Finite-element modeling of eddy-current nondestructive evaluation (NDE)

Quantitative eddy-current nondestructive evaluation is hampered by the lack of suitable models to describe the field/defect interaction. The governing partial differential equations admit of closed-form algebraic solution only in the most trivial of geometries. Finite-element methods hold out the hope of providing numerical solutions for more complex cases. In this paper certain effectively two-dimensional eddy-current problems are considered, previous analytical results being used to validate the computations wherever feasible. The extension to three-dimensional fields, which is outlined here, is treated in detail in a subsequent paper.     

Eddy current probe response to open and closed surface flaws

A general theory of eddy current flaw response, appropriate to both standard low frequency and ferromagnetic resonance (FMR) probes, is developed for simple two-dimensional and three-dimensional open and closed surface flaw geometries. This analysis, based on the assumption of a uniform interrogating field applied to the flaw by the probe, shows that flaw opening responses increase with the operating frequency of the probe. Experimental results using both types of probe confirm this result for realistic practical geometries, where variations of crack mouth opening displacement under load provide useful information about crack dimensions.     

Eddy current probe impedance due to a flaw: The influence of phase-shifted fields

An approximate formula for the change in impedance of an eddy current two-coil probe situated above a conducting medium with a volumetric flaw is derived in this paper. The primary field is generated by the superposition of two phase-shifted alternating currents of the same frequency. The Lorentz reciprocity theorem and a layer approximation are used to obtain the change in impedance. Numerical results for different values of the phase difference are presented. These results can be used for developing more selective eddy current testing methods.     

ORCHID—A computer simulation of the reliability of an NDE inspection system

CANDU pressurized heavy water reactors contain several hundred horizontally-mounted zirconium alloy pressure tubes. Following a pressure tube failure, a pressure tube inspection system called CIGARette was rapidly designed, manufactured and put in operation. Defects called hydride blisters were found to be the cause of the failure, and were detected using a combination of eddy current and ultrasonic scans. A number of improvements were made to CIGARette during the inspection period. The ORCHID computer program models the operation of the delivery system, eddy current and ultrasonic systems by imitating the on-reactor decision-making procedure. ORCHID predicts that during the early stage of development, less than one blistered tube in three would be detected, while less than one in two would be detected in the middle development stage. However, ORCHID predicts that during the late development stage, probability of detection will be over 90%, primarily due to the inclusion of axial ultrasonic scans (a procedural modification). Rotational and axial slip could severely reduce probability of detection. Comparison of CIGARette's inspection data with ORCHID's predictions indicate that the latter are compatible with the actual inspection results, though the numbers are small and data uncertain. It should be emphasized that the CIGARette system has been essentially replaced with the much more reliable CIGAR system.     

Eddy Current Signal Deconvolution Technique for the Improvement of Steam Generator Tubing Burst Pressure Predictions

Eddy current techniques are extremely sensitive to the presence of axial cracks in nuclear power plant steam generator tube walls, but they are equally sensitive to the presence of dents, fretting, support structures, corrosion products, and other artifacts. Eddy current signal interpretation is further complicated by cracking geometries more complex than a single axial crack. Although there has been limited success in classifying and sizing defects through artificial neural networks, the ability to predict tubing integrity has, so far, eluded modelers. In large part, this lack of success stems from an inability to distinguish crack signals from those arising from artifacts. We present here a new signal processing technique that deconvolves raw eddy current voltage signals into separate signal contributions from different sources, which allows signals associated with a dominant crack to be identified. The signal deconvolution technique, combined with artificial neural network modeling, significantly improves the prediction of tube burst pressure from bobbin-coil eddy current measurements of steam generator tubing.     

The relationship between radiating body forces and equivalent surface stresses: Analysis and application to EMAT design

An analysis of the radiation from a body force shows that it is equivalent to the radiation from a series of surface stresses defined by the moments of the body force taken with respect to the depth coordinate. As the body force becomes localized near the surface, the zeroth moment of the force dominates the radiation and is often thought of as an equivalent surface stress. However, under certain conditions, this can vanish, and the other moments must be considered. It is found that, as the order of the moment of a particular force component increases, the resulting radiation patterns alternate between those characteristic of a compressive surface stress and those characteristic of a shear surface stress, which have considerably different angular variations. Results of experiments in the development of EMAT transducers for nondestructive testing that support these results are cited, and important consequences in the design of inspection systems are indicated.     

Application of ferromagnetic resonance probes to the characterization of flaws in metals

This article presents some results obtained in the characterization of surface flaws by means of probes using ferromagnetic resonance of yttrium iron garnets (FMR probes). These experiments on artificial flaws show that FMR probes operate like eddy current probes for nonmagnetic materials and like magnetic field sensors for magnetic ones. Consequently, the working distance is larger for magnetic materials (1000–1500 µm) than for nonmagnetic ones (100–300 µm). FMR probes have good sensitivity to narrow flaws, good spatial discrimination, and are sensitive to flaw width and depth. Vector analysis allows the separation of distance and flaw effect by phase analysis on nonmagnetic materials. On magnetic materials this phase separation does not exist and another procedure is suggested. These results, and in particular those obtained on ferromagnetic materials, point to the possibility of replacing some eddy current or magnetic particle inspections by tests with ferromagnetic resonance probes.     

