Optimum eddy current excitation frequency for subsurface defect detection in SQUID based non-destructive evaluation

Detailed experimental studies have been carried out for the determination of optimum eddy current excitation frequencies for the defects located at different depths below the top surface of an aluminum plate. These subsurface defects were detected by using a highly sensitive superconducting quantum interference device (SQUID) based eddy current non-destructive evaluation (NDE) system. The signal to noise ratio was found to be significantly higher at the optimum excitation frequency, which depended on the depth of the defect. The optimum excitation frequencies have been evaluated for defects located at different depths from 2 to 14 mm below the top surface of the plate. The defect depth was varied in steps of 2 mm, while the overall total thickness of the stack of plates was kept constant at 15 mm. Each defect represented a localized loss of conductor volume, which was 60 mm in length, 0.75 mm in width and 1 mm in height. The experimental results show that the square root of the optimum excitation frequency is inversely proportional to the depth of defect.     

Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet is used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0–180°, and the setup has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils.     

Investigation of carbon fiber reinforced polymer (CFRP) sheet with subsurface defects inspection using thermal-wave radar imaging (TWRI) based on the multi-transform technique

A combined theoretical and experimental approach is reported using thermal-wave radar imaging (TRWI) for carbon fiber reinforced polymer (CFRP) with subsurface defects inspection. The multi-transform technique (Fourier transform, FT; Hilbert transform, HT; and cross-correlation, CC) is applied to extract the characteristics of thermal-wave signal. Experimental results indicate that the multi-transform technique of thermal-wave signal is available for detecting the subsurface defect. For the shallow defect (defect depth ≤1 mm), the delay time image of CC exhibits high contrast, and the phase image of FT has high SNR at the right frequency component. For the deep defect (defect depth 2.0 mm), the phase images of HT have both high contrast and large SNR value.     

Silicon Σ13(5 0 1) grain boundary interface structure determined by bicrystal Bragg rod X-ray scattering

The atomic structure of the silicon Σ13(5 0 1) symmetric tilt grain boundary interface has been determined using Bragg rod X-ray scattering. In contrast to conventional structural studies of grain boundary structure using transmission electron microscopy, this approach allows the non-destructive measurement of macroscopic samples. The interface was found to have a single structure that is fully fourfold coordinated. X-ray diffraction data were measured at Beamline I07 at the Diamond Light Source.     

Evolution of orientation distributions of γ and γ′ phases during creep deformation of Ni-base single crystal superalloys

The evolution of orientation distributions of γ and γ′ phases in crept Ni-base single crystal superalloys have been investigated by theoretical calculations with elastic–plastic models and by experiments. As creep deformation proceeds, the crystallographic orientation distributions for both phases are broadened as a result of the waving of the raft structure, which occurs to reduce the total mechanical energy. The broadening of the orientation distribution occurs in such a way that the 0 0 1 pole broadens isotropically while the h k 0 poles broaden preferentially along the 〈0 0 1〉 directions. Since the extent of the broadening increases almost linearly with the number of creep deformation, the measurement of the broadening by X-ray diffraction can be utilized in non-destructive methods to predict the lifetime of Ni-base superalloys.     

Defect characterisation from limited view pipeline radiography

This work presents a method of characterising pipeline defects using a small number of radiographs taken at different angles around the pipe. The method relies on knowledge of the setup geometry and use of multiple images, and does not require calibration objects to be included in the setup. It is aimed at use in situations where access is difficult such as in subsea pipeline inspections. Given a set of radiographs, a background subtraction method is used to extract defects in the images. Using a ray tracing algorithm and knowledge of the experimental setup, the range of possible locations of the defect in 3D space is then calculated. Constraints are applied on potential defect shapes and positions to further refine the defect range. The method is tested on simulated and experimental flat bottomed hole defects and simulated corrosion patch defects with lateral and axial sizes ranging from 12.5 to 33.8 mm and thickness between 3 mm and 16 mm. Results demonstrate a good, consistent ability to calculate lateral and axial defect dimensions to within ±3 mm of the true size. Defect thickness calculations are more difficult and as such errors are more significant. In most cases defect thickness is calculated to within 4 mm of the actual value, often closer. Errors in thickness are due to overestimation, meaning the calculation could be used to place a maximum limit on potential defect size rather than as an actual estimate of the thickness. This would still be useful, for example in deciding whether a defect requires further investigation.     

