Volume 65, Issue 5

The micromagnetic Barkhausen noise as a nondestructive testing method was employed in this work to analyze the fatigue behavior of high-strength steels by subjecting them to multi-stage load increase fatigue tests. The micromagnetic Barkhausen noise energy (MBN Energy ) was measured during fatigue tests and coupled with digital image correlation to evaluate the effect of static and cyclic stress as well as local surface strain on MBN Energy . The test samples were prepared from the forged bainite 18MnCrMoV6 and two ferrite-pearlite 38MnVS6 and 46MnVS5 steels. A part of the specimens was pre-strained before cyclic loading to analyze the effect of pre-deformation on cyclic strength considering measured MBN Energy . A significant increase in MBN Energy was observed by increasing the cyclic load on 18MnCrMoV6 specimens with bainite phases while this increase of MBN Energy was not significant or was not changed for ferrite-pearlite steels 38MnVS6 and 46MnVS5.

The creep resistance of light metals is of utmost relevance for application at elevated temperatures. This includes automotive components in power trains, engines, battery casings and transmission housings. The alloy design of temperature resistant light alloys for these applications depends heavily on the timely determination of the creep behavior. However, specialized facilities and testing equipment are required to perform creep experiments, which are not available in all labs. In contrast, deformation dilatometers are state-of-the-art at most research facilities with a metal forming department. These instruments can apply a constant force at high temperatures and are therefore, in principle, able to conduct creep experiments. To validate the applicability of such a deformation dilatometer for the development of magnesium alloys, short time creep experiments were conducted and compared with results from a standardized creep stand. Good agreement between the dilatometer and conventional constant force creep experiments was found. In this work, both methods are described in detail and possible limitations are discussed.

This paper compares two methods for transforming a stress or load spectrum into a damage-equivalent constant amplitude loading, i.e., the required fatigue strength (RFS) and the consistent equivalent strength (CES) concepts. Because of their shared fundamentals, i.e., the modified Palmgren-Miner hypothesis for a given allowable damage sum, the resulting RFS- and CES-amplitudes are comparable to each other, but not equal, when the slopes after the knee-points of the Woehler-lines differ. The RFS-method delivers a damage-equivalent stress as the knee-point of a fictitious bi-linear or linear Woehler-line by positioning of the Woehler-line for a given allowable damage sum. In contrast to this, the CES-method also results in a damage-equivalent stress as the knee-point, which depends on the consistent fatigue strength reduction by increasing cumulating damage until the allowable damage sum is reached. Thus, the main difference to the RFS-method is that the CES-method retains the constant amplitude loading slope after the knee-point. Both methods indicate a lowest necessary strength quality, but, like the calculated fatigue lives, they differ according to the calculation methods, and depend on the slopes of the Woehler-lines after the knee-points.

In the multiaxial low fatigue test study, four multiaxial non-proportional fatigue loading paths were used including rectangular, square, rhombic and circular. The effect of loading condition changes on the fatigue behavior and cyclic damage characteristics of A319 aluminum alloy was investigated by means of stress response curves, stress–strain hysteresis lines, fatigue fracture morphology, and dislocation substructure morphology in combination with material microstructure. The non-proportional hardening behavior exhibits strong loading path dependence, and the fatigue plastic deformation in the torsional direction is greater in the circular and rhombic paths, and the fatigue damage is more severe, resulting in a shorter fatigue life of A319 aluminum alloy. The axial directions under non-proportional multiaxial loading all have tensile and compressive asymmetric behavior. The differences in loading paths result in different fatigue crack expansion modes. The non-proportional additional hardening behavior under different loading paths is closely related to the dislocation substructure. In the rhombic and circular paths, the alloys have higher dislocation density and strong interaction between dislocations and precipitation phases, which is macroscopically expressed as stronger non-proportional hardening ability in the rhombic and circular loading paths.

This work presents a concurrent design and multi-objective optimisation framework of horizontal axis wind turbine blades, made of composite material, for low wind speed. The optimisation model aims to minimise the structural mass of the blade whilst simultaneously maximising the turbine power output, subjected to three constraints viz. blade tip deflection, and Tsai-Hill and von Mises criteria. The design variables are blade shape and details of the internal blade structure. The control points and polynomial interpolation technique were adopted to determine the blade shape while the airfoil types at blade sections remained fixed. The internal blade structure design variables include the thickness of ribs and spars and the carbon fibre thickness and orientations. The blade element momentum approach is utilised to calculate turbine power and structural loads, whereas a finite element method is employed for structural analysis. Twelve multi-objective metaheuristics algorithms are used to solve the proposed multi-objective optimisation problem while their performance is investigated. The results obtained show that the multi-objective cuckoo search algorithm is the most efficient method. This study is said to be the baseline for a future study on multi-objective optimisation which combines two design stages of the composite low-speed wind turbine blades.

