Volume 58, Issue 3

For the characterization of the deformation and damage behavior of the automotive lightweight steels ZStE340 and DP1000, high speed tension tests for various stress triaxialities with shear tensile specimens, smooth tensile specimens and notched specimens and static and dynamic Nakajima tests were performed. The deformation up to failure was recorded using high speed video cameras with high resolution in the necking zone and evaluated with 2D and in part 3D digital image correlation (DIC). The maximal achieved effective strain at the location of failure, determined by video analysis and fractography, was specified as failure strain. The results for various specimen geometries were plotted versus stress triaxiality as determined by FE simulation in order to receive failure diagrams. Overall, both materials show descending failure curves from shear loading up to tensile loading of notched specimens. Up to equibiaxial tension, failure curves are rising again. For uniaxial and equibiaxial tension, with increasing strain rate larger failure strains occured for DP1000, while no significant influence of strain rate on failure strain was observed for ZStE340. However, under shear loading with increasing strain rate, decreasing failure strains occur for both materials. This is probably due to an adiabatic temperature rise in a small localized zone which promotes shear failure. For shear crash tests a temperature rise of more than 200 K was measured by a high speed infrared camera. These results show the necessity of dynamic tests under different stress triaxialities to provide reliable input data for crash simulations and increase the ability to accurately predict failure. Especially shear failure must be investigated very carefully under crash loading, because here failure occurs at smaller failure strains than it would be expected under quasistatic loading.

An intensive failure investigation of the alloy 803 outlet pigtail tube, used in a steam hydrocarbon reforming furnace, was conducted in the current study, in order to clarify the failure root causes. In a fabrication stage, the outlet pigtail tube was unintentionally overheated during post weld heat treatment for solution annealing. The overheating temperature of about 1250 °C, exceeding a designed temperature of 1150 °C, was detected by thermal measuring devices. As a result of overheating, the grains of the material were remarkably coarsened. An average grain size of 598 μm was found for the overheated region, which was about 2.6 times larger than the grain size of material in the unaffected regions. Additionally, overheating resulted in the partial dissolution of Ti-rich stabilized carbides in the grain interiors. During service at a temperature between 800 and 850 °C, Cr-rich carbide networks were formed at the grain boundaries. A combination of the coarsened grains and the Cr-rich carbide networks at grain boundaries strongly resulted in a decrease of the materials cracking resistance. Therefore, the welded outlet pigtail tube failed by the occurrence of circumferential rupture after being in service for about 500 hours. Based on these evidences, the reheat cracking mechanism has to be anticipated as to be important for controlling the rupture of the welded outlet pigtail tube.

The study examines how, using wire electrical discharge machining (WEDM), the microstructural, mechanical and conductivity characteristics of the titanium alloy Ti6Al4V are changed as a result of heat treatment and the effect they have on machinability.Scanning electron microscope (SEM), optical microscope and X-ray diffraction (XRD) examinations were performed to determine various characteristics and additionally related microhardness and conductivity measurements were conducted. L 18 Taquchi test design was performed with three levels and six different parameters to determine the effect of such alterations on its machinability using WEDM and post-processing surface roughness (Ra) values were determined. Micro-changes were ensured successfully by using heat treatments. Results obtained with the optimization technique of artificial neural network (ANN) presented minimum surface roughness. Values obtained by using response surface method along with this equation were completely comparable with those achieved in the experiments. The best surface roughness value was obtained from sample D which had a tempered martensite structure.

The purpose of this study was to investigate the characteristics of magnesium alloy AZ31B and its resilience with regard to fatigue by means of spectrum loading for automotive applications. This is because magnesium is being used extensively in automotive components and indirectly, it is necessary to know the fatigue level of the material to have a clear picture when designing the use of the material. Experiments and simulations were used in this study, which involved the experimental testing of material stress and fatigue. The fatigue test, using hydraulic servo fatigue, was divided into two parts, one using the constant amplitude loading analysis, and the other, spectrum loading, to get the level of resilience of the alloy concerned. The simulations were conducted using fatigue analysis software to obtain the number of fatigue cycles of the material using the Coffin-Manson theory. The spectrum loading data was developed based on the percentage of the yield stress of the AZ31B alloy. The experimental results obtained show that the material properties of magnesium alloy AZ31B are approximately equivalent to the ASTM standard with an error of less than 20 %. The validation experiments and simulations for loading fatigue had almost the same effect, namely in the correlation ratios of 1 : 2 and 2 : 1. From the study, it can be seen that the resilience of AZ31B makes it appropriate for the use in automotive applications.

Selection of welding parameters is very important to increase the weld strength. Friction stir welding parameters affect the weld strength of aluminum alloy, such as high strength AA7075 plates. This paper presents an application of Taguchi method to friction stir welding strength of AA 7075 plates. An orthogonal array, the signal-to-noise ratio (S/N), and the analysis of variance (ANOVA) are employed to investigate friction stir welding parameter effects on the weld strength. From the ANOVA and the S/N ratio response graphs, the significant parameters and the optimal combination level of welding parameters were obtained. Confirmation experiments have been conducted to validate the optimized parameters.

