Volume 60, Issue 12

The reel-lay method is one of the most effective procedures for installing submarine pipelines. During the reel-lay installation process, pipelines are subjected to large-scale plastic strain because of cyclic bending/straightening processes. Thus, mechanical tests must be carried out to assess the effect of large-scale pre-strain on the girth weld of pipelines after cyclic bending/straightening processes. Tensile tests were performed on weld metals, the results showed that a slight increase in tensile and yield strength were caused by a pre-strain of 2.709 % (maximum strain region). A single-edged notched tension (SENT) specimen was examined via the unloading compliance method to evaluate the fracture toughness for the base metal, weld metal and HAZ. The fracture toughness test results showed that the weld metal exhibited the lowest fracture toughness followed by the base metal and HAZ. The fracture toughness of the base metal, the weld metal and HAZ all decreased after the cyclic bending/straightening with a pre-strain of 2.709 %.

Strain accumulation is easily formed in friction stir welded Mg alloys which significantly deteriorate the strength of the joints. However, the formation mechanisms of strain accumulation are not well understood. The present study aims to explore the different strain accumulations on the top and bottom side of welded samples using tensile and compressive tests. An effective procedure is provided for measuring strain accumulation. An interesting finding is that the measured strain accumulations differ from those determined by visual inspection. The analyses indicate that the different types of strain accumulation at both sides have a close relationship with the shear layers and textural variations in the stir zone. Additionally, a model for the formation of strain accumulation at the top and bottom side of the Mg weld joints is provided.

In this study, the weldability of lower strength St 37 grade steel plate with a higher strength steel plate (i. e., St 44) using the friction stir welding technique has been studied. St 37 grade steel sheets are widely used in various industries. However, the joining of St 37 steel plates with higher strength grades such as St 44 grade steel is usually required when the strength of St 37 grade is not sufficient. Therefore, microstructural and mechanical properties of friction stir welded St 37-St 44 butt joints were investigated by conducting optical microscopy, microhardness measurements, and tensile testing. A fine-grained recrystallized microstructure was observed in the weld regions of the joints produced. The highest tensile strength, i. e. 385 MPa, was exhibited by the joint produced at 630 rev × min −1 .

The experimental and numerically predicted fatigue strength of steel tire cord filaments used as reinforcing elements in automobile tires has been determined in this study by means of the pure bending test. The filaments had diameters of 0.20 and 0.25 mm and were produced under three distinct conditions of cold drawing strain quantified as 3.37, 3.68, and 3.71 %. All the fatigue tests were conducted at room temperature and at a frequency of 10 Hz via a custom manufactured pure bending tester with a fully reversible alternating strain value, R = ∊ min /∊ max , is −1. As the drawing strain value increased, the fatigue deformation limit increased parallel to an improvement in the mechanical properties. One can expect that an increase in drawing strain through a decrease in filament diameter will provide an improvement in the light weight design of cars.

The microstructure and thermal properties of a 39NiCrMo3 low alloy steel ingot obtained by melting in an electric arc furnace (EAF) and refining by electro-slag remelting (ESR) were experimentally investigated using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), thermal diffusivity measurements and differential scanning calorimetry (DSC). A phase diagram of the alloy steel investigated was calculated using Thermo-Calc software and a TCFE6 database. It was determined that the steel investigated displays a ferrite-very fine pearlite microstructure. The thermal conductivity of the investigated steel, measured in the temperature range from 25 to 400 °C, decreases from 44.3 to 38.3 W × (m × K) −1 .

Nondestructive testing (NDT) of spot welds still poses a great challenge for industrial mass production like automotive body manufacturing or rail vehicle constructions. In these production areas, ultrasonic-based NDT methods with pulse-echo technology have become established. Hitherto, these methods require a lot of experience from the staff, which often makes the results subjective. Furthermore, these methods reach their limits when testing spot welds with deep electrode indentations or huge differences between sheet metal thicknesses. Imaging analysis of the residual magnetic flux density shows great potential especially when addressing the restrictions of the ultrasonic-based NDT methods. Typical spot welds show a characteristic electrode indentation. This indentation influences the measureable magnetic flux density, which depends on the distance between the surface of the spot weld and the sensor. This paper will show the influence of electrode indentation on the test result and presents the resulting restrictions for imaging analysis of the residual magnetic flux density when testing spot welds.

