Volume 58, Issue 7-8

A wide range of industries including energy, chemistry, pharmacy, textiles, food and drink, pulp and paper, etc. is using stainless steels. Metastable austenitic steels such as used in power plants and chemical industry are subjected to cyclic mechanical and thermal loading in air as well as under the influence of corrosive media. This paper provides an overview on different nondestructive and electrochemical measurement techniques, which allow differentiating fatigue damage effects in total strain controlled multiple and constant amplitude tests with respect to damage appearance on surface, in subsurface area as well as in volume of specimens or components microstructure. In addition to conventional mechanical stress-strain hysteresis curves, electrical resistance, magnetic and open circuit potential measurements have been applied to characterize the cyclic deformation behavior of the metastable austenitic steel AISI 348 (X10CrNiNb18-9) in laboratory air and in distilled water. Based on these results obtained, the paper provides an outlook on the possibility for an efficient (remaining) fatigue life evaluation approach, which is adapted to the needs of the application areas.

Brackets, together with the arch wires, are widely used in orthodontics in order to align and to straighten teeth and also to position the teeth with regard to the patient bite. Different materials are employed for manufacturing such orthodontic devices. This paper is a study of two types of brackets. Commercial stainless steel and ceramic brackets were investigated by atomic force microscopy analyses. The tests were performed at room temperature with a relative humidity of 13 %. The topography and the friction parameters of the investigated brackets were determined at nanoscale. The results pointed out that the ceramic brackets are characterized by higher values of both topographical and frictional parameters (average roughness, friction force and friction coefficient) in comparison to the stainless steel brackets.

Kurzfassung Die stetig zunehmenden Anforderungen an Kraftstoffverbrauch, Schadstoffemission, sowie passive Sicherheit haben die Automobilindustrie vor Herausforderungen gestellt, die nur durch einen konsequenten Einsatz moderner hochfester Stahlwerkstoffe zu bewältigen sind. Obgleich eine generelle Eignung dieser Stähle für das Widerstandspunktschweißen (WPS) gegeben ist, kann es durch verschiedene externe Einflüsse im betrieblichen Umfeld zur Rissbildung in der Schweißverbindung kommen. Der Einfluss dieser Risse auf die mechanischen Eigenschaften ist derzeit nicht hinreichend genau erfasst, so dass häufig die Vorgabe einer rissfreien Schweißverbindung besteht. Die Kenntnis der Rissanfälligkeit der verarbeiteten Materialkombinationen sowie von ungünstigen Schweißparametern ist daher für viele Unternehmen von erheblicher wirtschaftlicher Bedeutung. Im Rahmen des FOSTA Projekts P921 „Entwicklung eines Verfahrens zur Bestimmung der Rissanfälligkeit von hochfesten Stählen beim Widerstandspunktschweißen“ wurde ein Ansatz zur Bestimmung der Rissanfälligkeit von WPS-Verbindungen hochfester Stähle entwickelt. Mittels einer hydraulischen Vorrichtung unter Zugbelastung wurden geschweißte, rissbehaftete Proben optisch ausgewertet. Das Verfahren ist geeignet, um ein Werkstoffranking bezüglich der Rissanfälligkeit beim Widerstandspunktschweißen für Werkstoffe aufzustellen.

This publication describes novel experimental and numerical investigations on the punching failure of carbon fiber-reinforced plastics. This kind of failure is relevant in joining processes as well as a characteristic failure type of mechanical fasteners between metals and carbon fiber-reinforced plastics like rivets or screws. So far, there is a lack of a precise phenomenological description of this failure mode. For this reason, this paper focuses on the experimental analysis of the damage and failure mechanism. As a result, the experiments show a failure evolution without any shearing of the fibers. This provides the basis for the accompanying simulation using the finite element method. Subsequently, the simulation of the experiments demonstrates that a modeling of a punching failure is possible by a combination of known fiber, inter-fiber and delamination failure criteria without extensions.

Fiber metal laminate (FML) composite materials are an economical alternative to engineering materials because of their superior properties. While assembling these composites, drilling is unavoidable. Drilling of such materials is a difficult task because of the drilling induced damage. Reducing this damage requires an optimization of the drilling parameters. The main focus on the optimization of the process parameters in drilling of fiber metal mesh laminate (FMML) composites lies in the application of the Taguchi procedure with Grey relational analysis (GRA). In the respective drilling experiments, coated drills were used for the FMML which was specially made with stainless steel wire mesh (SSWM) in the middle layer and woven glass fiber on top and on bottom layer using vacuum bag molding. The drilling parameters such as point angle, spindle speed and feed rate were optimized with concern of multiple performance characteristics, such as thrust force, delamination factor and roundness. The results of the GRA indicate that the feed rate is the most influential parameter. The experimental results illustrate that with the help of the optimal drilling conditions, the machining performance in the complete composite component production process can be improved.

