Volume 61, Issue 5

Cyclic temperature changes in the working layer of a roll during hot rolling cause thermal stress which leads to damage on the surface layer of the roll due to thermal fatigue. The pre-conditions of crack initiation are described. The intensity of crack propagation could be influenced by optimizing the chemical composition and physical-mechanical properties of the material on the working layer. This reduces the tendency of the material with respect to damage due to thermal fatigue and extends the lifetime of the rolls. The microstructure as well as the operational characteristics of the developed HSS working rolls resistant to thermal fatigue are presented.

The main objective of this study is to analyze the tensile strength of PLA parts manufactured through fused filament fabrication (FFF) using a commercially available 3D printer. This study is primarily focused on the effects of the tensile strength of specimens subjected to the influence of four factors; layer thickness, fill density, raster orientation and sample structure type. A 2 3 3 1 mixed-level factorial design approach was used, and the individual effects of the four main factors and their interactions determined. This experimental design had been implemented for two different infills: rectilinear and honeycomb. Specimens were printed at raster orientation angles of 30 °, 60 ° and 90 ° at a fill density of 50 % and 100 %. A layer thickness of 0.15 mm and 0.05 mm was chosen for printing the specimens. The samples were tested using a standard tensile testing machine with an extensometer to determine mechanical strength characteristics such as ultimate tensile strength, maximum force and maximum elongation. The data obtained was then analyzed using Minitab 13.20 software. The results showed that 30 ° raster orientation yields the highest mechanical properties at each individual layer when compared to 60 ° and 90 °. The fill density proves to be the most influential parameter on tensile strength, followed by the sample structure type. The results also found tensile strength to directly proporionate to layer thickness. By improving the material properties through the addition of layers as observed in the results, it will be possible to provide support for software developers, mechanical designers and engineers to reduce manufacturing time, material use and costs.

In this work, 10 mm thick AA7075-T651 aluminum alloy plates were friction stir welded. The welds were heat-treated using two different heat treatment techniques. Firstly, a special solutionizing method called cyclic solution treatment (CST), involving a repeated heating between 400 and 480 °C for 15 minutes and water quenching. Secondly, CST followed by artificial aging at 130 °C for 24 h. The effect of CST and artificial aging on the mechanical behavior of the welds was studied by dint of hardness, tensile, and impact tests. Weld microstructures were examined by optical microscopy and transmission electron microscopy (TEM). CST enhances hardness across the joint and homogenizes the hardness distribution while marginally improving the yield strength of the weld. CST followed by aging at 130 °C for 24 h was found to result in a significant improvement in the hardness and yield strength of the welded joint. Grain size in the weld nugget is not considerably altered after heat treatment. TEM studies revealed that a partial dissolution of coarse precipitates (MgZn 2 ) and the formation of metastable (MgAlCu) precipitates take place in the weld nugget after CST. In addition, CST followed by artificial aging causes a uniform distribution of stable MgAlCu precipitates in the weld nugget, which thus seems responsible for the enhancement in hardness and strength of the joint.

Thickness distribution has an important impact on the quality of thermoformed products. It has significant effects not only on durability but also on the oxygen and gas transmission rate of thermoformed packaging products. In this study, acrylonitrile butadiene styrene and polystyrene sheets were thermoformed using a lab scale thermoforming machine. Three different mold shapes were considered: conical, spherical and cylindrical. The thickness distribution of thermoformed acrylonitrile butadiene styrene and polystyrene samples for each mold was investigated based on geometric element analysis (GEA) and finite element analysis (FEA). Finite element analysis yielded more accurate results than geometric element analysis. Process parameters such as temperature distribution on the sheets before molding, have a crucial effect on thickness distribution for all configurations.

In the research for this contribution, the use of layered hybrid composites as materials for ballistic armor has been tested. As part of the study, ballistic tests of glass/aramid/carbon fiber laminated comp osite plates at different reinforcing angles, in various numbers and thicknesses were carried out and the results analyzed. Eight hybrid composite laminates, of various thicknesses 200 × 200 mm in size were produced by the hand lay-up method at angles of 0°, +/−45°. Plain and twill fabrics were used to analyze the ballistic performances at various fiber reinforcing angles. Ballistic tests were carried out at the Elazig Special Operation Branch Office, using a Beretta gun loaded with 9 mm FMJ (full metal jacket) bullets. These experiments with ballistic test equipment were conducted in accordance with international standards. During the ballistic experiments, the velocity and BFS (backface signature) was measured and evaluated. Test results suggest that five samples were successful according to NIJ 0101-06 standards. It was found that twill woven (TW) fabrics made for the study manifest a higher ballistic performance than plain-woven (PW) fabrics. UD (unidirectional), +/−45° UD fabrics were found to be more effective than 0° UD fabrics. The findings are indicative for the design of lightweight ballistic armor material.

