High-speed inspection of rails using ACFM techniques

High-speed ACFM tests were carried out using a rotary test piece that contained spark-eroded notches. The ACFM sensor detected the induced notches during inspection at 121.5 km/h. The recorded signal remained unaffected by the increases in inspection speed under constant lift-off. To simulate actual rail inspection conditions at high speed, further tests were carried out using a spinning rail rig and a set of rails that contained spark-eroded notches of various shapes and sizes up to a speed of 32 km/h. Although, the ACFM sensor successfully detected the majority of the notches, the signal obtained was affected by lift-off variations.     

Modelling and experimental measurements of idealised and light-moderate RCF cracks in rails using an ACFM sensor

Alternating current field measurement (ACFM) sensors can be used to detect surface breaking defects in metal components. In rails rolling contact fatigue (RCF) cracks form due to the wheel–rail contact stresses. These cracks are surface breaking and can have complex shapes. A COMSOL model has been developed for a commercial ACFM system and RCF cracks in rails. In this paper model results are compared to experimental measurements using an ACFM pencil probe for calibration defects machined into a rail and real RCF defects (light and moderate categories; <20 mm surface length) in a rail removed from service. X-ray tomography has been used to determine the size and morphology of the real RCF cracks for input into the model. It has been shown that the model can be used to determine the change in normalised Bx signal due to the presence of calibration defects machined into a new rail. The model has also been used to compare the experimental data for the real RCF cracks to the reconstructed model cracks and a semi-elliptical approximation to their shape.     

High temperature ultrasonic crack monitoring using SH waves

The prototype implementation of a permanently installable high temperature crack monitoring system is described. The system uses an array of waveguides to excite shear horizontal (SH) waves on the surface of the component that is to be inspected. The waveguide is made of poorly heat conducting stainless steel so that it can sustain a large temperature gradient. This ensures that conventional piezoelectric transducers can be used at one end of the waveguide to excite ultrasonic waves while the other end is in contact with the component to be monitored which is situated in a harsh environment at elevated temperature (up to 600 °C). Different crack monitoring strategies using SH waves are discussed. Simulation and experimental results for the array prototype on notched plates are presented. Furthermore, experimental results on samples containing fatigue cracks are shown and measurement results that were collected from a notched plate in a furnace at 550 °C are also presented.     

Investigation of the low-cycle fatigue mechanism for micron-scale monocrystalline silicon films

This study investigated the cyclic and static fatigue properties of 10 μm thick, deep reactive ion etched, monocrystalline silicon films. Stress–life fatigue curves and fatigue degradation rates vs. stress curves were generated at both 4 and 40 kHz, at 30 °C, 50% relative humidity (RH). A significant frequency effect was observed, with shorter fatigue lives and faster damage accumulation rates at 4 kHz. Static fatigue was also observed with shorter static lifetimes at 80 °C, 90% RH than at 30 °C, 50% RH. Fracture surface evaluation did not reveal any major difference between cyclically and statically fatigued devices. These experimental results confirm that the fatigue of micron-scale silicon is not purely mechanical. The study also proposes a fatigue scenario based on time-dependent subcritical crack growth to account for the low-cycle fatigue regime.     

Experimental and numerical investigation of an electromagnetic weld pool support system for high power laser beam welding of austenitic stainless steel

A three-dimensional turbulent steady state numerical model was used to investigate the influence of an alternating current (AC) magnetic field during high power laser beam keyhole welding of 20 mm thick stainless steel AISI 304 being modeled as an ideal non-ferromagnetic material. Three-dimensional heat transfer and fluid dynamics as well as the electromagnetic field equations were solved with the finite element package COMSOL Multiphysics 4.2 taking into account the most important physical effects of the process. Namely, the thermo-capillary (Marangoni) convection at the weld pool boundaries, natural convection due to gravity and density differences in the melt volume as well as latent heat of solid–liquid phase transitions at the phase boundaries were included in the model.It is shown that the gravity drop-out associated with the welding of thick plates due to the hydrostatic pressure can be prevented by the application of AC magnetic field between 80 mT and 135 mT for corresponding oscillation frequencies between 1 kHz and 10 kHz below the weld specimen. Experimentally, a value of the magnetic flux density of around 230 mT was found to be necessary to allow for single-pass laser beam welding without sagging or drop-out of melt for a 20 mm thick combination of austenitic stainless steel AISI 304 and ferritic construction steel S235JRC at an oscillation frequency of around 2.6 kHz.     

