Volume 60, Issue 1

This paper considers the mechanisms which can detrimentally affect the performance and reliability of materials in their technical applications. The basic deterioration mechanisms – electromigration, aging, biodeterioration, corrosion, wear, fracture – are reviewed and the methods of structural health monitoring and performance control are outlined.

The corrosion fatigue behavior of extruded magnesium alloys AZ31 and ME20 was characterized by means of load increase tests in 5 wt.-% NaCl solution. The results were correlated with the respective corrosion behaviors investigated in immersion tests and by means of microstructural investigations after precorrosion. ME20 exhibited a superior corrosion resistance compared to AZ31, probably as a result of an enhanced passivation tendency due to grain refinement because of the presence of rare earth alloying elements. Accordingly, no differences in the cyclic deformation behaviors of ME20 in air and 5 wt.-% NaCl solution were observed in the load increase tests, while AZ31 exhibited a reduction of the estimated fatigue strengths as well as signs of embrittlement due to the mechanical-medial load.

GTD-111 precipitation-strengthened nickel-based superalloy is widely used in blades of gas turbine engines which operate at high temperature and in a hot localized corrosion atmosphere. After long-term exposure to high temperature, γ' precipitate is known to exhibit catastrophic changes in size and distribution which cause deterioration of its properties and failure of the component. In this study, a damaged blade removed froma land-based gas turbine generator was subjected to nonpre-heat-treated GTAW and laser welding repair with various welding powers in the range of 135 to 295 J × mm −1 , followed by post-weld heat treatment (PWHT) at 1473 K for 7200 s and strain aging at 1118 K for 86 400 s. Results show no significant relationship between welding powers, size and area fraction of the γ' precipitate in the fcc γ matrix in both GTAW and laser-welded specimens. The final γ' precipitate size and distribution depend mainly on PWHT parameters as γ' precipitates in all GTAW and laser welded specimens showed similar size and area fraction independently of the heat input from welding. Unmixed zones are observed in all laser welding specimens which may cause preferential weld corrosion during service. Microcrack occurrence due to welding and PWHT processes is also discussed.

The effect of Cr content on the microstructure and mechanical properties of carbidic austempered ductile iron (CADI) was studied by means of optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM), Rockwell hardness testing, pendulum impact testing machine and ring-on-block wear tester. The results show that with the increase of Cr content the volume fraction of graphite and nodularity rate of as-cast ductile iron decrease gradually, but carbide content increases, and the diameter of graphite reaches the maximum followed by decrease. Then, the specimens were austenitized at 900 °C for 100 min and austempered at 300 °C for 100 min. The hardness and wear resistance of CADI reach maximum when Cr content is 1.42 wt.-% and then decrease with the increase of Cr content, and the impact toughness decreases gradually. Taking the impact toughness into consideration, the comprehensive mechanical property is best when Cr content is 0.96 wt.-%.

The widespread application of resistance spot welding in automotive body assemblies has made it mandatory for automotive manufacturers to examine the quality of the welding. A variety of destructive and nondestructive tests are applied and the weld nugget size is examined for inspection for weld quality. Because of the wide variety of welding parameters used in the spot welding, a welding diagram which gives the optimum weldability range for the welded material is required and extensively used. Due to the different chemical, physical and mechanical properties of different materials in dissimilar joints, it is difficult to evaluate the quality of the weld. In this study, the DP600/TRIP800 dissimilar steel couple was joined using resistance spot welding by combining various welding parameters including welding current, welding time and electrode force. The tensile shear test was applied for determining relationship between welding parameters and mechanical properties of weldment. For quality inspection, the weld nugget size and microstructure of welded sample was examined metallographically and the welding diagram dedicated to the optimum weldability range was also developed for dissimilar joints.

The aim of the present study is to investigate the effect of welding parameters both on the electrochemical corrosion behavior and tensile strength of pre- and post-electrochemical corrosion of friction welded dissimilar steels. The microstructural changes of AISI 1040/AISI 304L friction welded couples and also parent materials were analyzed by using scanning electron microscopy. The electrochemical behaviors of AISI1040/AISI304L joints were comparatively investigated by potentiodynamic polarization curve test and by electrochemical impedance spectra. Moreover, tensile strength experiments were carried out determining the behavior of friction welded joints of pre- and post-electrochemical corrosion and results indicated that the maximum tensile test value of the dissimilar welded pre-electrochemical corrosion was higher than those of post-electrochemical corrosion and was also very close to AISI 1040 parent material value.

