Volume 60, Issue 6

Magnetic Barkhausen noise (MBN) is a nondestructive method for residual stress analysis. A great advantage over most complementary residual stress analysis techniques is that measurements can be performed very fast. Barkhausen noise is affected by residual stress to the same extent as by the microstructure of the analyzed sample. Thus, a careful calibration considering the microstructure has to be carried out for quantitative residual stress determination. In the present work, the influence of slight changes in microstructure (i. e. grain size) was investigated for a mild steel. Calibration curves were determined by means of 4-point bending tests. Afterwards, the calibration curves were validated using tensile test samples with known applied stress. The tensile samples were manufactured from a different batch attaining a smaller average grain size than the calibration sample. The stress determined significantly differed from the nominal applied stress due to the differences in the grain size. After consideration of the grain size effect, satisfactory agreement between determined and nominal stress could be achieved. Finally, an MBN residual stress analysis was performed on a deep rolled sample. Complementarily, the residual stress state was determined by means of X-ray diffraction (XRD). Significant differences between the XRD and MBN results could be observed. The results indicate that the microstructure of the calibration sample must be identical with the microstructure of the sample to be analyzed to avoid erroneous residual stress results. Furthermore, the load direction during MBN calibration must be correctly chosen.

This paper aims to develop a new crash box with improved crashworthiness at reduced cost and weight as a base design for use in the automotive industry. Firstly, a baseline crash box model as presently used by the automotive industry was comprehensively examined by numerical crash analysis using Ls-Dyna software. Considering the initial design geometry, forty-five different crash box designs were developed by making changes in the geometry and wall thickness of the thin walled structures. The effects of the changes in wall thickness and geometry in alternative crash box designs on crash performances such as total energy absorption, peak crush force, mean crush force, specific energy absorption and crush force efficiency were investigated. The optimum crash box design obtained numerically was validated experimentally by means of the drop tower impact system. The numerical crash analysis results clearly agree with the experimental test results. In this study, a new crash box design at a lower cost and performing better in crashes compared with the other forty-six designs has been obtained and can be used in the automotive industry as an energy absorber. The results have revealed that crash box geometry, as well as the number and position of the spot welds and sheet-metal thickness have an important effect on crash performance, weight and cost of the crash boxes.

Glass fiber/epoxy composites exhibit varied behaviors in the Z direction (the direction perpendicular to the layers) and × direction (the direction parallel to the layers). The mechanical properties of these materials, subject to quasi-static compressive and tensile loadings, are investigated using a tensile testing machine and by means of the pasting strain gauge method. The results of the tests show that the compressive strength in the Z direction is higher than that in the × direction, whereas tensile strength exhibits the opposite. A split Hopkinson pressure bar was used to investigate the dynamic behavior of the composite. The strain rate-reinforcement effect of the material can clearly be found. In the Z direction and × direction, compressive strength attains its maximum at a strain rate of 2030 s −1 and 2100 s −1 respectively. The maximum compressive strength in the Z direction is 90 MPa higher than that in the × direction. At the same strain rate, the elastic modulus in the Z direction is higher than that in the × direction, which means that stiffness in the Z direction is higher than that in the × direction. The focus then turns to failure modes and fracture mechanisms of the composite. A scanning electron microscope shows that a macroscopic brittle fracture is mainly caused by fiber breakage and matrix cracking. However, shear failure of the matrix and delamination and buckling of the glass fiber laminates also lead to macroscopic shear failure.

In this paper, the effects of different quenching temperatures on the microstructure, hardness, and wear resistance of modified Al-bearing high boron high speed steel were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), a hardness tester, and an MM-200 block-on-ring wear testing machine. The results showed that the as-cast microstructure of modified Al-bearing high boron high speed steel consisted of martensite, ferrite, pearlite and continuous network eutectic borocarbides which were distributed at the grain boundary. The eutectic borocarbides mainly included M 2 B and M 7 (C, B) 3 . With an increase of quenching temperature, the matrix turned into martensite, and the continuous network structure of eutectic borocarbides were gradually broken and changed to the spherical; the secondary borocarbides M 23 (C, B) 6 were precipitated from the matrix. After quenching at 1150 °C, eutectic borocarbides were clearly visible as a broken network and spheroidal in shape, and the distribution was more uniform. When the quenching temperature was in a range of 1000 °C to 1150 °C, the hardness and wear resistance of the alloy obviously increased. When the quenching temperature attained 1150 °C, hardness reached the highest 64.3 HRC, and the wear resistance was best.