The effect of thiosulfate (S 2 O 3 2− ) with the different concentrations (100 g m −3 , 150 g m −3 , 200 g m −3 ) on the passivation of Zn alloys in artificial seawater at 353 K is investigated by using immersion tests, electrochemical measurements, and field emission scanning electron microscopy (FE-SEM) with EDX. It is found that the presence of thiosulfate in the solution can hinder the passivation. Potentiodynamic polarization results show that thiosulfate increases the current density at which the thin passive films with the low corrosion resistance are formed. Thiosulfate effect to retard the passivation, is different with the concentrations exposed to the respective Zn alloys based on the Al content. Due to the presence of various Zn and Al protective compounds at the surface, the passivation of Zn alloys occurs at the immersion time of 432 ks in the form of thin film. After the immersion time is 1037 ks, the passivation is still approximately as close as 432 ks inhibiting the film growth by the effect of thiosulfate and depassivation also would be occurred with the removal of the oxide thin film by the longer immersion time.

Mn–Cu alloys are metal materials, which can be used to reduce vibration and noise from machines. With their inner damping mechanism, the alloys show an excellent damping capacity. However, previous reports show that the damping capacity of the alloys often attenuates after placing at room temperature (RT) for a long time. In the present study, the as-cast Mn–Cu–Al–0∼3 wt% Sn alloys were held at RT for 18 months. Their crystallization morphology was observed using backscattered electron (BSE) technology. The phase distribution was characterized using electron backscatter diffraction (EBSD). Their phase structure was analyzed by using X-ray diffraction (XRD). Their starting martensite transformation temperature ( Ms ) and damping capacity were measured by using an inverted torsion pendulum device. It is observed for the first time that the FCT phases mainly form in the dendrites. With vacancies diffusing towards boundaries of phases and twins, the boundaries are pinned and the Ms point declines. Hence, the damping capacity attenuates within 18 months. Moreover, the addition of Sn can weaken the decline trend of Ms point and consequently the IF value of the Sn-contained Mn–Cu alloys attenuates less than that of the none-Sn alloy. This research could help us to understand how to cope with the damping attenuation of the Mn–Cu alloys.

In this paper, the variation of thermal stresses, strains and deformations created by combined thermal and mechanical loads is investigated in block braked railway wheels. Thermo-mechanical analyses of the railway wheel were performed by using finite element (FE) for the numerical analysis. Before performing FE analyses, input parameters, e.g. heat flow, heat conduction, heat convection, applied loads and boundary conditions were transferred from railway wheel conditions in reality. The thermal analysis of wheel structure in the analysis calculation was evaluated experimentally, and validated with numerical result used in current model. In this purpose, ANSYS Workbench module package was used to simulate the distribution of stress and temperature field over the wheel. Combined thermal and mechanically induced damage surfaces of the railway wheel in reality were compared with numerical results in ANSYS. It was concluded from the results that 3D simulations in this wheel model showed a significant effect of thermal loading rather than mechanical loading on wheel braking tread, and combination of thermal and mechanical loading caused the detrimental effect on railway wheel structures those used in this model.

The effect of two-step aging treatment on the fatigue behavior of welded AA 7020 Al. alloy was investigated. For fatigue tests to be carried out in atmospheric and seawater environments, a plane bending tester at a stress ratio of R  = 0 was used. Experiments in the control group were carried out with unwelded specimens at a strain rate of R  = −1 in air. The gradual heat treatment, two-step aging, significantly improved the fatigue and corrosion fatigue behavior of welded AA 7020 alloys. Unwelded specimens showed better fatigue properties than the welded ones. In the experiments in the seawater environment, pitting, and exfoliation corrosion occurred on the surfaces. It was determined that the corrosion pits reduce the fatigue life of welded and unwelded specimens. It was also observed that the corrosion sensitivity of the welded specimens increased in the heat affected zone and resulted in corrosion fatigue cracking.