The fatigue behavior of KMN with different heat treatments (marked as S and I) used in compressor blades was investigated in a very high cycle fatigue regime. It was found that the S-N curve of KMN-I had a slow decline after 10 6 cycles, while the fatigue limit was observed in KMN-S. In order to explain the difference between the two steels, mechanical property tests, metallographic observation, and SEM analysis were carried out. The results showed that surface fractures were mostly observed in KMN-I, while interior cracks initiated from nonmetallic inclusions which were identified as MnS by SEM. In addition, the surface defects were simplified as cracks using fracture mechanics and the fatigue strength model with surface defects was modified, which proved to be more in agreement with the experimental results than the Murakami model.

The surface topography of temper rolled strip consists of a range of spatial wavelengths with approximate normal distribution in height. Gaussian filtering methods can be applied in measuring and characterizing the topography of temper rolled strip. In this study, three Gaussian filtering algorithms, namely convolution algorithm, fast Fourier transform algorithm and fast recursive algorithm, are compared, in order to find out the most suitable one for the topography of temper rolled strip. A similar profile extension method based on the distribution of the topography of strip is proposed to eliminate the edge effect. The results show that the convolution algorithm is obviously beneficial for the computational efficiency. It is also found that the similar profile extension method can effectively eliminate the edge effect and improve the reliability of edge data. The Gaussian convolution algorithm, combined with the similar profile extension method, is able to effectively assess the surface topography of temper rolled strip.

Creep behavior of the ZK60 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress at high temperatures (175, 200 and 225 °C) under low stresses (25, 50, 75 and 100 MPa) and at low temperatures (100, 125 and 150 °C) under high stresses (100, 125, 150 and 175 MPa). The creep mechanism of the extruded ZK60 magnesium alloy is grain boundary sliding by dislocation climbing obtained at low temperature and high pressure. At high temperature and under low pressure, the creep mechanism of the extruded ZK60 magnesium alloy is grain boundary diffusion by grain boundary sliding. The initial microstructure of the extruded ZK60 magnesium alloy was composed of mainly Mg phase and a little MgZn phase. Under the conditions of low temperature and high pressure, MgZn 2 phase precipitated during creep deformation. Under the conditions of high temperature and low pressure, MgZn 2 phase and a small amount of Zr-Zn phase precipitated during creep deformation. The creep resistance of magnesium alloy ZK60 under the conditions of low temperature and high pressure is better than under the conditions of high temperature and low pressure.

Kurzfassung Der Einfluss der Oberflächenrauheit und die Anzahl der Messungen auf die Zuverlässigkeit und Streuung der Ergebnisse der mobilen Härteprüfung metallischer Werkstoffe wird kritisch untersucht. Dafür werden die häufigsten mobilen Härteprüfverfahren, UCI, Leeb und TIV an vier gängigen Stahlwerkstoffen mit abgestuften Rauheitswerten eingesetzt. Da die meisten Verfahren „blind“, d. h. ohne optische Kontrolle der Eindrücke arbeiten, ist die Zuverlässigkeit der Ergebnisse hier nur über die Streuung der Einzelmessungen kontrollierbar. Es wird gezeigt, dass die Rauheitsanforderungen gültiger Normen und Richtlinien oft unterschritten werden können. Dagegen sollte zur Sicherstellung einer hohen Messgenauigkeit, insbesondere bei großen Rauheiten, die Anzahl der Messungen jedoch erhöht werden.

Improvement of existing composite material to obtain new composite materials is the real challenge for most of the materials engineers. So, there are huge research endeavors emerging in the field of composites to grow new materials with upgraded mechanical, electrical and thermal properties. Among these, fiber reinforced polymer composites are the most charming as a result of their sufficient utilization in different applications which incorporates numerous mechanical, automotive and structural components. The present study investigates the mechanical characteristics of a Kevlar/glass fiber reinforced with Cloisite 30B nanoclay. Kevlar fibers have been widely used as impact-resistant reinforcement in composite materials. However, there are very few works about the effects of nanoclays on the impact strength of Kevlar/glass laminates. Nanoclay Cloisite 30B (2 wt.-%) was dispersed in the epoxy system. Such laminates manufactured with epoxy resin filled by 2 wt.-% of nanoclay showed the best performance in terms of elastic recuperation and penetration threshold. The opposite tendency was observed for the displacement at peak load. However, marginal benefits can be found when compared with the results obtained for laminates filled by 2 wt.% nanoclay. Kevlar/glass fibers were reinforced with different particulate fractions of chosen nanoclay and the specimens were investigated regarding mechanical properties such as tensile, impact and flexural strength.

