Frequency-Domain Laser Ultrasound (FDLU) Non-destructive Evaluation of Stress–Strain Behavior in an Aluminum Alloy

The evaluation of the stress–strain state of metallic materials is an important problem in the field of non-destructive testing (NDT). Prolonged cyclic loading or overloading will lead to permanent changes of material strength in an inconspicuous manner that poses threat to the safety of structures, components and products. This research focuses on gauging the mechanical strength of metallic alloys through the application of frequency-domain laser ultrasound (FDLU) based on a continuous-wave diode laser source. The goal is to develop industrial NDT procedures for fatigue monitoring in metallic substrates and coatings so that the technique can be used for mechanical strength assessment. A small-scale, non-commercial rig was fabricated to hold the sample and conduct tensile FDLU testing in parallel with an adhesive strain gauge affixed on the tested sample for independent measurement of the applied stress. Harmonic modulation and lock-in detection were used to investigate the LU signal sensitivity to the stress–strain state of ordinary aluminum alloy samples. A 1 MHz focused piezoelectric transducer was used to detect the LU signal. During the tensile procedure, both amplitude and phase signals exhibited good repeatability and sensitivity to the increasing stress–strain within the elastic regime. Signals beyond the elastic limit also revealed significant change patterns.     

Influence Analysis of Nonlinear Stress–Strain Behavior in Pulverizing Wheel of Fan Mill

Journal of Failure Analysis and Prevention - Recent failures of pulverizing wheels in fan mills of a steam turbine power plant show that the dovetail-groove area of the main components (the front...     

Temperature and Stress Dependence of Occurrence of Strain Burst in Cyclic Creep of AI-Mg Alloy

Cyclic creep tests of Al-Mg alloy have beenconducted at a wide range of stress andtemperature.When Mg content is beyond 0.45%,the strain bursts can be observed in the cyclic creepcurves.There exists a critical stress for the occur-rence of the strain burst,which increases with an in-crease of Mg content and is almost 60% of the ul-timate tensile stress.The occurrence of strain burstis proposed to be controlled by the easiness of thecross-slip of dislocations.For Al-Mg alloy,thedynamic strain aging plays an important role in thestrain bursts.     

Estimation of yttrium oxide microstructural parameters using the Williamson–Hall analysis

ABSTRACT

Yttrium oxide (Y₂O₃) particles were synthesised from a yttrium nitrate solution via ultrasonic spray pyrolysis (USP) method. The effects of temperature and precursor concentration on morphology and microstructural parameters were investigated. Y₂O₃ particles were characterised by scanning-electron-microscope energy-dispersive spectroscopy, Raman spectroscopy and X-ray diffraction analysis. Based on X-ray peak broadening, the crystallite size was calculated using the modified Debye–Scherrer (MDS) method. Furthermore, the crystallite size, crystal strain and the energy density of the crystal were evaluated using the Williamson–Hall (W–H) analysis integrated with the uniform deformation model, the uniform stress deformation model and the uniform deformation energy density model. A comparative evaluation of Y₂O₃ crystallite size using the MDS and W–H methods was carried out.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.     

Characterization of the Mechanical Stress–Strain Performance of Aerospace Alloy Materials Using Frequency-Domain Photoacoustic Ultrasound and Photothermal Methods: An FEM Approach

Determining and keeping track of a material’s mechanical performance is very important for safety in the aerospace industry. The mechanical strength of alloy materials is precisely quantified in terms of its stress–strain relation. It has been proven that frequency-domain photothermoacoustic (FD-PTA) techniques are effective methods for characterizing the stress–strain relation of metallic alloys. PTA methodologies include photothermal (PT) diffusion and laser thermoelastic photoacoustic ultrasound (PAUS) generation which must be separately discussed because the relevant frequency ranges and signal detection principles are widely different. In this paper, a detailed theoretical analysis of the connection between thermoelastic parameters and stress/strain tensor is presented with respect to FD-PTA nondestructive testing. Based on the theoretical model, a finite element method (FEM) was further implemented to simulate the PT and PAUS signals at very different frequency ranges as an important analysis tool of experimental data. The change in the stress–strain relation has an impact on both thermal and elastic properties, verified by FEM and results/signals from both PT and PAUS experiments.     