This research on the stiffness of glass fiber reinforced plastics (GFRP) pipes constructed according to various types of winding patterns and under varied loading conditions, proposes a model of stiffness degradation for GFRP pipes and the parameters for a model of stiffness degradation. Based on a hoop winding GFRP pipe fatigue test, it was found that the stiffness reduction rate of the first half a million cycles is 0.67 times that of the last 500,000 cycles when maximum alternating displacement is 5 % of the diameter of the GFRP pipe. For the same type of winding GFRP pipe, the stiffness reduction rate at a maximum alternating displacement of its diameter'of 10 % and 15 % yields 1.34 and 1.59 times the stiffness reduction rate of a GFRP pipe with an alternating displacement maximum of its diameter of 5 %, respectively. For GFRP pipes of varied types of winding modes, when the maximum alternating displacement is 5 % of the diameter of a GFRP pipe, the rate of stiffness degradation of the helical winding GFRP pipe and hoop-helical combined winding GFRP pipe is 1.50 and 1.28 times that of a hoop winding GFRP pipe, respectively.

The effect of cryogenic treatment on the microstructure, hardening behavior and wear resistance of subcritical treated GX200Cr13Ni6WMoMn was investigated. The specimens were improved for thermal shock resistance by having an austenitic matrix in place under all heat treatment conditions. After subcritical treatment, heat treatment increased the hardness and wear resistance of all the specimens since thin secondary carbides precipitated in the rich retained austenite. The amount of precipitated secondary carbides and martensite transformed by cryogenic treatment was greater than by air-cooling. The cryogenic process reduced the amount of austenite, but the austenite retained was not completely converted into secondary carbides. The abrasion resistance, toughness and hardness of the specimens were markedly improved by primary carbides and secon dary carbides after heat treatment for 1 h at 900 °C.

The standard aluminum alloy AlSi9Cu3(Fe) is the most common Al alloy used to produce castings by means of high-pressure casting technology and has wide application in the automotive industry, especially for motor mounts, gearbox housings, filter housings, generator housings, turbocharger housings, etc. The advantages of AlSi9Cu3(Fe) alloy are relatively good mechanical properties, strength and hardness, although it shows slightly lower elasticity and corrosion resistance. The artificial aging process or stabilization annealing can be used to improve the dimensional stability of castings, with the aim of achieving better mechanical properties as soon as possible. In this paper, the influence of natural aging time on the mechanical properties of AlSi9Cu3(Fe) alloy as well as on microstructure development was investigated. Specimens were prepared by high pressure die casting technology under optimized parameters. Chemical content was determined by optical emission spectroscopy. Thermodynamic calculation was performed by Thermo-Calc 5 software and transformation temperatures during dynamic measurement were followed by differential scanning calorimetry. Tensile properties after 3 and 30 days of aging, as well as 7 months, were determined by a tensile test at an MTS 810 instrument. Metallographic analysis was carried out by optical microscope and field-emission scanning electron microscope. Results showed a significant increase in tensile strength, yield strength and elongation with aging time and formation of (α Al + β Si ), Al x (Fe,Mn) y Si z , Al 2 Cu-Si, Al 5 FeSi and Al-Mg 2 Si-Cu phases.

An 6201Al alloy was rolled at room temperature (RT), liquid nitrogen and alcohol mixed temperature (LAT) and liquid nitrogen temperature (LNT). The effect of the rolling temperature on microstructure and mechanical properties was investigated by means of hardness measurements, tensile tests, optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Microstructural observations reveal that the dislocation tangles and dislocation density increased with decreasing rolling temperature. The dislocation accumulation was enhanced, and the formation of dislocation tangles and cells occurred during deformation under low rolling temperature. The substructures of the rolled sample at LAT and LNT are similar, consisting of dislocation cells, high dislocation density with networks, dislocation array, dislocation tangles and subgrains. Compared with rolled samples at RT and LAT, the rolled samples at LNT presented strengthened hardness and strength. The improvement of mechanical properties contributes to the fine substructures and accumulation of higher dislocation density during low temperature rolling. In addition, with decreasing rolling temperature, the average dimple size decreased.

Stainless steel is a possible measure for increasing the service life of reinforced concrete structures exposed to chlorides. For the prediction of service life it is crucial to quantify the corrosion resistance of the material used by determining the chloride threshold at the steel concrete interface inducing corrosion at the rebar. However, currently the corrosion resistance of stainless steel is expressed by its pitting resistance equivalent number (PREN) which considers only the content of chromium, molybdenum and nickel along with certain factors. The higher the PREN-value, the more corrosion resistant the steel. But this PREN-value cannot be transferred into a chloride threshold concentration. Therefore, this paper presents a test method, the so called “ECISS”-test (ECISS = European Commission for International Steel Standardization), which enables the evaluation of the chloride threshold of stainless steel. The focus is on the performance of this test with respect to its limits and opportunities. Test results of different types of stainless steels are included and discussed. The benefit of stainless steel with respect to the service life of reinforced concrete structures is highlighted.