In order to overcome the disadvantages of traditional test devices and methods, new accelerated test device and efficient test method for critical chloride concentration (Ccrit) initiating steel depassivation in cement-based materials were developed. Based on the theory of electromigration, a new accelerated test device for Ccrit was invented first by combining with the chloride electromigration system and electrochemical test system. Then the influences of applied voltage and continuous power-on time on electrochemical parameters including open-circuit potential, polarization resistance, and corrosion current density during the steel depassivation process were investigated. Moreover, an efficient test method for Ccrit was proposed based on the quantitative criterion in terms of corrosion current density. Analysis results show that Ccrit can be measured quickly and accurately based on the accelerated test device and method by selecting appropriate applied voltage (e.g., 3–6 V) and continuous power-on time (e.g., 48–72 h).

This paper aims to provide a comparison between properties of CuSn10 bronze alloy produced by two methods: FDM (fused deposition modelling) and casting. Wear behavior, mechanical performance and microstructure of CuSn10 alloy produced by FDM and casting methods were determined and compared. 3D printed parts were sintered at 875 °C for 5 h and casting was done at 1200 °C. The amount of porosity was determined by the metallographic cross-sectional examination. Vicker’s hardness, wear and tensile properties of samples were determined. As a result, the porosity value of the CuSn10 alloy produced by the FDM technique was determined as 5.07 ± 0.53%. Tensile strength values were 233.75 ± 18.48 MPa and 349.04 ± 5.10 MPa for CuSn10 alloys produced by FDM and casting methods respectively. While adhesive wear occurs in both production methods, it was observed that the FDM method reduced the wear resistance.

In order to investigate the wavelet time-frequency characteristics of fiber reinforced mortar under axial compression, acoustic emission signals associated with steel fiber reinforced mortar and polypropylene fiber reinforced mortar with five different fiber contents (0 to 2.0 vol%) under axial compression were recorded. A new method of multivariate statistical analysis of wavelet time-frequency maps based on Morlet wavelet transform is proposed to make signal processing more accurate. Results show in the 15% to 85% loading stage, Type 3 and Type 4 signals are abundant in fiber reinforced mortar than plain mortar, while Type 7 and Type 9 signals are abundant in the 85% to 100% loading stage. According to the temporal localization characteristics of signal frequency quantified by each type of signals, it can be inferred that the fibers improve the toughness of concrete materials. It is also found the strengthening and toughening effect of steel fiber is more sensitive to fiber contents than that of polypropylene fiber. The ratio of Type 1 signals can be used as a warning against the final failure of materials while that of Type 5 signals can be used to evaluate the anti-cracking effect of fibers.

In this study, production of dual phase steel-Al 1100 aluminium alloy laminated composite by explosive welding and investigation the microstructural and mechanical properties especially fatigue life of the composite and its components were aimed. Finite element analysis (FEA) was used to simulate the bending fatigue tests of the Al 1100-DP steel (LITEC 1050) bimetal. A semi-wave at the explosive interface with intermetallic was obtained as a microstructural result. Hardness was increased near the interface due to deformation and intermetallic formation at the explosive weld interface. The impact test results showed that the samples’ impact toughness was enriched by fabricating steel-Al laminated composite. The results also showed that the Al 1100-DP steel (LITEC 1050) bimetal composite properties were appropriate to the composite theory and satisfied strength-ductility relations accomplished. Additionally, the bending fatigue test and FEA results were compatible. Moreover, fatigue cracks were observed at the explosive weld interface between Litec DP steel and intermetallic but not at the Al-intermetallic interface.

Today, due to the frequent use of composite materials used in the aerospace and automotive industries, an intensive study is carried out on the joining methods of these materials. One method of joining composite or aluminum alloy materials is adhesive bonding. Studies on the development of the strengths of adhesively bonded joints focused on adhesive type, the bonding method, and hybrid joints. In the present study, a different method was used to increase joint strength. This study experimentally and numerically investigated the mechanical properties of single-lap joints bonded by a composite adhesive – obtained by adding fiber fabric to the adhesive. Adhesively bonded single-lap joints were produced using DP460 toughened adhesive type as the adhesives; AA2024-T3 aluminum alloy was used as the adherend, and carbon fiber and glass fiber fabric were used as the added fiber. In addition, for the numerical analysis of the presented study, the bulk properties of the composite adhesives were obtained. As a result, the use of fiber-reinforced adhesive in bonding joints significantly increases the failure load of the joint, as it prevents the progression of cracks formed in the joint. However, the increase rate in the failure load changes depending on the adhesive thickness and the type of fiber fabric. As follows, the failure loads of the fiber-reinforced joints increase between 5% and 41% compared to the non-fiber-reinforced joints. In addition, it was observed that the results obtained from the numerical analysis and the results obtained from the experiments were consistent with each other.