Synthesis and Characterization of ZnO Nanowires and ZnO–CuO Nanoflakes from Sputter-Deposited Brass(Cu_(0.65)–Zn_(0.35)) Film and Their Application in Gas Sensing

A novel method was presented for synthesis of ZnO and ZnO–CuO composites in the form of nanowires,nanorods and nanoflakes on oxidized silicon substrates. Further, the use of the synthesized nanostructures for gas sensing was demonstrated. Pure brass(Cu0.65–Zn0.35) films were deposited on oxidized Si substrate by radio frequency(RF) diode sputtering. Subsequently, these films having thickness in the range of 100–200 nm were oxidized in different oxidizing ambient in the temperature range of 300–550 °C.The effect of temperature, time and oxidizing ambient on the growth of nanostructures was investigated by scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and photoluminescence(PL) techniques. The nanostructures surface was analyzed by X-ray photoelectron spectroscopy(XPS). The synthesized nanowires had diameter in the range of 60–100 nm and length up to 50 μm. Based on these observations, the growth mechanism has been suggested. For the nanorods, the diameter was observed to be ~150 nm. Samples having dense nanowires, nanorods and nanoflakes were used as a gas sensing material. The performance of the sensor was investigated for different nanostructured materials for various volatile organic compounds(VOCs). It was observed that ZnO–CuO nanoflakes were more sensitive to VOC sensing compared to ZnO nanowires and nanorods.     

A Semiempirical Model for Post-Yield Stress Instability in the Stress–Strain Response of 3D Lattice Structures under Compressive Loads

Mechanical behavior of lattice structures is important for a range of engineering applications. Herein, a new semiempirical model is proposed that describes the entire range of stress–strain response of lattice structures, including the stress-instability region which is modeled as an oscillator. The model can be fit to individual stress–strain curves to extract elastic modulus, yield stress, collapse stress, post-yield collapse ratio, densification strain, and the energy absorbed per unit volume. The model is fit to 119 unique experimental stress–strain curves from 13 research papers in literature covering four different lattice designs, namely, octet truss, body-centered cubic with vertical members, body-centered cubic, and hexagonal. Manufacturing methods (additive and conventional) and materials (metals and polymers) were also included in the analysis. The fitted model yields several new insights into the compression behavior of previously tested lattice structures and can be applied to additional lattice designs. Among other results, analysis of variance (ANOVA) reveals that the octet truss lattice demonstrates the highest post-yield collapse ratio and the smallest normalized energy absorption per unit volume amongst the lattice structures investigated. The proposed model is a powerful tool for designers to quantitatively compare and select 3D lattice structures with the desired mechanical characteristics.     

Growth of GaN Nanotubes and Nanowires on Au–Ni Catalysts

Technical Physics Letters - Arrays of GaN nanowires (NWs) and nanotubes (NTs) have been grown by metalorganic vapor phase epitaxy using a gold–nickel film as the catalyst. The simultaneous...     

Evaluation of Young''''s Modulus and Residual Stress of NiFe Film by Microbridge Testing

Microbridge testing was used to measure the Young's modulus and residual stress of metallic films. Samples of freestanding NiFe film microbridge were fabricated by microelectromechanical systems. Special ceramic shaft structure was designed to solve the problem of getting the load-deflection curve of NiFe film microbridge by the Nanoindenter XP system with normal Berkovich probe. Theoretical analysis of load-deflection curves of the microbridges was proposed to evaluate the Young's modulus and residual stress of the films simultaneously. The calculated results based on experimental measurements show that the average Young's modulus and residual stress for the electroplated NiFe films are 203.2 GPa and 333.0 MPa, respectively, while the Young's modulus measured by the Nano-hardness method is 209.6±11.8 GPa for the thick NiFe film with silicon substrate.     

Single-Stage Models of Stress–Strain Curves Up to Maximum Load: Duplex Stainless Steel and High-Strength Steels

There is a need for developing an accurate and united mathematical model representing tensile engineering stress–strain curves of duplex stainless steels and high-strength steels up to maximum load.     

IR Spectra and Structure of Si–Al–O–N Phases Prepared by Carbothermal Reduction of Kaolin in Nitriding Atmosphere

Si–Al–O–N ceramics of different phase composition (dominated by the X-phase, -sialon, or 15R-sialon) were prepared by carbothermal reduction of kaolin in nitriding atmosphere at different kaolin-to-carbon ratios. The intensity of absorption bands in the IR spectra of the sialon phases was shown to correlate with their chemical composition. The high strength of the Al–O absorption in the IR spectrum of the 15Rpolytypoid was interpreted as evidence for the presence of AlSiNO₂bilayers (solid solution of Si and N in -Al₂O₃).     

Evaluation of Young＇s Modulus and Residual Stress of NiFe Film by Microbridge Testing

Microbridge testing was used to measure the Young＇s modulus and residual stress of metallic films. Samples of freestanding NiFe film microbridge were fabricated by microelectromechanical systems. Special ceramic shaft structure was designed to solve the problem of getting the load-deflection curve of NiFe film microbridge by the Nanoindenter XP system with normal Berkovich probe. Theoretical analysis of load-deflection curves of the microbridges was proposed to evaluate the Young＇s modulus and residual stress of the films simultaneously. The calculated results based on experimental measurements show that the average Young＇s modulus and residual stress for the electroplated NiFe films are 203.2 GPa and 333.0 MPa, respectively, while the Young＇s modulus measured by the Nano-hardness method is 209.6：1：11.8 GPa for the thick NiFe film with silicon substrate.     