The present study is aimed at finding an optimization strategy for the CNC pocket milling process based on regression analysis including differential evolution algorithm (DEA). Milling parameters such as cutting speed, feed rate and depth of cut have been designed using rotatable central composite design (CCD). The AISI 1050 medium carbon steel has been machined by a high speed steel (HSS) flat end cutter tool with 8 mm diameter using the zig-zag cutting path strategy under air flow condition. The influence of milling parameters has been examined. The model for the surface roughness, as a function of milling parameters, has been obtained using the response surface methodology (RSM). Also, the power and adequacy of the quadratic mathematical model have been proved by analysis of variance (ANOVA) method. Finally, the process design parameters have been optimized based on surface roughness using bio-inspired optimization algorithm, called differential evolution algorithm (DEA). The enhanced method proposed in this study can be readily applied to different metal cutting processes with greater and faster reliability.

Conventional aluminum (Al) and its alloys have been replaced by aluminum matrix composites (AMCs) due to their superior properties in the recent years. There are several attractive candidates as reinforcement for aluminum composites such as Al 2 O 3 , B 4 C and SiC. Nowadays, carbon based materials such as graphite, carbon nanotubes (CNTs) and graphene have attracted much attention owing to their above mentioned mechanical and physical properties. In this study, the wear behavior of Al graphene nanoplatelets (GNPs) composites reinforced with up to 2 wt.-% GNP were investigated. The addition of graphene up to 1 wt.-% decreased coefficient of friction. Experimental data were used to develop linear and nonlinear models. The experimental results are in good agreement with the results of the simulations based on the identified models.

Kurzfassung Im Vergleich zu metallischen Werkstoffen zeigen faserverstärkte Kunststoffe auch bei einfachen uniaxialen Beanspruchungen ein komplexes viskoelastisches Materialverhalten. Die Werkstoffmodelle sind schwer übertragbar und nur für eng eingeschränkte Werkstoffgruppen gültig. Der vorliegende Beitrag analysiert daher anhand von experimentellen Untersuchungen (Zug, Kriech-Erholung, Wöhler Tests) das mechanische Verhalten von carbonfaserverstärkten Sheet Molding Compounds (CF-SMC) in Abhängigkeit von Temperatur, Probendicke und zahlreichen weiteren Einflussfaktoren. Darüber hinaus wird der Fokus auf die Beschreibung des Schwingfestigkeitsverhaltens unter Berücksichtigung der Steifigkeitsdegradation gelegt. Auf Basis umfangreicher Prüfergebnisse werden unterschiedliche Ansätze zur Werkstoffmodellierung vorgestellt und diskutiert.

Kurzfassung Stumpfschweißverbindungen austenitischer Werkstoffe sind häufig Teil von Komponenten der Kraftwerkstechnik oder des chemischen Apparatebaus. In diesem Bereich unterliegen die Bauteile zyklischen mechanischen und thermischen Lasten. Vornehmlich bedingt durch hohe Temperaturschwingweiten, resultieren hohe elastisch-plastische Dehnungsschwingweiten. Die Bereiche Grundmaterial und Schweißnaht zeigen ein unterschiedliches plastisches Deformationsverhalten, die daraus resultierende metallurgische Kerbe kann versagensmaßgebend sein. Im Rahmen der begleitenden numerischen Untersuchungen wurde ein Modell entwickelt, um die metallurgische Kerbe abzubilden.

Kurzfassung AFP-Stähle (ausscheidungshärtende ferritisch-perlitische Stähle) bieten gegenüber geschmiedeten Vergütungsstählen mitunter erhebliche Kostenvorteile, stoßen aber bislang auf Vorbehalte bei Anwendungen, bei denen eine schlagartige Belastung nicht ausgeschlossen werden kann. Insbesondere sicherheitskritische Bauteile der Radaufhängung und -lenkung müssen so dimensioniert werden, dass ein Versagen im Betrieb sicher ausgeschlossen werden kann.

Among all the raw materials for aluminum electrolytic capacitors, there is no doubt that the anode foil is one of the most important raw materials influencing the performance of capacitors. The voltage, capacitance, thickness and impurity content of the anode foil have a direct effect on the performance of capacitor. The cost of anodes also contributes to the total cost of capacitors by a high percentage. Thus, for the manufacturers of capacitors, minimizing the anode foil costs needs also to be taken into account besides its performance. Against such a background, this article evaluates the anode foil of a new supplier to be used in a 400 V medium and in a high voltage aluminum electrolytic capacitor with the purpose of minimizing the product costs while achieving the same performance. The capacitors with new supplier's anode foil are compared with that using the current anode foil by evaluating the performance by the change of the various electrical properties.