Ultrasonic evaluation of geometrical and surface parameters of rough defects in solids

Ultrasonic measurements from rough cracks were carried out using both broad-band and narrow-band methods. An analysis is suggested to determine parameters of the crack quantitatively such as size, shape, rms surface roughness, and distribution function of the surface roughness. Ultrasonic measurements of the parameters compare very well with the actual parameters of the defect.     

Recent improvements to the application of the volume integral method of eddy current modeling

A previous attempt by Dunbar to model eddy current NDE data using the volume integral equation method produced qualitative agreement with experimental data, but true verification was not possible due to the fact that the results were not independent of the discretization of the flaw and there were numerical problems encountered in calculating the unperturbed electric field. We have overcome these problems by calculating the integrals of the Green's tensor more accurately, especially in the vicinity of any singularities, and by using a fast Hankel transform to calculate the incident electric field. Further improvements were made by considering a linear variation of the electric field, instead of taking it to be constant over each cell used in the discretization. Results obtained from rectangular flaws in a half-space and in a thin plate are presented.     

Boundary layer equations and stretching sheet solutions for the modified second grade fluid

A modified second grade non-Newtonian fluid model is considered. The model is a combination of power-law and second grade fluids in which the fluid may exhibit normal stresses, shear thinning or shear thickening behaviors. The equations of motion are derived for two dimensional incompressible flows. The boundary layer equations are derived from the equations. Symmetries of the boundary layer equations are calculated using Lie Group theory. For a special power law index of m = −1, the principal Lie algebra extends. Using one of the symmetries, the partial differential system is transferred to an ordinary differential system. The ordinary differential equations are numerically integrated for the stretching sheet boundary conditions. Effects of power-law index and second grade coefficient on the boundary layers are shown and solutions are contrasted with the usual second grade fluid solutions. The shear stress on the boundary is also calculated.     

Use of Eddy Current Technology to Assist in the Evaluation of the Fatigue Damage of Electrical Conductors

The article deals with the evaluation of fatigue damage on overhead conductors which have been in service for a period exceeding 50 years. Under wind excitation, electrical transmission lines are subjected to aeolian vibrations that may induce wire fatigue of conductor at the exit of supporting equipment. This fatigue phenomenon is a fretting fatigue problem which is located at contact points between wires belonging to adjacent wire layers or between external layer wire and the supporting equipment. This article reports the methodology followed: 1) in selecting wires with the more severe damaged contact point using eddy current technology and 2) in comparing, through a metallographic examination, the contact points of those selected wires coming from in situ worn conductors and from laboratory fatigue tested virgin conductors used as reference.     

Characterization of localized surface elastic defects by nonspecular reflection at the Rayleigh angle

The relationships between characteristics of elastic defects and nonspecular features of bounded ultrasonic beams reflected at the Rayleigh angle from a liquid-solid interface are investigated. The results can serve as a theoretical basis for interpretation of Rayleigh angle nonspecularly reflected beam profiles as characterization of localized surface elastic defects.     

Reverse problem of the transient gradiental flow of a Shvedov-Bingham plastic through a flat channel and a cylindrical pipe

Analyzed is the reverse problem of the transient gradiental flow of a Shvedov-Bingham plastic through a flat channel and a cylindrical pipe. Equations are derived for determining the distribution of tangential shearing stresses and for the pressure gradient as a function of time, these equations to be solved by an iteration scheme which is shown here.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 4, pp. 725–729, April, 1973.     

Dozor roughness testers for workshop noncontact inspection of the surface of machined parts

Reflectometric results are given for a nonparallel incident beam at various wavelengths. A number of different original portable testers have been developed on the basis of those results for checking the roughness parameter R_a of outside surfaces. The devices are described and their specifications are given.Translated from Izmeritelnaya Tekhnika, No. 11, pp. 21–25, November, 2004.     