Es wurden Al-TiO 2 -Gr-Komposite mittels Rührgießens hergestellt, die verschiedene Gehalte an TiO 2 - und Graphit-Partikeln enthielten. Die Proben wurden hergestellt, indem die TiO 2 - und Gr-Partikel geschmolzenem Aluminium zugegeben wurden, dann wurde die Schmelze bei einer bestimmten Temperatur für eine vorgegebene Zeit gerührt. Mit den Gussstücken wurden REM- und EDAX-Analysen durchgeführt. Anschließend wurden die Auswirkungen der Zugabe von Verstärkungen auf die mechanischen Eigenschaften und die Bearbeitbarkeit der Komposite untersucht. Die mikrostrukturelle Untersuchung der Endprodukte erwies, dass synthetisierte Komposite mit gut verteilten Verstärkungspartikeln mittels Rührgießen herstellbar sind. Die Bruchanalyse der Zugproben wurde ebenfalls mittels REM-Analyse durchgeführt. Die Ergebnisse zeigen, dass die Zugfestigkeit der Komposite mit steigendem Gewichtsanteil der TiO 2 - und Gr-Partikel zunimmt. Die Bearbeitbarkeitsstudien zeigen, dass die Zugabe der Verstärkungen die jeweils wahre Axialspannung σ z , die wahre Umfangsspannung σ θ , die wahre hydrostatische Spannung σ m und die Spannungsverhältnisse σ z / σ eff , σ θ /σ eff und σ m /σ eff beim Kaltstauchen erhöht.

A food processing plant supplied different heat exchanger plates to investigate the cause of their premature corrosion. In the elder version of the system, heat exchanger plates of grade 316/316L worked for five years before they became unsuitable for use. The new production line includes two heat exchangers working interchangeably to allow breaks for regular cleaning. An increased permeability during hydrogen test showed that corrosion of new plates started in one of the heat exchangers after two weeks, and in the second one after few months. Leaks manifested themselves by a regular pattern of corrosion pitting, occurring exactly in contact points of adjacent plates. A detailed study of the microstructure features of the austenitic steel plates was performed using scanning electron microscopy and metallography. It was found that plates corroding prematurely had surface impurities containing chlorides. A characteristic feature of the corrugated plate surface is secondary recrystallized microstructure with thick and pronounced austenite grain boundaries. Pitting was identified as the dominant material degradation mechanism promoted by fretting corrosion of adjoining plate ribs.

Hadfield steel, due to its high manganese content, is difficult to drill and work-hardens very quickly. In this study, Hadfield steel material was drilled with micro-size deep holes using the electrical discharge machining (EDM) technique, and hole diameter values were examined for specific machining parameters. Experiments were carried out with three different discharge currents (6, 12 and 24 A), three different electrode rotational speeds (200, 400 and 600 rev × min -1 ), three different pulse durations (12, 50 and 100 µs), a fixed dielectric spray pressure (40 bars) and a fixed pulse interval (3 µs). It was determined that the hole profiles obtained following the tests are directly related to the machining parameters, and that the resulting average overcut (AOC) and taper (Tp) values increased with discharge current, electrode rotational speed and pulse duration. Analysis of variance (ANOVA) conducted demonstrates that pulse duration is the dominant parameter affecting AOC, whereas pulse duration has the highest effect on Tp. When determination coefficients and normal probability plots were compared for the mathematical models obtained from analyses conducted for the prediction of test values, it was observed that the models obtained by quadratic regression analysis exhibited a better performance than the models produced by linear regression analysis.

This experimental investigation deals with the evaluation of abrasive wear behavior of Al alloy and Al 2 O 3 particle reinforced Al alloy metal matrix composites on a pin-on-disc test rig, where the specimens were slided against a 600 grade SiC P emery paper. The specimen preparation and experimentation have been carried out according to the ASTM G-99 standard. A plan of experiments based on the Taguchi design of experiments was developed to acquire data in controlled way. An orthogonal array and the analysis of variance were employed to investigate the percentage of contribution of various process parameters like sliding speed, applied load, sliding distance and their interactions affecting the abrasive wear volume loss of the materials. The correlations between the various factors affecting the abrasive wear behavior of composites were obtained by using multiple linear regression equations. The experimental results showed that the sliding distance has more contribution on the abrasive wear followed by applied load for both materials. The predicted volume loss of the specimens was found to lie close to that of the experimentally observed ones.

