Issue 2

Glass fiber-reinforced polymers (GFRP) are highly suitable for use in transportation industry in order to achieve the targets of energy and resource efficiency. In this context, due to its high specific strength, GFR-epoxy (GFR-EP) has already been implemented in a wide range of applications. However, in cases of energy efficiency and damage tolerance, GFR-EP shows disadvantages compared to GFR-polyurethane (GFR-PU). The aim of this study is the comparative characterization of the quasi-static and cyclic deformation behavior of GFR-PU and GFR-EP with similar layer setup. The mechanical properties have been investigated in instrumented tensile, interlaminar shear strength and compression after impact tests. In addition, the tests were combined with varying temperatures (−30 °C, RT, +70 °C) with respect to aerospace applications to determine the material property development under low and elevated temperatures. In cyclic investigations, the fatigue properties have been estimated by resource-efficient multiple step tests and validated in constant amplitude tests. Hysteresis and temperature measurements were applied in order to investigate the damage processes. It could be shown that polyurethane exhibits improved damage tolerance by significantly reducing delamination area under impact loading, whereas epoxy leads to optimized properties under elevated temperature. Furthermore, epoxy generally underlines higher capabilities under cyclic loading, which is due to void content of polyurethane.

The Au-Ge alloy is useful for bonding applications in microelectronic and optoelectronic packages. Also, recent investigations show that Au-Ge system could present a lead-free solder candidate. The results of characterization of the eutectic alloy from binary Au-Ge system are presented in this paper. Experimental research was done using thermal analysis (differential thermal analysis, DTA) and structural investigation by light optic microscopy (LOM) and scanning electron microscopy with EDS (SEM-EDS). The eutectic temperature of 359 °C was detected using thermal analysis, while eutectic structure was confirmed using microstructural analysis. Obtained results were compared with recent literature references and good mutual agreement was noticed.

Metallic multilayer materials consisting of hundreds or thousands of layers offer a high potential for broad applications in modern technology. A uniform and gradual thinning of layers can be realized by using alloys with different crystal structures in combination with the efficient and high-performance technology of hot pack rolling. However, investigations on fatigue properties, especially to evaluate the influence of the number of layers, are still missing. In the present study, the fatigue behavior of metallic multilayer materials consisting of austenitic and ferritic stainless steels AISI 304 and AISI 430 with 100 and 1400 layers are characterized by applying a time-efficient load increase procedure. Therefore, instrumented stepwise load increase tests were performed to define suitable loading parameters for a convenient comparison of fatigue properties in constant amplitude tests. A benefit of the complex production process leading to 1400 layers was verified concerning the investigated load level in the range of low cycle fatigue with a significant improvement by the factor of 3.5. The alternating current potential drop method for measurements of change in voltage was determined to be most suitable to detect microstructural changes at an early state of fatigue damage for multilayer materials. Microstructures as well as fractured surfaces were investigated using light and scanning electron microscopy to evaluate the results of the two technological manufacturing routes as well as the crack and failure behavior.

In-situ testing inside a scanning electron microscope is a helpful tool for detailed analyses of small sized specimens with respect to their mechanical properties and the correlated microstructural alterations. Thus, this test method is used to analyze the tensional properties of thin 316L (1.4441) wires used for microscale components, e. g., like coronary artery stents. Tensile tests were conducted on unnotched and circularly notched 316L wires (Φ 0.95 mm) with a special focus on the number of grains within the cross section as well as the notch geometry. Four combinations of notch width (2 and 4 mm) and notch depth (diameter at notch root: 0.5 and 0.75 mm) were chosen. Notch depth and notch shape were adjusted by means of electrochemical polishing. Previous investigations showed, that oligocrystalline structures exhibit a different mechanical behavior compared to polycrystalline ones or single crystals. There are only a few data available on mechanical testing of oligocrystalline structures with respect to varying notch geometries. Depending on the notch geometry, grain size and, therefore, the number of grains within the notch cross section widely scattering yield- and tensile strength as well as failure elongation values were measured. However, the transition criterion between poly- and oligocrystalline behavior could be quantified to be 6 to 7 grains within the cross section.