A complex approach to the development of the method and equipment for thermal nondestructive testing of CFRP cylindrical parts

Composite materials are being increasingly used in high-tech industries, such as aerospace, automotive manufacture and building inspection. Thermal nondestructive testing (TNDT) has become an accepted method for composite inspection. However, the majority of investigations have dealt with flat or slightly-curved composite components with a thickness of up to 5 mm. Particular studies have been devoted either to NDT modeling with an emphasis on some theoretical issues, or they have been based exclusively on experimental results. There has been some recent interest in the use of composite materials in the nuclear industry. Some critical parts, including centrifuge components, have been made of carbon fiber reinforced polymer (CFRP) composites. The working conditions in a centrifuge include radioactivity and high rotational speed, and the composites used in centrifuges must have very uniform thermal properties and must be free of defects.This paper describes a complex approach to the TNDT of cylindrical parts made of CFRP by starting from thermal properties measurement, theoretical modeling and preliminary experiments, and finishing with the technical requirements for the development of practical equipment capable of operating in both laboratory and industrial conditions.The objects tested were CFRP cylinders with a diameter of 150 mm and a wall thickness of 4–6 mm, and they contained some artificial defects of varying size and depth. Both one- and two-sided test procedures have been analyzed for spot, line and uniform heating. Ultrasonic excitation has also been used as an alternative stimulation technique.In a one-sided test, the depth detection limit has been about 4 mm. Similar results have been observed in the case of ultrasonic stimulation, but the practical implementation of ultrasonic IR thermography to the inspection of cylindrical parts requires further exploration.In a two-sided test, even fairly mild heating resulted in the reliable detection of all defects independent of their size and depth.In all test cases, the highest signal-to-noise ratio occurred after applying the technique of principal component analysis.     

A freeform surface manufacturing approach by integration of inspection and tool path generation

Freeform surfaces have been widely used in various engineering applications. Increasing requirements for the accuracy of freeform surfaces have led to significant challenges for the manufacturing of these surfaces. A method for manufacturing of freeform surfaces is introduced in this paper by integrating inspection and tool path generation to improve manufacturing quality while reducing manufacturing efforts. Inspection is conducted by comparing the digitised manufactured surface with the design surface to identify the error regions. In this new inspection technique, the areas on the manufactured surface that are beyond the design tolerance boundaries are used as the objective function during the localisation process, in order to minimise post-inspection machining efforts. The tool path generation methods are then selected based on the geometric characteristics of the identified error regions, for creating tool paths to remove the errors. Computational efficiency, machining efficiency, and quality are considered in this integrated method.     

Contribution of ultrasonic surface rolling process to the fatigue properties of TB8 alloy with body-centered cubic structure

The effect of ultrasonic surface rolling process(USRP) as a severe plastic deformation technology was investigated on the evolution of microstructure, residual stress and surface morphology of TB8 alloys with body-centered cubic structure. Stress-controlled rotating-bending fatigue tests indicated increased fatigue strength in USRP samples prepared using different number of passes compared to the base material, which was attributed to the presence of gradient structure surface layers. Five subsequent USRP passes resulted in the highest fatigue strength, due to the optimal surface properties including higher extent of grain refinement, larger compressive residual stresses, "smoother" surface morphology and increased micro-hardness. However, the effect of USRP technology on improving fatigue strength of TB8 alloy was not significant in comparison with that of other titanium alloys(for example, Ti6 Al4 V), which was attributed to the notable surface residual stresses relaxation revealed from measurements on postfatigued USRP samples. Electron backscatter diffraction analysis confirmed that fatigue crack initiation occurred in the larger grains on the surface with high Schmid factor. Small cracks were found to propagate into the core material in a mixed transgranular and intergranular mode. Further analysis indicated that grain growth existed in post-fatigued USRP-treated TB8 samples and that the average geometrically necessary dislocations value reduced after fatigue loading.     

A comparative study of the yield criteria applicable to replicated foams

In this article, first some published evidence for yielding of aluminum and polymeric replicated foams and a numerical study on regular porous structures are revisited in which a kind of twist in the shape of yield surface along the hydrostatic axis was reported. Next, motivated by this observation, some existing appropriate yield functions and a new extended generalized failure criterion capable of capturing this effect are presented. Finally, using the experimentally measured yield clouds of aluminum replicated foams from the literature, the efficiency, inherent features, and limitations of these criteria are thoroughly assessed, compared, and discussed.     

CdZnTe单晶的机械抛光及其表面损伤层的测定

研究了CdZnTe单晶片的机械抛光工艺.采用SiO2和MgO进行分步机械抛光后的晶片光亮平整,在光学显微镜下观察没有划伤,采用New View5000TM测得抛光后晶片的表面粗糙度Ra为8.752nm.采用X射线摇摆曲线的半峰宽表征了表面损伤程度.通过分析不同时间腐蚀后晶片的质量和半峰宽值,计算出机械抛光产生的表面损伤层厚度约为26.7μm.     

镁合金表面铝合金化层组织及性能研究

使用消失模铸造方法在镁合金表面制备了一层铝合金化层,并对合金化层的金相组织、成分、相组成及腐蚀性能进行了分析.结果表明,当真空度为0.08MPa,铝粉粒度为150μm时,镁合金表面生成的铝合金化层厚度为750μm,新相覆盖率达到76.2%;合金化层呈菊花状或网状,与基体为冶金结合,其主要组成物相为Mg17Al12、Mg...