The aim of this work is to study the effect of welding and rotational speed on peak temperature, duration of temperature exposure and their influence on the tensile strength of a friction stir welded aluminum alloy. AA6061-T6 plates 6 mm thick were friction stir welded at various welding and rotational speeds. The temperature during the process was measured. The temperature of the FSW process was predicted using the thermal model and compared with the experimental results. The tensile specimens were prepared from welded plates and tested. The results show that failures occur at the transition between the thermo-mechanically affected zone and the heat affected zone where it possesses minimum hardness. When welding speed increases, peak temperature and the duration of temperature exposure at the failure location decreases and yield and ultimate strength increase. When rotational speed increases, peak temperature and the duration of temperature exposure at the failure location increase and yield and ultimate strength decrease.

Kurzfassung Im Rahmen des Projektes „ELOBEV“ (Erforschung von elektrolytischen Beschichtungssystemen für Verbindungselemente aus höchstfesten Werkstoffen), gefördert vom Bundesministerium für Bildung und Forschung, wird eine Prüfmethodik für die Bewertung der Gefahr einer wasserstoffunterstützten Kaltrissbildung für Hilfsfügelemente erarbeitet. Dazu wird die Wasserstoffaufnahme während der kathodischen Beladung unter dem Einfluss verschiedener Parameter untersucht. Die Bestimmung der materialspezifischen Wasserstoffdiffusionsgeschwindigkeit und Anwendung von Gleichungssystemen sollen Aufschluss über das Wasserstofftransport- sowie Absorptions- und Desorptionsverhalten von vergüteten Stählen unter Einbeziehung unterschiedlicher Überzugskonzepte geben. Auf Basis dieser Ergebnisse soll in Abhängigkeit der Wasserstoffkonzentration sowie des Wasserstoffverteilungsprofils und der Beanspruchung mittels geeigneter Proben Erkenntnisse zu der Empfindlichkeit auf die Ausbildung wasserstoffunterstützter Kaltrisse gewonnen werden.

The effect of cutting parameters, such as depth of cut, feed rate, spindle speed, cutting fluid flow rate and the number of layers deposited on the surface roughness and flank wear of a cathodic arc evaporation coated TiAlN/WC-C tungsten carbide cutting tool insert was investigated during CNC turning of AISI 1015 mild steel. The analysis of variance was applied to determine the critical effect of the cutting parameters. Taguchi orthogonal test design was used to optimize the process parameters. The depth of cut and deposition of layers altered the surface roughness (49 and 32 %, respectively) of the inserts. However, cutting speed, feed rate and the flow rate of the cutting fluid contributed only minimally to the surface roughness. In contrast, the number of layers deposited (48.66 %) and cutting speed (30.93 %) were the dominant factors affecting tool wear. The optimum cutting conditions for achieving the desired surface roughness consist of a medium level of cutting speed, a low depth of cut level, a low feed rate, high flow rate of cutting fluid and multilayer deposition. Minimal tool wear was achieved for the process parameters such as low cutting speed, low depth of cut, medium feed rate, medium flow rate of cutting fluid, and a multilayer deposited tool.

Kurzfassung Die Computer-gestützte Laminografie (CL) wurde als komplementäre Methode zur Computertomografie für die dreidimensionale Bildgebung von lateral ausgedehnten Objekten entwickelt. Ursprünglich für medizinische Zwecke verwendet, wurde diese Methode kürzlich als zerstörungsfreie nicht-invasive Methode nicht nur in der Materialforschung, sondern auch mit steigendem Interesse für kunsthandwerkliche und historische Objekte eingesetzt. Hier wird die Computer-gestützten Laminografie mit polychromatischer Neutronenstrahlung an einer historischen Tsuba eingesetzt, einem Stichblatt eines japanischen Schwerts. Eine Analyse der Lötstellen gibt Rückschlüsse auf das Herstellungsverfahren. Zudem wurden unterschiedliche Materialsysteme gefunden, vermutlich um dem Tsuba lokal eine höhere Stabilität zu verleihen. Die Messungen wurden an der Imaging-Beamline CONRAD-2 an der Neutronenquelle BER 2 des Helmholtz-Zentrums Berlins (HZB) durchgeführt.