Duplex stainless steel is common in thick-walled components such as longitudinal welded pipes in the oil and gas industries and as parts of various machines in the chemical and food industries. Electron beam welding is a very suitable method for welding such components. Due to the high power density of the electron beam combined with the decreased evaporation temperature in a vacuum atmosphere, steel with a sheet thickness of up to 150 mm can be welded in one pass. In the case of the electron beam welding of duplex stainless steels, vacuum atmosphere in the working chamber causes a nitrogen effusion from the weld pool. The microstructure of the resulting weld is characterized by an unacceptable high ferrite content, which is the main reason for both low impact toughness and low pitting corrosion resistance. This work focuses on the influence of nitrided weld edge surfaces on metallurgical properties of the resulting welded joint. The aim here is to investigate the effect of increased nitrogen content at the weld edges on nitrogen loss during EB-welding. The welds produced within the experimental work were characterized by means of microstructural analysis and the use of optical and energy dispersive X-ray spectroscopy. It was shown that nitrided weld edge surfaces can compensate nitrogen loss from the weld pool and decrease the ferrite content in the resulting weld.

Aluminum foam could be used as a defense against explosion and shock wave. Its energy absorption capability is an important indicator to evaluate its blast resisting ability. But since the impedance of aluminum foam is much lower than that of metal, it is hard to measure its exact stress-strain relation by means of the traditional Split Hopkinson pressure bar (SHPB) method. To evaluate the energy absorption characteristic of aluminum foams of varied cell sizes, an improved SHPB method is proposed. This improved method can enhance the accuracy of the stress-strain curve of aluminum foam and by using a longer striker, might increase the strain on the samples. Two aluminum foams of different cell sizes were selected. The experimental results show that the cell size of the aluminum foam and the strain rate have a significant effect on the compressive characteristics and energy absorption. Smaller cell aluminum foam is stronger than that with larger cells due to fewer flaws in the microstructure. Aluminum foam of a smaller cell size can absorb more energy than larger cell aluminum foam due to higher plateau stress. The energy absorption of smaller cell aluminum foam increases by 42 % at strain rate 3579 s −1 compared with quasi-static compression while larger cell foam increases 55 % at a strain rate of 1586 s −1 .

In this study, the bending behavior of a thin-walled aluminum square tube (AST) were analyzed using the finite element (FE) method, and reinforcing arrangements were decided for the composite beams based on the FE results. Accordingly, bending behavior of thin-walled ASTs with internal cast polyamide (PA6) and external glass fiber reinforcement polymers (GFRP) were investigated via a quasi static three-point bending test experimentally. Moreover, bending performance under impact loading was also investigated experimentally. The results revealed that local buckling has a decisive influence on bending performance of the tube, and the contribution of inner reinforcement is more effective than outer reinforcement. The reinforcements provide 575 % and 312 % increases in bending load and impact energy, respectively. The developed plastic-metal hybrid-composite structure is promising especially for critical supporting parts in vehicles. It is thought that the combination of these materials will offer new focus of attention for designers seeking more appropriate composite beams with high bending load and impact resistance.

