Band 60, Heft 7-8

In this paper, the effect of conventional steel, new generation DP-TRIP steels, AA7108 – AA7003 aluminum alloys, AM60 – AZ31 magnesium alloys and crash-box cross-sections on crash performance of thin-walled energy absorbers are investigated numerically for the lightweight design of vehicle structures. According to finite element analysis results, crash performance parameters such as total energy absorption, specific energy absorption, reaction forces and crush force efficiencies are compared for the above-mentioned materials. The energy absorption capability of steel energy absorbers is better than that of aluminum and magnesium absorbers. On the other hand, the energy absorption capacity per unit mass of energy absorbers made from lightweight materials is higher than that of steel energy absorbers. This advantage of lightweight alloys encourages automobile manufacturers to use them in designing structural vehicle components.

Weight-optimized component design as well as a reliable estimation of the lifetime of metallic materials and components require a comprehensive understanding of fatigue processes and a systematic investigation of the underlying fatigue behavior. This becomes even more important when designing highly loaded components such as wheels of high-speed passenger railway systems. Typically, mechanical stress-strain hysteresis measurements and increasingly different types of temperature and electrical resistance measurements are used to characterize the fatigue behavior and fatigue processes. Here, electrical resistance measurements provide significant information as they allow the detection of microstructural changes, e. g., through changes in dislocation density and structure. In addition, electrical resistance measurements can be considered in load increase and constant amplitude tests with inserted load-free sequences and in service load tests to characterize damage progress. In this paper, characteristic values of the change in electrical resistance were determined for ICE R7 wheel steel specimens and correlated with dislocation density, which was load- and cycle-dependent and determined through transmission electron microscopy.

3D printing, using material extrusion (a.k.a. FDM), is increasingly used for final parts, and not only prototypes. This increases research into printed material strength. For such research to be reproducible, many parameters must be controlled. Assessed here are several such parameters currently underappreciated in the literature: variation between printers, printing toolpath, void content, and bed placement. Through systematic testing (n = 780), the first three parameters are proven to be statistically significant. With these controls, the effect of layer height, print speed, nozzle temperature, and print orientation on strength as previously reported is accurately reproduced, and contradictory prior results regarding print orientation are explained. A single material was used: polylactic acid, benchmarked by injection molding and testing of specimens with comparable geometry to the printed specimens.

In this study, the stress concentration effect of single subsurface (D type) pit types and undercutting (E type) pit types with various pit depth and diameter has been investigated with respect to uniaxial loading by conducting a series of 3D stress analyses systematically. Finite element analyses have been conducted in order to determine the stress concentration factor of pits. Pit aspect ratio q/a and a/2c are the main parameters affecting the value of the stress concentration factor for subsurface types of pits. e/a and e/2c are also pit aspect ratios affecting the value of the stress concentration factor for undercutting types of pits. Some equations have been made to estimate the stress concentration factor depending on the geometrical parameters from the results of stress analyses. The stress concentration factor can be calculated using an equation obtained with a correlation coefficient (R 2 ) of 0.993 for D types (subsurface) pits. The stress concentration factor can also be calculated using an equation obtained with a correlation coefficient (R 2 ) of 0.995 for E types (subsurface) pits.

A procedure a for multi-step characterization of microstructural features observed in austempered ductile cast iron (ADI), was proposed based on the results of microscopic examinations, a local X-ray microanalysis and EBSD-based crystallographic evaluation. The polyphase complex microregions having an increased content of alloying elements such as Mo, Cr, Mn in comparison with that estimated for the alloy's matrix, were observed in the examined material. As a first step, a classification of microregions was performed based on their morphology and specific distribution in the alloy's microstructure. Subsequently, by using established criteria for the selection of microanalysis data for a further microregions discrimination, several groups of particles characterized by a particular ratio of the alloying elements, were distinguished. The particles were classified based on the specific range of the carbide stabilizers concentration revealed, then were ascribed in situ to two types of carbides identified by means of EBSD analysis. Furthermore, the effect of heat treatment conditions on the evolution of the chemical composition of the microregions was examined experimentally and discussed.

Unlike the uniaxial creep test results, the interpretation of small punch (SP) test results is not straightforward due to the complex deformation behavior of SP specimens. Accordingly, the reliability of correlations of SP creep test results with corresponding uniaxial tests is still not clear. One crucial point is that the initial yielding at the beginning of an SP creep test results in the hardening of the material before creep sets in and is thought to affect the creep properties. In this study, to clarify this effect, first an accurate numerical model of the SP creep test has been developed following which this model was used to study the influence of test parameters such as specimen thickness, punch diameter and the radius of the receiving hole on the initial yielding. Varying these parameters changes the deformation mechanisms and thus has a strong effect on the deflection-time curves along with the amount of initial plastic deformation. It was observed that decreasing specimen thickness and the punch diameter increased the initial yielding. Conversely, increasing the radius of the receiving hole resulted in higher initial yielding.