Source separation techniques applied to the detection of subsurface defects in the eddy current NDT of aeronautical lap-joints

This paper investigates the application of two source separation techniques, principal component analysis and independent component analysis, to process the data from the inspection of riveted lap joints by eddy currents. An eddy current array sensor is designed for the rapid inspection of lap-joints and used to test a set of flawed rivet configurations featuring 1–10 mm notches, buried down to a 4 mm depth. Implementation methods are proposed for processing such eddy current data by means of both the considered source separation techniques. The signal processing results obtained from the experimental data are compared in terms of source separation efficiency and detection using a receiver operating characteristic approach. In the light of this study, both the techniques appear to be efficient. However, the principal component analysis provides better defect detection results, especially for deeply buried defects.     

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.     

Detection and 3-D positioning of small defects using 3-D point reconstruction,tracking, and the radiographic magnification technique

In this paper the capability of a 3-D point reconstruction algorithm based on multiple hypothesis tracking is experimentally explored on a setup consisting of a microfocus X-ray source and a digital detector array. The algorithm is verified to detect and 3-D position steel particles with diameters 0.16–0.26 mm radiographed behind 4.7 mm Inconel 718 at two to ten times magnification. At ten times magnification the algorithm is verified to detect and 3-D position with an average error of 0.1 mm pore defects with diameters 0.05–0.25 mm in 4.7 mm thick titanium alloy laser welds.     

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ～70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (～350–850 J), creating pressure pulses with initial duration of ～3 ns and amplitudes of up to ～145 GPa. The laser beam, with a spot radius of ～1 mm, created a crater of significant depth (～135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ～24 GPa for the monocrystalline to ～150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.     

A comparative study of high resolution cone beam X-ray tomography and synchrotron tomography applied to Fe- and Al-alloys

X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterization and evaluation of materials. Due to measurement speed and quality, XCT systems with cone beam geometry and matrix detectors have gained general acceptance. Continuous improvements in the quality and performance of X-ray tubes and XCT devices have led to cone beam CT systems that can now achieve spatial resolutions down to 1 μm and even below. However, the polychromatic nature of the source, limited photon flux and cone beam artefacts mean that there are limits to the quality of the CT-data achievable; these limits are particularly pronounced with materials of higher density like metals. Synchrotron radiation offers significant advantages by its monochromatic and parallel beam of high brilliance. These advantages usually cause fewer artefacts, improved contrast and resolution.Tomography data of a steel sample and of two multi-phase Al-samples (AlSi12Ni1, AlMg5Si7) are recorded by advanced cone beam XCT-systems with a μ-focus (μXCT) and a sub-μm (nano-focus, sub-μXCT) X-ray source with voxel dimensions between 0.4 and 3.5 μm and are compared with synchrotron computed tomography (sXCT) with 0.3 μm/voxel. CT data features like beam hardening and ring artefacts, detection of details, sharpness, contrast, signal-to-noise ratio and the grey value histogram are systematically compared. In all cases μXCT displayed the lowest performance. Sub-μXCT gives excellent results in the detection of details, spatial and contrast resolution, which are comparable to synchrotron-XCT recordings. The signal-to-noise ratio is usually significantly lower for sub-μXCT compared with the two other methods. With regard to measurement costs “for industrial users”, scanning volume, accessibility and user-friendliness sub-μXCT has significant advantages in comparison to synchrotron-XCT.     

Characterization of subsurface defects in aeronautical riveted lap-joints using multi-frequency eddy current imaging

The authors present an original eddy current imager (ECI) designed for the fast and accurate non-destructive evaluation of defects buried next to rivets in aeronautical lap-joints. The ECI is associated to a signal processing method based on a principal component analysis (PCA) followed by a maximum likelihood (ML) approach. The PCA was implemented using EC images obtained with selected excitation frequencies. These images are considered as resulting from a linear mixing of different sources including the presence of rivets and defects, and the PCA is used to separate these sources thanks to an eigen decomposition of the EC data covariance matrix. As a result, the defect signatures are enhanced and used to implement an automatic defect characterization. This characterization is carried out by the means of an ML approach which allows the length and depth of the defects to be estimated. The method was implemented for the evaluation of a laboratory made riveted lap joint mock-up featuring buried defects. It was experimentally optimized and successfully implemented for the characterization of calibrated defects ranging from 2 to 10 mm in length and 2 to 8 mm in depth.     