Core loss characteristics of Fe-based amorphous alloys

Relations of core losses with the peak inductions and magnetizing frequencies of Fe-based amorphous alloys were investigated. Core losses of these Fe-based amorphous alloys were measured under sinusoidal induction at the frequency range from 50 Hz to 1 kHz and various inductions from 0.6 T to 1.0 T. The results indicated that the core losses of Fe-based amorphous alloys significantly depended on their components. The core loss separation calculated by the scaling theory suggests that the core loss at a frequency of 1 kHz and an induction of 1 T can be reduced to 7.77 W/kg and the percentage of the eddy-current loss can be lower than 17% by adjusting the components of Fe-based amorphous alloys.     

Influence of excitation frequency on structural and electrical properties of spray pyrolyzed CuInS2 thin films

This paper reports the cost effective deposition of the copper indium sulfide (CuInS₂) thin films under atmospheric conditions via ultrasonic spray pyrolysis. Structural and electrical properties of these films have been tailored by controlling the nozzle excitation frequency and the solution loading. Smoother films have been obtained via 120 kHz excitation frequency compare to the 48 kHz. Band gap energy of the films has also been tailored via excitation frequency. UV–vis–NIR analysis revealed that films deposited at 48 kHz excitation frequency had lower band gap energies. Although, both excitation frequencies resulted chalcopyrite structure, crystallinity of the CuInS₂ films was better for 120 kHz. On the other hand, better optical absorption in visible and near infrared region was observed at 48 kHz. Moreover, room temperature electrical conductivity of the samples deposited at 48 kHz excitation frequency was higher than that of samples deposited at 120 kHz. Temperature dependent electrical conductivity data showed that variable range hopping mechanism can be used to explain the conduction of spray pyrolyzed CuInS₂ thin films. Electrical mobility as high as 48 cm²/Vs has been observed for the sample deposited from 0.51 ml/cm² loading at 48 kHz excitation frequency. This value is very close to the mobility of vacuum deposited thin films like amorphous silicon, which is one of the most commonly used semiconductor in electronic and energy applications.     

Longitudinal strain monitoring of rail using a distributed fiber sensor based on Brillouin optical correlation domain analysis

A fiber-optic distributed sensor system is configured to measure the longitudinal strain distribution of a rail in real time. The system is based on the Brillouin correlation domain analysis (BOCDA), in which the variation of local Brillouin frequency (ν_B) is measured that linearly depends on the strain applied to the optical fiber. In the test measurement, the longitudinal strain distribution along a 2.8 m rail is measured under different loading conditions with a spatial resolution of 3.8 cm and an accuracy of ±15 με.     

Properties of pulsed reactive DC magnetron sputtered tantalum oxide (Ta2O5) thin films for photocatalysis

Tantalum oxide (Ta₂O₅) is a wide band gap semiconductor, known for a wide range of applications in many areas. The present investigation reports the effect of pulsing on the physical and photocatalytic properties of Ta₂O₅ thin films. The samples are prepared at room temperature on quartz and ITO substrates by pulsed reactive direct current (DC) magnetron sputtering. The pulsing frequency is varied between 5 kHz and 100 kHz. The photocatalytic activity is measured by the Rhodamine B dye. The thicknesses of all films were kept constant of ~ 500 nm. The microstructure obtained by X-ray diffraction is amorphous for all the samples. A lowest surface roughness of 4.62 nm for 50 kHz pulsing frequency is seen in atomic force microscopy measurements. The calculated relative density, optical band gap and the surface work function varies with the pulsing frequency. The sample prepared at 50 kHz pulsing frequency shows high photocatalytic activity: 2.59 × 10¹³ number of Rhodamine B molecules were oxidized per an incident photon flux of 1.78 × 10¹⁶ photons/s at 254 nm. The pulsed-Ta₂O₅ thin films were compared with continuous DC-Ta₂O₅ films and showed that pulsing (target power) gives the enhanced film properties, leading to better photocatalytic properties.     

Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10⁵–10⁹ cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.     