Thermal insulation concrete with recycled coarse aggregate (RATIC) provides an excellent alternative to energy saving and the reuse of waste concrete in buildings. In this paper, experiments were conducted to investigate the mechanical properties of the RATIC material after exposure to elevated temperatures, including failure feature and residual strengths. The effects of temperatures and the replacement percentage of recycled coarse aggregate (RCA) on the compressive and tensile strengths of the RATIC were first assessed experimentally. Then, the formulas for both compressive and tensile strengths were developed as a function of temperatures using the regression analysis. The test results show that the RATIC is prone to explosive spalling due to the compact structure of cement paste and low thermal conductivity. The compressive strength of the RATIC increases slightly under the temperature of 400 °C and then decreases significantly with an increasing exposed temperature. In contrast, the splitting tensile strength of the RATIC decreases significantly after the material was exposed to a high temperature of 200 to 800 °C. With the increasing RCA percentage, the RATIC experiences more strength deterioration in tension than compression, while little difference was observed regarding thermal exposure and failure modes of the RATIC.

In this study, the rut susceptibility of warm mix asphalt (WMA) was evaluated by laboratory material characterization tests that included repeated creep testing using universal testing machine (UTM-5P) on WMA and conventional hot mix asphalt (HMA) mixtures. With regard to the fact that aggregate makes up a high proportion of volume and mass of mixtures, this parameter is considered as an important constituent of asphalt concrete. So, one aggregate source, three gradations and three different percentages of Sasobit® content were used to manufacture mixtures. Four creep parameters, namely, flow number (FN), creep stiffness (CS), creep strain slope (CSS) and secondary initial point (SIP), were used to analyze the test results. The primary objective of this research effort was to conduct a comparison between permanent deformation of HMA and WMA mixtures and also to evaluate the effect of aggregate gradation and different Sasobit® contents on introduced creep parameters. Analysis of data shows that the creep behavior of both asphalt mixtures (HMA and WMA) was influenced by aggregate gradation and also by Sasobit® content. Comparison of test results between HMA and WMA exhibits that, in general, WMA mixtures exhibit improved rut resistance compared to conventional mixtures. Also, it is concluded that creep strength of WMA mixes are more susceptible to change in aggregate gradation compared to HMA mixes, while increasing in Sasobit® content produce more stable mixtures. In addition, it is determined that finer aggregate gradations were more influenced by change in the Sasobit® content.

In this study, machinability behavior of AA7075 aged at different time lengths was examined experimentally and by using artificial neural network prediction model. The hardness values were measured after the heat treatment processes. Homogenized reference samples and aged samples were machined by turning processes. On the one hand, the wear occurring on the cutting edge during machining, and the cutting forces depending on cutting speed and surface roughness were investigated. Surface roughness values for each reference material and aged sample were measured using processing parameters. Acquired surface roughness values formed a surface roughness prediction model by using artificial neural networks. The results showed that the surface roughness of the samples decreases while the cutting speed of the lathe increases. In the prediction model formed by using surface roughness acquired after the machinability tests, cutting force, cutting speed and aging process were used as input parameters. Surface roughness as a result of machinability tests were used as output parameters of the proposed prediction model. High coefficient of determination, R 2 rate, obtained in the formed prediction model showed that the model is successful in the prediction of surface roughness.

Fusion welding techniques are not recommended for the welding of EN AW 2024 aluminum alloy due to its dendritic structure, porosity, slug inclusion and distortion, etc. To prevent a decrease in weld joint strength, solid state welding techniques represent a more suitable alternative. Friction stir welding is commonly used for welding of aluminum alloys due to the welding temperature which is below melting point. The aim of the present study is to determine the optimal friction stir welding parameters using Taguchi-based Grey relational analysis. The tool pin shape, tool rotational speed and welding speed were considered as process parameters, while the ultimate tensile strength and percentage of elongation were considered as performance characteristics. The optimal level of each welding parameter was determined by the highest mean Grey relational grade of each parameter. The predicted optimal welding condition was confirmed by experiments. The effect of each parameter on the Grey relational grade was investigated using the probability value and the percentage contribution obtained from the analysis of variance. The results of the Grey relational analysis showed that the optimal friction stir welding parameter combination was the pin shape at first level with tool rotational speed of 500 rpm and welding speed of 80 mm × min −1 (PS 1 TRS 1 WS 2 ). The evaluation of analysis of variance results indicated that the parameter with the highest contribution to the Grey relational grade value was the tool rotational speed, 39.79 %, and also, it has a large effect, at a 90 % significance level. An investigation of the macrostructure of each welded joint exhibited a sound weld joint for the triangular shaped pin. The tool rotational speed of 800 and 1000 rpm produced the lowest quality in welded joints due to a range of defects in the nugget zone.