Investigation of electron beam melting and refining of titanium and tantalum scrap

In this paper obtained experimental and theoretical data for Ti and Ta electron beam melting regeneration from waste products are presented. Different technological regimes and methods are realized and the obtained results are discussed. Element analyses of the impurities’ concentration of the materials before electron beam melting and refining (EBMR) and of the ingots after EBMR are compared. Statistical approach is applied for optimization of process parameters for Ti. Material losses less than 1% and oxygen concentration less than 400 ppm after EBMR of Ti scrap are achieved at 11.5–12 kW beam power and 0.09–0.14 mm/s casting velocity. The optimal process conditions and purification data for Ti refining by minimization of all impurities’ concentrations and the material losses at the same time are obtained at 11.25 kW electron beam power and 0.0835 mm/s casting velocity. For the performed experiments the best purification of Ta (99.985) is obtained at 24 kW beam power and 0.029 mm/s casting velocity, the residence times on the front side of the feeding block and in the liquid metal pool are 2 min and 5 min, respectively.     

Nonpolar ZnO film growth and mechanism for anisotropic in-plane strain relaxation

Using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, we investigated the strain relaxation mechanisms for nonpolar (1 1 ?2 0) a-plane ZnO epitaxy on (1 ?1 0 2) r-plane sapphire, where the in-plane misfit ranges from ?1.5% for the [0 0 0 1]ZnO6[1 ?1 0 ?1]sapphire to ?18.3% for the [?1 1 0 0]ZnO6[?1 ?1 2 0]sapphire direction. For the large misfit [?1 1 0 0]ZnO direction the misfit strains are fully relaxed at the growth temperature, and only thermal misfit and defect strains, which cannot be relaxed fully by slip dislocations, remain on cooling. For the small misfit direction, lattice misfit is not fully relaxed at the growth temperature. As a result, additive unrelaxed lattice and thermal misfit and defect strains contribute to the measured strain. Our X-ray diffraction measurements of lattice parameters show that the anisotropic in-plane biaxial strain leads to a distortion of the hexagonal symmetry of the ZnO basal plane. Based on the anisotropic strain relaxation observed along the orthogonal in-plane [?1 1 0 0] and [0 0 0 1]ZnO stress directions and our HRTEM investigations of the interface, we show that the plastic relaxation occurring in the small misfit direction [0 0 0 1]ZnO by dislocation nucleation is incomplete. These results are consistent with the domain-matching paradigm of a complete strain relaxation for large misfits and a difficulty in relaxing the film strain for small misfits.     

Surface segregation of Pt in γ′-Ni3Al: A first-principles study

We performed first-principles slab calculations based on the density functional theory to investigate the surface segregation behavior of Pt on the clean (1 0 0), (1 1 0) and (1 1 1) surfaces of γ′-Ni₃Al. The results clearly indicate that Pt has a strong tendency to surface segregate in all three low-index surface orientations. Such a conclusion is in complete agreement with a recent experimental finding that Pt segregates to the clean Ni₃Al(1 1 1) surface. The tendency of Pt to surface segregate also helps to explain why Pt addition is beneficial to the oxidation resistance of Ni₃Al.     

A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy

Mixed 93W–4.9Ni–2.1Fe powders were sintered via the spark plasma sintering (SPS) and hybrid spark plasma sintering (HSPS) techniques with 30 mm and 60 mm samples in both conditions. After SPS and HSPS, the 30 mm and 60 mm alloys (except 60 mm-SPS) had a relative density (> 99.2%) close to the theoretical density. Phase, microstructure and mechanical properties evolution of W–Ni–Fe alloy during SPS and HSPS were studied. The microstructural evolution of the 60 mm alloys varied from the edge of the sample to the core of the sample. Results show that the grain size and the hardness vary considerable from the edge to the core of sintered sample of 60 mm sintered using conventional SPS compared to hybrid SPS. Similarly, the hardness also increased from the edge to the core. Furthermore, the 60 mm-HSPS alloy exhibited improved bending strength of 1115 MPa when compared to that of 60 mm-SPS, 920 MPa. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–Ni–Fe, however in the 60 mm-SPS alloy peeling of the grains was also observed which diminished the properties. The mechanical properties of SPS and HSPS 93W–4.9Ni–2.1Fe heavy alloys are dependent on the microstructural parameters such as tungsten grain size and overall homogeneity.     