Inspection of stress corrosion cracking in welded stainless steel pipe using point-focusing electromagnetic-acoustic transducer

Point-focusing electromagnetic-acoustic transducers (PF-EMATs) for shear-vertical (SV) waves were developed for crack inspection of stainless-steel pipes. The transducer has improved defect detectability by accumulating SV waves generated by concentric line sources at a focal point in phase. An optimum frequency for defect detection was found to be 2 MHz, with which a crack of 0.5 mm depth near a weld was clearly detected. The EMAT exhibited defect detectability comparable to that of a conventional phased-array piezoelectric transducer, indicating that this new EMAT is highly practical for the non-contacting evaluation of stress-corrosion cracking in stainless steels.     

Ultra-precision finishing of micro-aspheric mold using a magnetostrictive vibrating polisher

Demands of micro-aspheric glass lenses are increasing in optical devices such as digital cameras and blu-ray players. In this paper, a novel vibration-assisted polishing machine using a magnetostrictive vibrating polisher is proposed and developed to improve the efficiency, surface roughness and stability of finishing. The magnetostrictive vibrating polisher can generate a radius of 30 μm circular vibrating motion at frequency 9.2 kHz. From the polishing experiments, a smooth removal function was obtained. The form accuracy was improved to less than 100 nm P–V and the surface roughness was reduced to 3.3 nm Rz (0.4 nm Ra).     

Electro/magnetic shielding effectiveness of soft magnetic Fe80Nb6B14 amorphous alloy

Application of the Fe₈₀Nb₆B₁₄ amorphous alloy to electromagnetic shielding was examined in detail using different experimental techniques. For shields made of the optimized (annealing at 700 K/1 h) amorphous ribbons the shielding effectiveness b was measured versus frequency f and shield thickness h. It was shown that for h = 200 μm in the frequency range 2 MHz < f < 15 MHz (the near-zone, electric field) b decreases from 55 dB to 20 dB. In the frequency range 0.2 kHz < f < 10 kHz (the near-zone, magnetic field) b > 20 dB. The best shielding effectiveness, i.e. b > 100 dB was obtained for electromagnetic field in the frequency range 200 MHz < f < 1000 MHz (the far-zone).     

Design of modified electromagnetic main-flux for steel wire rope inspection

This paper presents the design and construction of modified main-flux equipment for wire rope inspection which has advantages over the in-service inspection and indirect axial-flux measurement used by ordinary main-flux and return-flux methods. The equipment can be adjusted high electromagnetic field strength to produce leakage filed from flaws of various large-diameter ropes. Unique coil sensors connected in series were employed and performed well when sensing leakage fields of the radial, axial and tangential directions. In addition, a standalone system has a simple setup for service inspection. Due to the complicated structure of the wire rope, electromagnetic field theory for anisotropic structures with high electromagnetization was used to design the equipment. The characteristics of the leakage field, in terms of localized fault and loss of metallic cross-sectional area, were distinguished mathematically. The sensor orientation to detect leakage fields was also determined. Performance of the system was then verified experimentally and shown good resolution and repeatability. Furthermore, the equipment is sufficiently sensitive to detect a smallest surface flaw of 1×2 mm at 5 mm equipment lift off. The inspection signals were processed and shown locations, levels and deterioration quantities.     

New dual-focalization ferroelectret-based array for air-coupled ultrasonic inspection of textiles

This work presents a design methodology for half-curved airborne ultrasonic arrays based in cellular ferroelectret film. The geometry of the array proposed allows us focus naturally in the vertical plane and electronically in the horizontal one, obtaining similar spatial resolution in both directions. Theoretical predictions and simulated results were validated with a developed array prototype designed to operate at frequencies between 50 kHz and 300 kHz. The potential of the device is shown by inspecting different textile samples in transmission mode. This multitransducer design is a low cost alternative to the use of composite 2D arrays in noncontact ultrasonic inspections.     

A method for measuring the guideway straightness error based on polarized interference principle

This paper presents a new method for measuring guideway straightness error based on the polarized interference principle and affords a new way to measure straightness error in real-time with high precision. Firstly, the method is demonstrated and analyzed in theory, and then the layout of the optical modulator and the polarization angle detecting unit are discussed in details. Finally, a calibration process is introduced with linear function based on the least-square method. Calibration results show that the correlation coefficients R² of the fitting curves are above 0.9999 and the standard error of the estimated value is less than 0.2 μm. The theoretical analysis of the relationship between the straightness error and polarization angle is verified. The range of measuring straightness error is above 0.5 mm with 0.5 μm resolution. The system uncertainty (k=3) is less than 1 μm after the measurement system is calibrated. Experimental results demonstrate that this method possesses the advantages of minimizing the effects caused by the variation of light intensity and the shape and surface error of the guideway. The measurement accuracy is considerable with the autocollimator having the characteristic of high reliability and accuracy. It will have a prospective application in the industrial measurement field.     