This article presents the optimization of process parameters in reducing the risk of failure due to wrinkling and fracture in deep drawing of a rectangular cup. Blank holder force, friction coefficient, punch radius and initial blank shape were chosen as design parameters. Wrinkling and fracture criteria were used as objective functions. Two-stage optimization approach was used. In the first stage optimization, optimum blank shape was determined using the Taguchi optimization method. In the second stage optimization, optimum parameter values were determined for the selected blank shape by the first stage optimization using genetic algorithm. In the second stage of optimization, response surface models based on 3-level full factorial design of experiments were constructed. Finite element simulation results were used for creating response surface models. Parameters obtained from optimization study were confirmed by conducting deep drawing simulations using commercial code DYNAFORM 5.9.2. Optimum results showed that proposed optimization methodology can solve wrinkling and fracture problems in the sheet metal forming process.

In this work, microstructures were developed on a polymer surface using an embossing process by utilizing the transverse section of a wood as a biotemplate/mold. Microstructured polymers obtained by the proposed practical method were characterized by optical microscope for rough imaging, by scanning electron microscope for morphological properties and by Fourier transform infrared spectroscopy for surface chemistry. Utilization of a fluoropolymer eliminates the anti-stick surface pre-treatment and facilitates intact demolding of polymer structures from micro-voids of the wood. The height of the polymer microstructures was measured as 4 to 6 μm. Dimensions of the wood micro-voids can be tuned via acid/base treatment process. A drastic increase in heights of polymer microstructures (26 μm) was observed for the sample stamped by the acid/base treated biotemplate. The proposed method, which delivers well-organized microstructure patterns on fluoropolymer surfaces, can be utilized in the production of specialty surfaces, such as antibacterial, stick/anti-stick and water repellent.

In the present work, a measurement system has been designed and developed to measure friction and wear for two wheeler brake liners. Non-asbestos organic brake liners were evaluated regarding friction and wear by pressing against aluminum alloy brake drum. Each brake liner was subjected to four different applied loads (400, 500, 600 and 700 N) with six different braking times (2, 4, 6, 8, 10 and 12 minutes). The velocity of the brake drum was fixed at 1000 rpm for all test conditions. The drum temperature was measured by a thermocouple embedded in the brake drum and coupled with programmable logic controller (PLC) to record the temperatures of brake drum during braking. Test results obtained from the friction test were drum temperature (°C), friction coefficient (μ) and wear volume (cm 3 ). From the test results, it was concluded that the increase of friction coefficient at the beginning of braking is due to the sharp edges such as zircon being ploughed onto the wear surfaces and also due to the increase in the contact area of the brake drum. The wear volume increased exponentially for the applied load more than 600 N and braking time more than 4 minutes. Below the applied load of 600 N, the wear volume increased linearly. The degradation of the organic components in the composition increased with temperature and this resulted in the reduction of composition bonding between the powders and structure integrity, thus increasing wear exponentially.

Joint replacement surgeries are doubtlessly the gift of science and technology for human welfare. In the cemented knee replacement joint, the stiffer implant carries the majority of the load, which is actually carried by the bone itself before implantation. The resulting implant induced stress-shielding and subsequent bone remodeling causes bone resorption at the proximal bone under the tibia tray. The aim of this study is to optimize the material of the cemented tibia tray using functionally graded material FGM. The target is to find the optimal material and the optimal gradation direction for the cemented tibia tray. The results showed that the optimal vertical FGM changed from titanium at the lower tibia stem to bioglass at the upper layers of the tibia tray. The vertical FGM reduced the stress shielding by 56 and 80 % in cancellous epiphyseal and diaphyseal bone, respectively, compared to cemented titanium tibia tray. However, the optimal horizontal FGM changes from titanium at the stem core to bioglass at the rim of tibia tray layers. This horizontal gradation reduced the stress shielding in cancellous epiphyseal bone by 62 %. However, the stress shielding in cancellous diaphyseal bone did not change compared to cemented titanium tibia tray.

Owing to their superior mechanical and physical properties, carbon nanotubes (CNT) seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. In most of the experimental results up to date, however, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. In the present paper, the influence of single wall carbon nanotube agglomeration on the effective stiffness is analyzed by using an Eshelby’s inclusion model. Analytical expressions are derived for the effective elastic stiffness of single wall carbon nanotubereinforced composites with the effects of agglomeration. The present study not only provides the important relationship between the effective properties and the morphology of CNT-reinforced composites, but also may be useful for improving and tailoring their mechanical properties. In addition, a multiscale Monte Carlo finite element method (MCFEM) was used for determining mechanical properties of polymer nanocomposites (PNC) that consist of polymers reinforced with single-walled carbon nanotubes (SWCNT). Specifically, the method uses a multiscale homogenization approach to link the structural variability at the nano/micro scales with the local constitutive behavior. Subsequently, the method incorporates a FE scheme to determine the Young’s modulus and Poisson ratio of PNC. The use of the computed properties in macroscale modeling is validated by comparison with experimental tensile test data.

In this study, aluminum metal matrix composites were fabricated using the powder metallurgy technique with different ratios of alumina (Al 2 O 3 ) and graphite as reinforcing elements. The obtained workpieces were then drilled using different machining parameters and the thrust force was measured using a force dynamometer. The thrust force data were modeled using a gene programming soft-computing technique and a function was obtained using the input parameters.