Predictions of Phase Distribution in Liquid–Liquid Two-Component Flow

Ground-based liquid–liquid two-component flow can be used to study reduced-gravity gas-liquid two-phase flows provided that the two liquids are immiscible with similar densities. In this paper, we present a numerical study of phase distribution in liquid–liquid two-component flows using the Eulerian two-fluid model in FLUENT, together with a one-group interfacial area transport equation (IATE) that takes into account fluid particle interactions, such as coalescence and disintegration. This modeling approach is expected to dynamically capture changes in the interfacial structure. We apply the FLUENT-IATE model to a water-Therminol 59^® two-component vertical flow in a 25-mm inner diameter pipe, where the two liquids are immiscible with similar densities (3% difference at 20°C). This study covers bubbly (drop) flow and bubbly-to-slug flow transition regimes with area-averaged void (drop) fractions from 3 to 30%. Comparisons of the numerical results with the experimental data indicate that for bubbly flows, the predictions of the lateral phase distributions using the FLUENT-IATE model are generally more accurate than those using the model without the IATE. In addition, we demonstrate that the coalescence of fluid particles is dominated by wake entrainment and enhanced by increasing either the continuous or dispersed phase velocity. However, the predictions show disagreement with experimental data in some flow conditions for larger void fraction conditions, which fall into the bubbly-to-slug flow transition regime. We conjecture that additional fluid particle interaction mechanisms due to the change of flow regimes are possibly involved.     

Stress–singularity analysis in space junctions of thin plates

The stress singularity in space junctions of thin linearly elastic isotropic plate elements with zero bending rigidities is investigated. The problem for an intersection of infinite wedge-shaped elements is considered first and the solution for each element, being in the plane stress state, is represented in terms of holomorphic functions (Kolosov–Muskhelishvili complex potentials) in some weighted Hardy-type classes. After application of the Mellin transform with respect to radius, the problem is reduced to a system of linear algebraic equations. By use of the residue calculus during the inverse Mellin transform, the stress asymptotics at the wedge apex is obtained. Then the asymptotic representation is extended to intersections of finite plate elements. Some numerical results are presented for a dependence of stress singularity powers on the junction geometry and on membrane rigidities of plate elements.     

Varistor properties and microstructure of ZnO–BaO ceramics

Ceramics in the system ZnO–BaO have been investigated for possible use as varistors. Specimens were prepared by the mixed oxide route, and were sintered at temperatures in the range 1000–1400°C. The electrical properties were determined using d.c. A and impulse techniques. The ZnO–BaO ceramics have a two-phase microstructure comprising a ZnO phase and a Ba-rich grain boundary phase. Due to liquid phase sintering, the average grain sizes for the ZnO–BaO ceramics are large (typically 35–55 m for samples sintered at 1300°C). This results in low breakdown fields, (1000 V cm-1). The maximum non-linearity exponent obtained for ZnO–BaO ceramics (14) is higher than that for binary ZnO–Bi2O3 ceramics. However, the high water solubility of the Ba-rich phase may restrict the use of ZnO–BaO ceramics.     

Change in antibacterial characteristics with doping amount of ZnO in MgO–ZnO solid solution

Antibacterial activity for MgO–ZnO solid solution was studied by measuring the change in electrical conductivity with bacterial growth. MgO–ZnO solid solution powders were prepared by heating at 1400°C for 3 h in air. A single phase with cubic type structure was obtained in the weight ratio range (MgO/ZnO) of 4.0 and 1.5, but the ratio of 0.67 resulted in a ZnO phase in addition to solid solution. After milling the solid solution powders by planetary ball mill, the average particle size and the specific surface area of these powders became 0.1 μm and 26 m²/g, respectively, which were used in the test of antibacterial activity. From the results of antibacterial tests, the activity increased with increasing the powder concentration in the medium. With increasing the doping amount of ZnO in MgO–ZnO solid solution, it was found to show a decrease in the antibacterial activity against Escherichia coli and Staphylococcus aureus. The pH value in physiological saline at the powder concentration of 2.5 mg/ml showed the alkali region above 10.0, and decreased with the increase of ZnO amount in solid solution. The decrease in antibacterial activity, therefore, was associated with the decrease of pH value in medium     

Design of Schottky barriers in ZnO–TiC interface and its application in high sensitivity detection of aniline

Journal of Materials Science: Materials in Electronics - The Schottky barrier at the interface between the semiconductor phase and the metal phase has excellent rectification effect which can...