Kurzfassung Die thermografische Rekonstruktion von Messsignalen einer Infrarot-Kamera hat große Vorteile gegenüber einer Ultraschall-Prüfung: Es wird kein Koppelmedium benötigt, und die Temperatur kann an vielen Pixeln der Kamera parallel gemessen werden. Der Hauptnachteil im Vergleich zu Ultraschall ist die sich stark verschlechternde Auflösung mit zunehmender Tiefe der abzubildenden Strukturen. In dieser Arbeit wird demonstriert, wie Ultraschall-Rekonstruktions-Methoden für die thermografische Rekonstruktion genutzt werden können. Vor der Ultraschall-Bildrekonstruktion wird aus dem gemessenen zeitlichen Temperaturverlauf jedes Kamera-Pixels eine virtuelle Schallwelle berechnet. Wie hier gezeigt, ist diese virtuelle Welle die inverse eindimensionale Rekonstruktion an der Position dieses Pixels. Diese lokale Transformation beinhaltet die gesamte Irreversibilität des Diffusionsprozesses und kann für beliebige zwei- und dreidimensionale Probenformen verwendet werden. Es werden zweidimensionale Rekonstruktionen aus numerischen Simulationen und experimentelle Ergebnisse gezeigt, bei denen parallele Stahlstäbe in einer Epoxidharz-Matrix eingebettet sind und durch Induktion von Wirbelströmen pulsförmig erhitzt werden.

When making components out of fiber-reinforced plastics, the composite material itself is formed during the manufacturing of the component, so that suitable methods for process monitoring and quality assurance of each manufactured component regarding its structural integrity are necessary. An alternative approach to the commonly used ultrasonic testing is pulse thermography, which is suitable for the inspection of large areas. But this method is restricted regarding inspection depth, on one side, and thermal contrast and depth resolution with increasing depth, on the other side. In this paper we compare several data processing methods which can be used to overcome the limitations of this method and to optimize the contrast for detection of deeply buried defects. (1) differential absolute contrast, (2) thermographic signal reconstruction, (3) pulse phase thermography, (4) principle component thermography and (5) high order statistics are compared regarding their capability of detecting defects, e. g. delaminations, in different depths. As a measure for the achievable contrast of the different methods designed to detect various defects we compared the respective signal-to-noise-ratio. Additionally, we discuss further approaches for improving the detection capability by applying different evaluation methods in sequence or combining them with image processing methods based on geometric filters. For this purpose a CFRP specimen with 18 artificial defects was examined.

Mechanical and microstructural properties of quenched steel are directly related to tempering time and temperature. In many applications, conventionally quenched and tempered steel is widely used for acquiring high strength and toughness. The present study was carried out to investigate the variation in mechanical properties, observation of diminished energy consumption and evaluation of the microstructural properties in SAE-AISI 6150 steel components by induction heating, compared with those of steel tempered by conventional method. Induction quenched and tempered steel provides a shorter process time, less energy consumption and improved mechanical properties through the inhibition of grain growth. In this study, quenching and tempering processes were carried out on medium and low frequency induction units and by using a conventional electrical resistance furnace for the sake of comparison. It was observed that cementite particles began changing their shape from spherical to fine-grained in the induction tempered samples. The sample tempered by low frequency induction manifests superior mechanical properties and offers a potential advantage for significant cost savings.

This study investigated the atmospheric corrosion behavior of carbon steel and galvanized steel framing samples at residential buildings in Southwest China over a three-year period of exposure. The study was begun in 2010 when four monitoring sites were established and test samples installed in a case study area at Dujiangyan City, Sichuan Province. Particularly, corrosion behavior, including corrosion morphology, corrosion rate and the composition of corrosion products of the selected steel samples, were analyzed under local atmospheric conditions. The results showed that the corrosion rate of the steel varied considerably at the four different monitoring sites due to significant temperature and moisture differences at the locations. By increasing the testing cycles, the corrosion rates of the steel tended to decrease, and the galvanized coating of the steel surface could effectively protect the base metal from corrosion in the atmospheric environment. The main factors that might affect corrosion behavior seem to be relative humidity, wet condition and the fluctuation of relative humidity. These findings could help municipal authorities and local communities make sound management strategies in the future regarding the treatment and re-use of discarded construction materials.