Amorphous magnetoelastic sensors for the detection of biological agents

Freestanding amorphous magnetoelastic (ME) biosensors were fabricated by two ways. One type with larger size, 2000 × 400 × 15 μm, 1000 × 200 × 15 μm and 500 × 100 × 15 μm, was made from an ME Fe40Ni38Mo4B18 ribbon, the other with smaller size 200 × 40 × 4 μm was manufactured by dual beam sputtering and non-traditional microelectronic fabrication techniques. Both platforms were immobilized with JRB7 phage and were developed for the real-time in vitro detection of Bacillus anthracis spores. The experimental results show that the measured sensitivity of the ME sensors agrees with theoretical predictions and the specificity of ME sensors coated with JRB7 phage for B. anthracis spore species is excellent. The 200 × 40 × 4 μm biosensor was found to have a detection limit of 10² cfu/ml and sensitivity of 13.1 kHz/decade.     

Frictional heating model for efficient use of vibrothermography

This paper investigates vibrothermography for the detection of fatigue cracks in steel compact tension specimens using combined experimental and numerical analyses. First, a numerical modal analysis is carried out to predict the optimal excitation parameters. A coupled thermo-mechanical model is then built to simulate the thermographic inspection. The model predicts the detection of cracks as short as 0.1 mm that is also confirmed experimentally using a commercial infrared camera with a maximum error of 2.13% on the temperature distribution. The model reveals that the specimens’ temperature increases at the crack vicinity according to the excitation frequency and is modulated due to the nonlinearity induced by the crack. The model also shows that the stress at the crack tip is lower than the material's yield stress, which makes the test truly non-destructive.     

Pure carbon microwave absorbers from anion-exchange resin pyrolysis

A series of carbon materials [C_F-x (where x denotes carbonization temperature)] have been prepared by pyrolysis of an anion-exchange resin at different temperatures (500–700 °C). X-ray diffraction and Raman spectroscopy suggest the presence of tiny crystalline domains in these materials, whose content is strongly determined by carbonization temperature. The microwave absorption of these materials is examined in the frequency range of 2–18 GHz, and it is found that the reflection loss characteristics are highly sensitive to the carbonization temperature. At a thickness of 2 mm, C_F-600 exhibits the best microwave absorbing ability with a maximum reflection loss of ?20.6 dB at 16 GHz, and a bandwidth exceeding ?10 dB in the range 13.5–18 GHz. It is concluded that dielectric loss in cooperation with better matched characteristic impedance results in the excellent microwave absorption of C_F-600. Furthermore, a reflection loss exceeding ?10 dB can be obtained in the range of 7–18 GHz by manipulating the thickness from 2 to 3.5 mm, and the maximum can reach ?37.0 dB at 10.8 GHz with a thickness of 2.8 mm. These materials may be used as light-weight and highly effective microwave absorbers over a wide frequency range.     

A new electromagnetic acoustic transducer (EMAT) design for operation on rail

Electromagnetic acoustic transducers (EMATs) can emit and receive ultrasound on a conducting sample without contact, but are usually kept within 3 mm lift-off from the sample surface, to achieve a sufficient signal-to-noise ratio (SNR). There are scenarios under which EMATs must scan a sample at high speed, with the EMAT-sample separation varying by more than the standard lift-off range, such as for detection of gauge corner cracks in rail. A new EMAT has been designed that allows the low weight and flexible EMAT coil to skim over the sample surface, while the heavier and bulkier magnet behind the coil has a lift-off that can vary over 10 mm whilst still achieving a reasonable SNR. In experiments conducted with the EMATs mounted on a train, scanning a rail, they were demonstrated as being sufficiently robust, with an SNR sufficient for defect detection.