Advanced materials such as super alloys, ceramics and composites are not easy to machine by conventional machining techniques to achieve high surface quality, which is the primary customer requirement for the end products. To meet these challenges, electrochemical machining (ECM) is being employed to achieve higher metal removal rate and lower surface roughness and has wide applications in components made for the aerospace and automotive industries, and for surgical components in order to ensure reliable quality. Surface quality is an important factor controlled in ECM. The significant variables affecting quality characteristic measures in ECM are electrolyte concentration, applied voltage, electrolyte flow rate and tool feed rate. According to central composite second order rotatable design (CCD), experiments were conducted. Sensitivity analysis was performed and compared with the relative impact of input parameters on surface roughness R a in order to verify measurement errors. From the results, it was found that a change in tool feed rate affects the R a significantly more than other parameters. A fuzzy logic model was established and the results obtained were compared with the experimental values and found to be close to the experimental values. Further, through dimensional analysis, a semi-empirical model was developed for the variation in surface roughness. This mechanistic model was developed using dimensional analysis for prediction of surface roughness beyond the range of the significant input parameters.

The synthesis of Al/TiB 2 metal matrix composite using the in situ technique has been performed in the present study. Two halide salts, namely potassium hexa fluro titanate (K 2 TiF 6 ) and potassium tetra fluro borate (KBF 4 ) were introduced into an Al 6061 melt at 820 °C adding different amounts of TiB 2 particles, i. e. 4, 6 and 8 wt.-%. The stirring and holding times were maintained for all the casts at 30 minutes. The metal matrix composites were tested with respect to tensile strength and hardness in the as-casted condition. Tensile strength and hardness were found to increase with an increase of TiB 2 . The size and uniformity of the TiB 2 particles in the composites was corroborated with scanning electron microscopy (SEM) and X-ray diffractometry (XRD), confirming the presence of TiB 2 in the composites.

In the current study, the effect of SiC and fly ash particles on the mechanical and tribological behavior of Al 7075 composites has been studied as a replacement for a conventional bearing material (SAE 65 bronze). Al 7075 composites were prepared with the addition of various percentages of reinforcement materials by using the stir casting process. Hardness, tensile strength and wear behavior of the composites were studied and compared with a control specimen and SAE 65 bearing bronze. The hardness of Al 7075 matrix reinforced by SiC particles increases when compared with the base matrix due to the presence of the ceramic phase. The co-efficient of friction considerably decreases with the addition of SiC and fly ash into the base matrix. Specimens were subjected to wear tests under different load conditions and in accord with three wear mechanisms wear groove (or oxidation), abrasion and delamination, evaluated. The abrasion results revealed that the wear resistance of reinforced Al 7075 composites increased compared to both a control specimen and to SAE 65 bearing bronze.

Leachate in landfill contains a great number of toxic and harmful substances and results in long-term contamination of the underground environment once it leaks. Therefore, constructing an impermeable layer at the bottom of sanitary landfills is the key to preventing the contamination induced by leachate leakage. Bentonite is an impervious liner material that is currently widely used in landfills. However, the performance of bentonite as an impervious material is likely to be greatly affected when leachate contains a high salt content. By modifying additives using bentonite, a salt-resistant bentonite can be acquired which satisfies the impermeability requirement imposed by a saline environment. Additionally, by carrying out various tests (including a free swell index test, compaction, hydraulic conductivity, heavy metal adsorption, and direct shear tests), the performance of developed bentonite and the mixture of weathered soil and modified bentonite were verified. The swelling capacity of salt-resistant bentonite in saline water was nearly equal to that in a fresh water environment. After salt-resistant bentonite was mixed with weathered soil in a proportion of 1 : 9, the hydraulic conductivity of the mixture in saline water was less than 1 × 10 −7 cm × s −1 , satisfying the basic requirement for use as a landfill liner material. Moreover, the relationship between the shear strength or the cracks in the liner soil and the initial moisture content of the mixed soil can be obtained by conducting direct shear and drying shrinkage tests on the mixture of weathered soil and modified bentonite. The results are expected to provide data support for the design of engineering construction works in a geo-environmental context.

The influence of the concentration of sodium hydroxide solution on the tensile bond strength of alkali-activated slags (AAS) used as a tunnel fireproof coating material was studied under the simulated combined action of vibration and wind suction. A test method involving the simultaneous application of vibration and suction was developed and utilized. The test results indicate that as the concentration of sodium hydroxide solution in the AAS increases from 1 to 10 weight percentage, the bond strength of AAS coating improves under the coupling action of vibration and suction. Furthermore, the cumulative pore volume and the average pore diameter in the AAS are reduced and the ratio of harmful pores to total pores decreases. As the concentration of sodium hydroxide solution increases more than 10 weight percentage, however, there is no further benefit in terms of the pore structure and bond strength.