Electromagnetic characterization of grouting materials of bridge post tensioned ducts for NDT using capacitive probe

The use of a capacitive probe as a non-destructive investigative technique for controlling the post tensioned ducts of bridges has gained increasing acceptance in France since several years. A field campaign measurement made in 2006 has shown the performance of the capacitive probe developed by the IFSTTAR. Nevertheless, some results are not understood, and in particular the behaviour of the probe when cement exudation product are present in the duct. Without an accurate knowledge of the electromagnetic properties of those products it is impossible to quantitatively assess the results of our capacitive probe. This paper reports the development of a coaxial transmission line feature. It was designed to allow the evaluation of a large type of material (liquid, paste or granular) over a large frequency range (50 MHz–4 GHz at maximum). A calibration scheme developed before at the Fresnel Institute was used. Using a two port S parameter instrument, the complex permittivity and magnetic permeability were evaluated by frequency domain measurement. The electromagnetic characterization of cement paste, cement exudation products and injection wax has brought us some key results in the interpretation of the capacitive probe signal. The results of this characterization were then used in a 3D semi analytical modelling of the problem. The studies of configurations with exudation products are presented and compared to experimental results obtained with our capacitive probe on laboratory duct.     

Electrochemical short crack effect in environmentally assisted cracking of a steam turbine blade steel

Measurement has been made of the corrosion fatigue short crack growth rate in a 12Cr steam turbine blade steel subjected to low frequency trapezoidal loading in aerated and deaerated 300 ppb¹ Cl⁻ and 300 ppb ${\text{SO}}_4^{2-}$, simulating early condensate chemistry. No difference in growth rate compared to that for long cracks was observed in deaerated solution but significantly enhanced growth rate was obtained in aerated solution for a short crack of length less than 250 μm. Complementary stress corrosion cracking tests were conducted but to ensure crack development at modest applied stresses the environment adopted was aerated 35 ppm Cl⁻, representing a severe system upset. In this case, the growth rate of the short crack was up to 20 times higher than that for a long crack (>6 mm), even though the crack length had reached 1.6 mm. An explanation for both sets of data based on the difference in potential drop between a short and long crack is expounded.     

The relationship between microstructure and fracture toughness of zirconia toughened alumina (ZTA) added with MgO and CeO2

The aim of this research is to investigate the mode of crack propagation in zirconia toughened alumina (ZTA) added with MgO and CeO₂, respectively. The mode of crack refers to the toughening mechanism of the materials. Different ZTA compositions containing MgO and CeO₂ as sintering additives were prepared using pressureless sintering at 1600 °C. Each sample was subjected to Vickers indentation with 294 N load and the cracks that propagated were observed with SEM. The ZTA with an addition of 0.7 wt.% MgO showed a crack deflection with a fracture toughness value of 6.19 ± 0.26 MPa · √m. On the other hand, the ZTA with CeO₂ addition of 0.5 to 7 wt.% showed both crack bridging and deflection, and produced 5.78 ± 0.16 MPa · √m to 6.59 ± 0.23 MPa · √m fracture toughness values, respectively. The fracture toughness of the ZTA–MgO–CeO₂ compositions is higher due to crack bridging and crack deflection. The toughening mechanisms of crack deflection and bridging hinder crack propagation since more energy is required to make the crack propagate. However, the formation of CeAl₁₁O₁₈ phase was observed; this consequently decreases the hardness and fracture toughness of the ZTA–MgO–CeO₂ compositions.     

Long-term monitoring of atmospheric corrosion at weathering steel bridges by an electrochemical impedance method

Weathering steel corrosion was monitored for one to two years under natural atmosphere by an electrochemical impedance technique. Two identical comb-shape weathering steel sheets embedded in epoxy resin were used as monitoring probe electrodes at two different bridges in Japan. Impedances at 10 kHz (Z_10kHz) and 10 mHz (Z_10mHz) were automatically measured every hour. Coupons (50 × 50 × 2 mm³) prepared from the same steel sheets were exposed together to measure the corrosion mass loss. The average (Z_10mHz)⁻¹ value for half to one year exposure correlated well with the average corrosion rate determined from the corrosion mass loss.