Processing conditions can significantly influence the structure and properties of polymer nanocomposites. In the present study, melt mixed high density polyethylene (HDPE)/multi-walled carbon nanotube (MWCNT) nanocomposites were prepared via twin-screw extrusion and then compression molded (CM). The effect of heating temperature, pressing time and cooling rate on the structure, electrical and mechanical properties of the CM nanocomposites was systematically investigated. Volume resistivity tests indicate that the nanocomposite with 2 wt.-% MWCNTs, which is in the region of the electrical percolation threshold, is very sensitive to the CM parameters such that heating temperature > pressing time > cooling rate. Generally, the resistivity of nanocomposites decreases with increasing heating temperature and pressing time. Interestingly, the electrical resistivity of the rapidly cooled nanocomposite with 2 wt.-% MWCNTs is about 2 orders lower than that of the slowly cooled nanocomposite which is attributed to the lower crystallinity and smaller crystallites presenting less of an obstacle to the formation of conductive pathways. The tensile properties of the nanocomposite with 2 wt.-% MWCNTs are also influenced by the compression molding parameters to some extent, while those of the nanocomposites with higher MWCNT loading are insensitive to the changes in processing conditions. The modulus of the nanocomposites increases by about 25 to 50 % and 110 to 130 %, respectively, with the incorporation of 2 and 4 wt.-% MWCNTs, which agrees well with the theoretical values predicted from Halpin-Tsai and Mori-Tanaka models. This work has important implications for both process control and the tailoring of electrical and mechanical properties in the commercial manufacture of conductive HDPE/MWCNT nanocomposites.

For the reduction of the distortion and the residual tensile stresses in welded seams on carbon manganese steels, low transformation temperature materials (LTT) were developed. These materials use the volume expansion effect during martensitic transformation. The volume expansion counteracts volume shrinkage during cooling. The positive effects of the low transformation temperature alloys on the residual tensile stresses were demonstrated in various investigations. The low transformation temperature materials were, so far, used as filler material in arc welding processes in large volumes. The use of modular thermal fields of the electron beam welding processes offers the potential of a temporally activated use of compressive stress induction by phase transformation of the low transformation temperature alloys. The aim is to exert influence in situ on the welding residual stress state and thus on the distortion of complex parts. The metallographic analysis of an electron beam welded seam in unalloyed steel with low transformation temperature filler material is demonstrated. The evaluation is made via electron backscatter diffraction (EBSD) and is shown in the first part. The second part will show the effect on near surface residual stresses examined by hole-drilling method in combination with an optical evaluation by electronic speckle pattern interferometry.

AA7075-T651 plates were friction stir welded using in-process cooling with the intention of investigating variations in microstructure and hardness across the joint in through thickness direction. The variations were studied using optical microscopy, transmission electron microscopy (TEM) and hardness tests. Water cooled welds exhibit better performance in terms of hardness compared to air cooled welds. However, both welds were found to display considerable reduction in the hardness in the heat-affected zone (HAZ). Furthermore, hardness across the joint enhances as the survey proceeds away from the surface of the specimen to the root of the weld. The weld nuggets in both welds reveal fine recrystallized grains and the grain size was found to dwindle from surface to root of the joint along the thickness of the weld. TEM studies disclose that the weld nugget suffers from dissolution of precipitates and the HAZ consists of precipitate-free zones, which are considered accountable for lower hardness in the HAZ. It is concluded that defect-free welds can be made with thick plates of AA7075-T651 using in-process cooling and furthermore, water cooled welds exhibit better hardness values.

Numerous experimental works have been devoted to studying the influence of different shot peening parameters on the surface material conditions in different metals in the last decades. Most of the research work has been focused on experimental determination of the residual stress and its depth profile by means of X-ray diffraction and corresponding electro-polishing of the surface layers or byhole drilling method. This state of knowledge has led to the development of phenomenological models to describe the surface material conditions qualitatively. Since there are quite a number of parameters, which could influence the residual stress field after shot peening, covering the whole possible process parameters combinations with the purpose of experimentally determining the residual stress profiles could be difficult. A deeper insight into the residual stress states after shot peening could be possible on the basis of sound physical principles by means of numerical approaches. In this study, shot peening of high strength steel S690QL has been modeled and simulated. The results have been compared with the residual stress depth profiles determined by X-ray, synchrotron, neutron diffraction and hole drilling method.

The effective characterization of the particle distribution in SiCp/Al composites is performed by ultrasonic imaging. The required corrections and processing of ultrasonic signals for 3D reconstructions are developed. The spatial distribution of the SiCp particles and their aggregation are obtained and the reliability of the imaging results is verified by metallography. The correlation of ultrasonic attenuation and velocity to the SiCp content are analyzed. For the quantitative analysis of the SiCp particle distribution, the velocity imaging method is more accurate than the attenuation method. The resulting 3D ultrasonic imaging method is a suitable tool for the characterization of SiCp particle aggregations in Al matrix.