In the present work, the scientists were successful in obtaining a friction surfaced coating of mild steel over mild steel, mild steel over stainless steel (AISI 304) and stainless steel over stainless steel using a low-cost conventional milling machine. A 2 2 factorial method with three replications was used to design the experiments. The influence of variation in rotational speed (1400 rpm and 1000 rpm) and table feed rate (4.17 mm/s and 2.67 mm/s) on coating width, thickness and interface temperature of the coating and substrate was investigated. Multi-response optimization was performed and the developed model was validated by confirmatory experiments. The results showed that, with an increase in rotational speed and table feed rate, width and thickness of the coating decreases, while temperature increases. An infrared (IR) camera provided thermographs that elaborated the distinct stages of heat dissipation during the process of coating formation. The bond integrity and strength of the coating was analyzed by bend tests and Vickers micro hardness test. The results revealed that a higher hardness value was obtained at the interface of the coating as compared to coating and interface.

Al-Si metal matrix composites have generally been manufactured using casting methods. Powder metallurgy has been used as an alternative manufacturing technique to obtain more homogeneous and segregation-free products. In this study, 2 wt.-% TiB 2 particle reinforced Al-7 wt.-% Si composites were manufactured using high energy ball milling, cold pressing (at 450 MPa) and pressureless sintering (at 570 °C for 2 h under Ar flow) techniques. The effects of different milling processes, such as mechanical alloying at room temperature and/or cryomilling in an isolated polycarbonate cylinder soaked in liquid nitrogen or sequential milling, on the Al-7 wt.-% Si-2 wt.-% TiB 2 powders and corresponding bulk products were investigated. The microstructural, physical and mechanical properties of the composites sintered from the mechanically alloyed, mechanically alloyed and cryomilled, and sequentially milled powders were significantly improved as compared with those of as-blended ones. The highest density, the highest microhardness and the lowest wear rate were obtained in a composite sintered from mechanically alloyed and cryomilled powders at 92.38 %, 214.14 ± 41.17 HV and 3.8 × 10 −3 mm 3 ·N −1 × m −1 , respectively.

In-situ compaction and sintering of nanopowders are of great interest with respect to industrial production because they save time and energy. As a key study, Al 2 O 3 -1 wt.-% GNP composites were mixed in a V-type mixer for 2 hours at 35 cycle/min to obtain homogeneous mixtures. Mixed graphene nanoplatelets and alumina nanoparticles were compacted and sintered in-situ by applying uni-axial pressure in a graphite die, heated and generated by the ultra high-frequency induction system (UHFIHS). The graphite die was heated to 1500, 1550, 1600 and 1650 °C for 5, 10 and 20 minutes under a vacuum. The effect of sintering temperature and time on density, surface roughness, hardness and the microstructure of in-situ sintered nano graphene-alumina compacts were investigated.

In order to generate a material data base for computational welding mechanics, temperature and strain-rate dependent stress-strain experiments were performed by using a Gleeble ® 3500 testing system. The object of the investigation was HSLA transformable steel S960QL and related solid phases as bainite, martensite and austenite. For the production of these solid phases, the base material was heat treated according to an average weld temperature cycle which was extracted within the heat affected zone of a thermal numerical weld simulation of a GMA weld. The hot tensile tests were carried out via cost-saving flat specimen geometries. Two experimental series with different strain-rates were conducted, where the longitudinal strain-rate was controlled by specification of the transversal strain-rate applying Poisson's-ratio. Subsequently, the resulting stress-strain curves were approximated in accordance with the Ramberg-Osgood-materials law. Consequently, it is shown that the temperature and strain-rate dependent stress-strain behavior of metals can be successfully characterized by means of a Gleeble ® -system. However, this requires a control of the longitudinal strain-rate by specification of the transversal strain-rate. The related experimental procedure and the method of evaluation are explained in detail. With regard to all tested solid phases, a significant strain-rate dependency can only be observed upwards from temperatures of 400 °C. Based on experimental results, Ramberg-Osgood-parameters will be presented to describe the stress-strain behavior of steel S960QL and related solid phases for temperatures between 25 °C and 1200 °C. Furthermore, the use of cost-saving flat specimen-geometry appears reasonable.

In the field of optically excited thermography, flash lamps (impulse-shaped planar heating) and halogen lamps (modulated planar heating) have become established for the specific regimes of impulse and lock-in thermography. Flying-spot laser thermography is implemented by means of a rasterized focused laser, e. g. for crack detection (continuous wave operation) and photothermal material characterization (high-frequency modulated). The availability of novel technologies, i. e. fast and high-resolution IR cameras, brilliant innovative light sources and high-performance data acquisition and processing technology will enable a paradigm shift from stand-alone photothermal and thermographic techniques to uniform quantitative measurement and testing technology that is faster and more precise. Similar to an LED array, but with irradiance two orders of magnitude higher, a new type of brilliant laser source, i. e. the VCSEL array (vertical-cavity surface-emitting laser), is now available. This novel optical energy source eliminates the strong limitation to the temporal dynamics of established light sources and at the same time is spectrally clearly separated from the detection wavelength. It combines the fast temporal behavior of a diode laser with the high optical irradiance and the wide illumination area of flash lamps. In addition, heating can also be carried out in a structured manner, because individual areas of the VCSEL array can be controlled independently of each other. This new degree of freedom enables the development of completely new thermographic NDT methods.

