Papers by Keyword: Hysteresis Loop

Abstract: Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L1₂ γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

Abstract: In the paper the change of the magnetic flux density under single harmonic distortion is discussed. Presented results show the dependence of the value of the magnetic flux density in the toroidal magnetic core made from the Ni80Fe20 tape of the phase angle of higher harmonic in relation to the main harmonic of distorted magnetizing current. Moreover, the influence of higher harmonic depends from it frequency and it becomes undetectable above 15th higher harmonic, even if its level reaches 50% of the RMS value of the main harmonic of distorted magnetizing current. Laboratory tests were carried out for the magnetic toroidal core of iCT with a current ratio equal to 300 A / 5 A. The oscilloscope is used to measure waveforms of the excitation current and the secondary voltage through the voltage probes. Build in numerical integration is used to determine the magnetic flux density from secondary voltage. In the case of tested 5^th higher harmonic the highest value of the magnetic flux density is obtained for phase angle equal to 90° between main and higher harmonics, while the lowest is obtained for 270°. This depends from the initial phase of the magnetic field strength and results from the integration of distorted secondary voltage with the particular content of higher harmonics.

Abstract: BiScO₃-xPbTiO₃ (BS-PT) of perovskite type structure (with x~0.64) were prepared by solid state ceramic process with addition of calcium fluoride (CaF₂) in different molar ratios (0.5, 1.0 and 4.0 mol%). The doped compositions were found to have different symmetry compared with parent material. The 0.5 mol% doped sample was found to possess enhanced rhombohedral phase as compared to other compositions while he Curie temperature (T_c) was not affected appreciably. Small dielectric peaks were observed, indicating phase transition, at temperature lower than T_c in all the samples except 4.0% doped composition. At room temperature doping reduced the loss factor. d₃₃ and P_r reduced rapidly with doping. The characteristics of BS-PT ferroelectric ceramics became harder with the addition of CaF₂.

Abstract: Double rare-earth (La; Sm/Gd) substituted Aurivillius family of Bismuth Layered Structured Ferroelectrics (BLSF) namely Bi_2.6Sm_0.2La_0.2TiNbO₉ (BSLT; sample-A), Bi_2.6Gd_0.2La_0.2TiNbO₉ (BGLT; sample-B), single phase ceramics were prepared by solid state route. In addition, intergrowth (x BSLT - (1-x) BGLT, where x=0.49; sample-C) and solid solution (BSLT_­x - BGLT_y; where x + y=0.4; sample-D) materials were prepared. Dielectric, ferroelectric and Raman spectroscopic properties were studied on the said above materials. The X-ray diffraction analysis and Raman spectra revealed well-formation of stable structure. Though, the sample-C and sample-D have lower coercive field, compared to the sample-A and sample-B, but they exhibited sharp hysterisis loop. Therefore the instrinsic defects of sample-D inhabits more sensitivity towards the ferroelectric behaviour. The results were corroborated to the impedance and dielectrical data. The results were consistent with the SEM micrographs and complex impedance plots. An attempt is made to understand the effect of rare-earth ions on A-site of layered-pervoskite structure, defined as: (Bi₂O₂)²⁺(A_n-1B_nO_3n+1)^2-.The term n represents number of pervoskite blocks interleaved with the bismuth oxide layers.

Abstract: Scaling of minor hysteresis loops in the amorphous alloy Co₆₆Fe₃Cr₃Si₁₅B₁₃ with a very high initial permeability (more than 150000) and low coercivity (about 0.1 А/m) has been studied. In weak magnetic fields and in the region of maximal growth of permeability a similarity of minor loops was detected. Analytical expressions for hysteresis losses are derived which provide a good accordance of calculations with the results of measurements.

Abstract: Series of glass samples with composition (72.5-x)TeO₂-15MgO-10Na₂O-2.5Nd₂O₃-xNiO where 1.0 ≤ x ≤ 2.5 mol% are prepared using melt quenching method. The glasses are characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD spectra confirmed the amorphous nature of prepared glasses. TEM images manifest the existence of NiO NPs which are homogeneously distributed in the glass matrix. The magnetization M(H) curve reveals that the glass displays an anomalous hysteresis behavior at room temperature. It is observed that M(H) is dependent on the NiO content. The initial curve of magnetization lies positively as a function of the magnetic field under 1000 H (O.e).

Abstract: Aseismic design of modern structures imposes a new requirement on structural engineers. Structural systems should withstand even very huge earthquakes. This goal cannot be achieved by standard design methods applying a linear elastic approach. An advanced aseismic design applies energy dissipating anti-seismic devices. During seismic event, these devices are exposed to a large plastic strain. The code EN 15129 is the standard on anti-seismic devices applicable in Europe. Mentioned standard defines a special material requirement imposed on devices working as energy absorbers. Material verification is possible only experimentally. In compliance with the instructions contained in the code EN15129, several cyclic tests of the materials S235 and DD11 have been used. Evaluation of the previous research and the current test results have proved that structural steel S235 is not applicable to the anti-seismic devices. As an alternative, steel DD11 has been suggested for this application. The test results have shown that the steel DD11 is applicable in specified range of target strain amplitudes.

Abstract: The pitting corrosion behavior of AISI 202 stainless steel (SS) – a low-nickel, austenitic SS grade, was investigated by means of cyclic voltammetry (CV) technique complemented by Scanning Electron Microscopy (SEM). From the starting potential, the current density decreases and changes its sign at the corrosion potential (Ecorr). The anodic response exhibits a well-defined anodic peak followed by a passive region. A noticeable increase in the anodic current density was observed after reaching the breakdown potential (Eb). The second anodic peak which may be attributed to onset of oxygen evolution was also observed. Moreover, the cyclic voltammograms revealed that hysteresis loop is absent for all the studied concentrations, indicating that AISI 202 SS in citric acid is highly resistant to pitting corrosion as also supported by the results of SEM. It was found out that the critical current density (icrit) increases with increasing citric acid concentration.

Abstract: At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

Abstract: Nickel nanoparticles are incorporated in Sm³⁺ doped zinc phosphate glass having a composition of 40ZnO-(58-x)P₂O₅-1Sm₂O₃-xNiO, with x = 0.0 and 2.0 mol% prepared by melt quenching technique. Sample characterizations are made by means of X-ray diffraction, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). X-ray diffraction shows that the glass is in amorphous state. The transmission electron microscopic (TEM) image reveals the existence of Nickel nanoparticles having a particles size in the range of 5.5 – 21.8 nm. The magnetization M(H) curve reveals that the glass displays an anomalous hysteresis behavior at room temperature. It is observed that the initial curve of magnetization lies positively as a function of magnetic field under 1000 H (O.e).