Polymer materials have many fields of application, such as in agriculture and agribusiness, medicine, consumer science, the automotive industry, for sports equipment etc. These materials are generally produced by injection molding, in-mold shaping and extrusion. A machining process is essential however both for small quantity production for complex and accurate shapes. Moreover, it is known that the demand for machining, especially with respect to the drilling of polymer materials has increased. In this study, the cutting temperature for drilling pure polyamide and carbon black reinforced polyamide was investigated as well as the effects of drill point angle, cutting speed, and feed rate on cutting temperature making use of Taguchi and ANOVA statistical methods. Among the selected drilling parameters, the most significant was found to be the drill point angle and the least effective the feed rate.

In this study, duplex stainless steel was coated with Ni 3 Al powder via self-propagating high-temperature synthesis (SHS) and the effect of the sintering temperature on the coating characteristics was investigated. An SEM electron microscope with EDX (energy dispersive X-ray spectroscopy) was used to examine the microstructure of samples that were coated according to the SHS method and likewise to detect the element content of the phases in the structure. Microhardness tests were conducted on the samples to determine the mechanical properties of the coated material. As a result of these studies, it was found that duplex stainless steel can be reliably coated with an Ni 3 Al intermetallic compound. The increase in sintering temperature within the given range significantly decreased the size of the disconnected area which completely disappeared.

Carbon nanotubes (CNTs) and other nanoparticles such as silicon carbide (SiC), alumina (AL 2 O 3 ) are used to improve the material properties of polymers and in general are considered to be highly potential fillers. However, questions concerning the proper type and amount of CNTs, e. g., single-wall CNTs (SWCNT), double-wall CNTs (DWCNT), or multi-wall CNTs (MWCNT) and other nanoparticles such as silicon carbide, alumina and their influence are still to be answered. This work studies the effect of the amount and types of nanofillers on the mechanical properties of epoxy based nanomaterials and their functionalization. In the present work, epoxy resin was modified with different types of nanofillers including SWCNT, DWCNT, MWCNT, SiC and AL 2 O 3 . The amount of CNTs (SWCNT, DWCNT and MWCNT) used for each type of CNTs were 0.1, 0.3 and 0.5 wt.-%. By contrast, the amount used for (SiC), and (Al 2 O 3 ) measured 0.5, 1 and 1.5 wt.-% for each type. Tensile test were conducted for each group to investigate the mechanical properties of epoxy based nanomaterials. The results show that CNTs of 0.3 wt.-% is optimal compared with other amounts, e. g. 0.1 and 0.5 wt.-%. The tensile strength at 0.3 wt.-% SWCNT/E, DWCNT/E and MWCNT/E increased by 44 %, 37.4 % and 35.9 %, respectively compared to the neat epoxy. For (SiC), and (Al 2 O 3 ), the results show that an amount of 1.5 wt.-% is optimal compared with other amounts, e. g. 0.5 and 1 wt.-%. The tensile strength at 1.5 wt.-% for Sic/epoxy and Al 2 O 3 /epoxy nanocomposite increased by 25 %, and 43 %, respectively compared to the neat epoxy. Finally, the results show that the (SWCNT) is optimal compared to the other nanofillers.

Metal matrix composites have higher resistance compared to ceramic and polymer matrix composites. In this study, an aluminum 5754 alloy was fortified using 10 % SiC to obtain a metal matrix composite with properties superior to the original alloy. An improvement of mechanical properties was obtained by reinforcement with aluminum matrix composites which can be produced with various techniques. Mechanical properties of the composite material (Al/SiCp) were initially investigated, and then, the metallurgical properties were determined using an optical metal microscope and SEM. Furthermore, the percent of elongation, tensile strength and Vickers hardness are the input parameters for the mechanical properties. The tensile strength as an output response parameter of Al/SiCp materials was analyzed using experimental Taguchi technique design as an orthogonal array. In the present investigation, experiments were carried out to compare experimental results with predicted optimal values. ANOVA analysis revealed that the percent elongation-PE (%) was the input parameter having the highest statistical influence on tensile strength. It could be shown that the composite material was resistant against both static and dynamic loads. It was also shown that silicon carbide mixed homogenously with aluminum.