The aim of this study was to investigate the effect of seawater on bearing strength of glass-epoxy laminated composites. The ratio of the edge distance to pin diameter (E/D) was changed from 1 to 5 and that of the specimen width to pin diameter (W/D) was selected as 3 and 4 for this study. The hole of the specimen was subjected to a traction force induced by a pin. Laminated composite plates were manufactured at the Company Izoreel Composites. Tension tests were performed on pinned joint specimens with a stacking sequence of [0/90] 3S . The specimens were immersed in seawater of the Aegean Sea having a salinity of about 3.5 wt.-% in a glass aquarium at laboratory for 1, 3, 6, 9 and 12 months. For each period of immersion time, failure modes and maximum failure loads of the specimens were found experimentally. The maximum reduction of the bearing strength was obtained as 38% for E/D = 3 and W/D = 3 due to the seawater effect.

Plasticized and unplasticized chitosan films were successfully prepared by thermomechanical treatment, as a possible alternative route to solvent casting method. Acetic acid and lactic acid were used as solvents and glycerol was used as plasticizer with a fixed concentration of 25 wt.-%. The properties of the prepared samples were investigated demonstrating the effects of acid type and the addition of glycerol. Microstructure analysis results revealed a homogeneous and cohesive matrix, indicating a smooth surface without pores, cracks and irregularities. Unplasticized samples with lactic acid showed lower stiffness, higher elongation at break, more thermal stability, higher water uptake and water vapor permeability compared to the samples prepared with acetic acid. The introduction of glycerol affected all properties of the samples, but the samples prepared with lactic acid were more influenced. The rheological and mechanical properties (tensile strength and elongation at break) were improved when glycerol was added, resulting in a ductile behavior with a small plastic deformation and higher elongation at break compared to unplasticized films. On the other side, thermal properties were negatively affected by a decrease in the thermal stability. The water uptake and WVP measurements verified that the hydrophilic character of the material was enhanced by the addition of glycerol.

Graphene-based conductive films have already attracted great attention due to their unique and outstanding physical properties. In this work, in order to develop a novel, effective method to produce these films with good electrical conductivity, a simple and green method is reported to rapidly and effectively reduce graphene oxide film using a low temperature heat treatment. The reduction of graphene oxide film is verified by XRD, FT-IR and Raman spectroscopy. Compared with graphene oxide film, the obtained reduced graphene oxide film has better electrical conductivity and its sheet resistance decreases from 25.3 kΩ × sq −1 to 3.3 kΩ × sq −1 after the heat treatment from 160 to 230 °C. The mechanism of thermal reduction of the graphene oxide film mainly results from the removal of the oxygen-containing functional groups and the structural changes. All these results indicate that the low temperature heat treatment is a suitable and effective method for the reduction of graphene oxide film.

During the shear forming process, precision, repeatability and reproducibility are significant factors to be considered while rotating the workpiece to shape it in the form of the mandrel. Final thickness of the material is dependent on the initial thickness and mandrel angle. The projected area of the final product is nearly equal to the area of the initial workpiece. On the other hand, high feed ratios may lead to cracking and low feed ratios reduce the workability and thickness of the material by causing overexpansion of the material in the radial direction. As the spinning ratio increases, the process becomes difficult. If the process is not carried out cautiously, some defects, such as wrinkling, tearing or cracking, may occur in the spun part. These are caused by applying unacceptable feeds and rpms to the material and diameter. Problems such as wrinkling, tearing or cracking can be addressed by applying the optimum rpm and velocity values with respect to the material being worked on. Several methods have been used to predict the behavior of the material during spinning. For example, during shear spinning, the surface roughness and required force can be determined by regression analysis. In this study, the correlation among optimum rpm, velocity, mandrel angle, and the initial and final thicknesses of metals during shear formation was evaluated.