In the present investigation, the influence of B 4 C p particles on the mechanical and tribological behavior of Al 359 composites has been studied. B 4 C p particle reinforced Al 359 composite samples were prepared by stir casting process. Hardness, tensile strength and wear behavior of the composites were studied and compared with a control specimen. Hardness of B 4 C p particles reinforced Al 359 matrix increases compared to base matrix due to the presence of the ceramic phase. Coefficient of friction considerably increases with up to 20 wt.-% addition of B 4 C p in base matrix. Specimens were subjected to wear tests under different load conditions and the following five different wear mechanisms such as wear groove, abrasion, delamination, oxidation and plastic deformation were evaluated. The abrasion results prove the increase in wear resistance of B 4 C p reinforced composites compared to a control specimen.

The aim of this paper is the modeling and the corresponding experimental validation of angular deep drawing parameters on the tensile strength, wall thickness and height of the square profile cups. The full factorial experimental method was used to collect the data. FE analysis results were compared with experimental measurements. As a consequence, it was observed that angular deep drawing dies gave better results with different angle values than conventional dies with higher drawing ratios. In general, the product quality increased by an increase of the die/blank holder angle. FEM model results were in good agreement with experimental measurements.

The stability and reliability of a vibration test bed are significantly influenced by the mechanical property of its structure. In order to reveal the mechanical property of the structure and its effect on the strength and stiffness, a finite element model of the damper vibration test bed frame has been created, the loads and constraints have been imposed, and the static solution has been carried out, from which the stress states and the maximum deformation of the test bed frame have been obtained. Moreover, according to the determined stress distribution, the strength and stiffness of the test bed frame have been extracted. The simulation results show that the maximum deformation, which is 0.0717 mm, is within the safety range and meets the stiffness safety condition. The maximum stress is 59.6 MPa and meets also the strength security condition. Finally, the minimum section stress has been compared to the theoretical prediction, and the results agree very well.

Carbonized chestnut shell was investigated as a low cost adsorbent for the adsorption of methylene blue (MB) from aqueous solution. Some parameters like the effects of initial dye concentration, initial pH value of solution and contact time of phases were investigated. The equilibrium was attained in 120 minutes. The isotherm models of Langmuir, Freundlich and Dubinin Raduskhevich (DR) were applied to the experimental data and Langmuir and DR adsorption models fit better than the Freundlich model. The maximum MB mono adsorption capacity was found to be 5.13 mg × g −1 obtained by Langmuir model. This value of carbonized chestnut shell was comparable to the adsorption capacities of some other adsorbent materials for dyes. The pseudo first order, second order and intra particle diffusion models were chosen for analyzing adsorption rate data. The adsorption reaction progressed according to the pseudo second order kinetic and Weber Morris diffusion was also effective in the adsorption phenomena. The thermodynamic parameters were also calculated.

In order to study the reinforcement mechanism of the single fluid alkalization method and to consider the possibility of using the permeation variation of loess, direct shear tests of loess were performed before and after the strengthening process of single fluid alkalization, and the resistivity of loess was measured. The test results indicate that the shear strength of alkaline loess samples is significantly higher than those without alkaline, due to a series of physical and chemical reactions occurring after the injection of lye. With the increase of the electrical current frequency, the vertical and horizontal resistivities of loess decrease quickly at first and then become stable. When the electrical current frequency is below 50 kHz, even a slight change of the current frequency will greatly affect the measured resistivity values of loess. When the electrical current frequency is above 50 kHz, the resistivity of loess seems no longer be affected by the current frequency. The vertical resistivity and the horizontal resistivity decrease with increasing curing time and finally remain stable during the single fluid alkalization process, in which the decreasing trend occurs in the initial stage. The average formation factor and the average shape factor decrease with increasing curing time and become stable eventually. The electrical anisotropy index increases rapidly with increasing curing time at first and eventually stabilizes. All tests indicate that during such single fluid alkalization process the use of the resistivity procedure to study the loess structure changes is an effective method.

In this paper, available structural models and equations for predicting the effective thermal conductivity (k e ) of recycled aggregate thermal insulation concrete, including series, parallel, Maxwell-Eucken, eEffective medium theory, and Gong's model are discussed and extended to three phases by considering the effect of glazed hollow beads (GHBs). The solid porosity of GHBs is estimated and used in the equations. A unified model for calculating k e values is proposed, where the fractal dimension of GHBs is considered. A simple expression characterizing the fractal dimension of GHBs is derived. Nine batch specimens with different concrete mixtures are tested to determine the k e values which are compared with the predicted values. The predicted values from Unified-2 model are found to be in good agreement with the experimental results.