Kurzfassung Im Bauwesen werden Polymerbeschichtungen auf Beton häufig eingesetzt um zum einen, ein bestimmtes Aussehen zu schaffen und zum anderen, das Bauteil vor Alterung, Verschleiß und Schädigung zu schützen. Für praktisch alle Ziele ist die Wirkung von der eigens dafür definierten Schichtdicke der Polymerbeschichtung abhängig. Daher wird die Dicke der Beschichtung nach erfolgtem Schichtauftrag überprüft. Für den in diesem Zusammenhang anspruchsvollen mineralischen Untergrund Beton stehen bislang allerdings nur zerstörende Prüfverfahren zur Verfügung. Aus diesem Grund wurden im Rahmen des Projektes IRKUTSK ein auf aktiver Thermografie basierendes Verfahren sowie ein Gerät für den vor-Ort-Einsatz entwickelt, mit dessen Hilfe eine zerstörungsfreie Schichtdickenbestimmung möglich ist. Hier wird ein kurzer Einblick in das zur Schichtdickenbestimmung entwickelte Thermografieverfahren gegeben. Die Besonderheiten bei der quantitativen Auswertung, die durch die Teiltransparenz der Polymerbeschichtungen auftreten, werden erläutert. Die Funktion des Verfahrens für einlagige Systeme wird anhand von Labormessungen mit verschiedenen optischen Quellen zur thermischen Anregung illustriert.

The objective of this study was to investigate the synthesis and characterization of aluminum alloy AA8011 reinforced with various amounts (0, 4, 8 and 12 wt.-%) of fly ash (FA) particles by the stir casting method. The wettability of fly ash particles in the matrix was improved by adding 0.5 wt.-% of Mg into the solution. Fly ash particles were combined with the semi-solid aluminum solution. The microstructure and mechanical properties of the fabricated aluminum matrix composites were analysed. The optical and SEM with EDX revealed a homogeneous dispersion of fly ash particles in the aluminum matrix composites. The aluminum matrix composites were characterized by a homogeneous distribution of fly ash particles having clear boundaries and good bonding qualities to the aluminum matrix. Mechanical properties such as hardness and tensile strength improve with an increase in weight percentage of fly ash particulates in the AMCs.

An experimental study was performed to investigate the high temperature compressive properties and failure mechanism of three-dimensional (3D) five-directional braided composites. The compression tests of 3D braided composites were conducted at six different temperatures. The failure modes, stress-strain curves, compressive modulus and strength at high temperature were analyzed and compared with those at room temperature. The results showed that temperature has a visible impact on these parameters. The 3D braided composites failed due to shear fracture, and the stress-strain curves exhibited obvious linear elasticity and a brittle failure feature at 20 and 50 °C, while the failure was characterized by fiber/matrix interface debonding, fiber bundles loosening and by outward expansion, and the stress-strain curves exhibited an obvious non-linear trend and a plastic failure feature at 70, 90, 110 and 130 °C. The compressive modulus and strength decreased as temperature increased. The compressive modulus and strength decreased by 92.9 % and 94.1 % as the temperature increased from 20 °C to 130 °C, which was mainly due to the degradation of the matrix properties and fiber/matrix interface properties at high temperature.

This study focuses on the fabrication of aluminum alloy 8011 with 8 % fly ash (FA) composite (AA8011-8 % FA), using the stir casting method. The dry sliding wear characteristics of the composite were investigated at various sliding parameters. A three-level central composite design experiment was developed using response surface methodology with various parameters such as load, time, and sliding velocity and varied in the range of 5 to 15 N, 5 to 15 min, and 1.5 to 4.5 m × s −1 , respectively. Dry sliding wear tests were performed as per the experimental design using a pin-on disc at room temperature. The regression result obtained indicated that the developed model performed well in relating the wear process parameters and predicted the wear behavior of the composite. The surface plot showed that the wear rate increases with increase in load, time, and sliding velocity. Moreover, the surface morphology of the worn-out composite was examined using a scanning electron microscope.

Numerous scientific reports devoted to the synthesis of carbon nanostructures (CNS) are published as a way of searching for industrially feasible processes. Researchers try to find efficient and at the same time cheap, affordable and easy-to-use sources of carbon, catalysts and methods of carbon nanotubes production. The present review article highlights research on the synthesis of CNS from gas and liquid feedstock which appears to be the most amenable for large-scale production. Carbon sources, types of catalysts used as well as methods for production of various carbon nanostructures are sequentially described.