Hole diameter, roundness error, and fiber overlap determine the form quality of a drilled hole on a fiber reinforced composite material. Using support plates is one of the methods to ensure hole form quality. In this study, the effects of support plates on hole form quality in the micro-drilling of CFRP laminates have been investigated. A comprehensive experimental study was carried out under variable cutting speeds and feeds. Three different setups were employed to inspect the effects of the support plates. In the first setup, bare CFRP laminates were drilled. In the second setup, aluminum entry board and phenolic backing board were used. In the third setup, brass entry board and wooden backing board were preferred to inspect the effect of different support plate materials. A large number of holes was drilled per experiment to be able to identify the tool wear effect on the process outputs. Diameter, roundness error, and fiber overlap were measured by using the hole images. Finally, variations of diameter, roundness error, and fiber overlap were evaluated for each setup. It was concluded that support plates improved the process outputs and made the process less sensitive to a change of process variables and tool wear.

Surface roughness plays an important role in the performance of finished structures. The surface quality obtained is enormously affected by cutting parameters. Therefore, the purpose of the present study is to examine the surface roughness value of aluminum 7075 workpiece material during milling operation by considering three steps: (1) the multi-nonlinear regression (MNLR) modeling basis of Taguchi design, (2) optimization based on signal to noise ratio (S/N), and (3) probabilistic uncertainty analysis depending on Monte Carlo technique as a result of depth of cut, cutting speed and feed rate. The depth of cut of 0.2 mm, cutting speed of 900 m × min −1 , and feed rate of 0.1 mm × tooth −1 were determined as Taguchi-optimized conditions with a surface roughness of 0.964 μm. In order to justify the surface roughness predicted under optimized conditions in relation to the predicted Taguchi method, three repetitive verification experiments were performed and surface roughness of 0.964 μm ± 0.3 % was achieved. The best-fit MNLR method with an R 2 pred (predicted regression coefficient) of 98.02 % is useful for calculating the success of estimating the outcome variable. Monte Carlo simulations were found to be quite effective for identifying the uncertainties in surface roughness that could not be captured by means of deterministic methods.

Turkey is one of the most earthquake-prone countries in the world. The studies on the damaged buildings with reinforced concrete structural systems, especially done after the 1999 Gölcük earthquake and other earthquakes in the past, showed that the negative effects of the structural irregularities in the buildings come to light through the impact of earthquake load. They also revealed that the regularity of structural systems is one of the main principles in the design of earthquake resistant reinforced concrete structural systems. In this study, two-story single-span 1/3 scaled reinforced concrete test specimens having soft story irregularity (interstory stiffness irregularity) and insufficient earthquake resistance were tested by applying different strengthening methods. The purpose of strengthening applications is to eliminate the adverse effects of the soft story irregularity. Reversed-cyclic lateral load was applied on the test specimens to simulate the earthquake effect. While the upper stories in all the test specimens had infill walls, the lower stories had been strengthened using different types of steel bracings. The first frame was strengthened with K-type shaped (<>), the second one with straight V-type (V) and the third with turned sideways V-type (>) steel diagonal elements. As a reference to these three strengthened frames, the following two frames, which were taken from another study, were used; a frame with both stories having infill walls and a frame with infill walls on the upper story, which is empty on the lower story. As a result of the study, load and displacement histories, hysteresis and envelope curves and energy dissipation graphs of these three strengthened reinforced concrete frames were obtained. Furthermore, all the experimental results, obtained graphs and the data from the reference frames were compared and interpreted. The results show that the frame strengthened with K-type steel bracings is the best solution to reduce the effect of soft story irregularity.

To find a quick method to quantify the depth of hardening in order to optimize the production process at thyssenkrupp Presta Schönebeck GmbH, micromagnetic characterization after surface heating is used. The difference between the following method and other proceedings is that the depth is measured on the opposite surface of the heated side. For this research process the “MikroMach“-system is used, which contains the four analyzing methods: eddy current, Barkhausen-noises, harmonic content and superposed permeability. The results of reference measurements are evaluated by statistical methods, thereby defining the parameters which manifest a high correlation and a low variation towards hardening depth. Based on these characteristic variables, the MikroMach software starts a multiparameter analysis to find a polynomial. As a result of this, a reference is given which is used to classify the components into functioning or defective goods. After that, the system was tested in the production area to validate the characteristic polynomial. As a result of these tests, it is shown that the classification of components is possible. In addition to that, the system showed unexpected influences of divergent heating parameters. But there are differences between the conventional depth of hardening and the quantitative measuring by using micromagnetic characterizations. The differences are caused by the quality and numbers of references in calculating the polynomial.