Showing papers in "Ndt & E International in 2014"

in situ measurement of pavement thickness and dielectric permittivity by GPR using an antenna array

Abstract: We present a ground penetrating radar (GPR) system, which uses an antenna array for in situ measurements of the thickness and dielectric permittivity of an asphalt pavement layer. We calibrated the antenna array by considering the antenna phase center and the antenna offset. The results of the laboratory measurements demonstrate that the proposed calibration method can greatly improve the accuracy of the velocity and thickness estimations when compared with the conventional method. A field measurement conducted on a highway pavement shows that the error of the thickness estimation of the asphalt layer is less than 6 mm (10%).

Crack detection limits in unit based masonry with terrestrial laser scanning

Abstract: This paper presents the fundamental mathematics to determine the minimum crack width detectable with a terrestrial laser scanner in unit-based masonry. Orthogonal offset, interval scan angle, crack orientation, and crack depth are the main parameters. The theoretical work is benchmarked against laboratory tests using 4 samples with predesigned crack widths of 1–7 mm scanned at orthogonal distances of 5.0–12.5 m and at angles of 0°–30°. Results showed that absolute errors of crack width were mostly less than 1.37 mm when the orthogonal distance varied 5.0–7.5 m but significantly increased for greater distances. The orthogonal distance had a disproportionately negative effect compared to the scan angle.

Approach to identify cracking in asphalt pavement using GPR and infrared thermographic methods: Preliminary findings

Abstract: Pavement condition is a factor of major interest due to its direct contribution to safety and comfort of the users of the road. Consequently, road inspections imply the evaluation of different parameters such as roughness of the pavement, skid resistance, and presence and condition of cracks. Although the first two parameters can be quantitatively evaluated with different sensors, the latter is usually only qualitatively assessed by visual inspection. This paper deals with this drawback through the combined application of Ground Penetrating Radar and Infrared Thermography to the detection and characterization of cracks in pavement and their origins.

Optimization of pulsed thermography inspection by partial least-squares regression

Abstract: This paper introduces and tests a statistical correlation method for the optimization of the pulsed thermography inspection. The method is based on partial least squares regression, which decomposes the thermographic PT data sequence obtained during the cooling regime into a set of latent variables. The regression method is applied to experimental PT data from a carbon fiber-reinforced composite with simulated defects. The performance of the regression technique is evaluated in terms of the signal-to-noise ratio. The results showed an increase in the SNRs for 96% of the defects after processing the original sequence with PLSR.

Velocity effect analysis of dynamic magnetization in high speed magnetic flux leakage inspection

Abstract: The investigation described in this paper focuses on the velocity effect of dynamic magnetization and magnetic hysteresis due to rapid relative motion between magnetizer and measured specimens in high-speed magnetic flux leakage (MFL) inspection. Magnetization intensity and permeability of ferromagnetic materials along with the duration of dynamic magnetization process were analyzed. Alteration of the intensity and distribution of magnetic field leakage caused by permeability of specimen were investigated via theoretical analysis and finite-element method (FEM) combined with the actual high-speed MFL test. Following this, a specially designed experimental platform, in which motion velocity is within the range of 5 m/s–55 m/s, was employed to verify the velocity effect and probability of a high-speed MFL test. Preliminary results indicate that the MFL technique can achieve effective defect inspection at high speeds with the maximum inspection speed of about 200 km/h being verified under laboratory conditions.

Noncontact detection of fatigue cracks by laser nonlinear wave modulation spectroscopy (LNWMS)

Abstract: Nonlinear wave modulation spectroscopy (NWMS) has been used to detect nonlinear ultrasonic signatures produced by fatigue cracks in materials. It is done by generating ultrasonic waves at two different frequencies and measuring their modulation. A proper choice of two distinct frequencies for a given structure plays a significant role in NWMS. This paper, instead of using input signals at two distinct frequencies, takes only one broadband pulse signal as the driving input, which can be generated by a laser beam. With a broadband excitation, material nonlinearity exhibits modulation at multiple peaks in a spectral plot due to interactions among various input frequency components of the broadband input. A feature called sideband peak count (SPC), which is defined as the ratio of the number of sideband (modulation) peaks over a moving threshold to the total peak number in the specified frequency band, is extracted from the spectral plot to measure the degree of material nonlinearity. The basic premise of the proposed laser nonlinear wave modulation spectroscopy (LNWMS) is that this SPC value will rise as the level of material nonlinearity increases. A noncontact laser ultrasonic system has been built for LNWMS measurement by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic wave generation and a laser Doppler vibrometer for ultrasonic wave detection. The proposed LNWMS technique has been successfully tested for detecting fatigue cracks in metallic plates and aircraft fitting-lugs having complex geometries.

Laser lock-in thermography for detection of surface-breaking fatigue cracks on uncoated steel structures

Abstract: This paper develops a new noncontact laser lock-in thermography (LLT) technique for detection of surface-breaking fatigue cracks on uncoated steel structures with low surface emissivity. LLT utilizes a modulated continuous (CW) wave laser as a heat source for lock-in thermography instead of commonly used flash and halogen lamps. LLT has the following merits: (1) the laser heat source can be precisely positioned at a long distance from a target structure thank to its directionality and low energy loss, (2) a large target structure can be inspected using a scanning laser heat source, (3) no special surface treatment of the target structure is necessary to generate and measure thermal wavefields, (4) thermal image noises created by arbitrary surrounding heat sources can be effectively eliminated and (5) the use of a low peak power laser makes it possible to avoid surface ablation. The LLT system is developed by integrating and synchronizing a modulated CW laser, a galvanometer and an infrared camera. Then, a fatigue crack evaluation algorithm based on a holder exponent analysis is proposed. The performance of the proposed LLT technique is validated through thermal wavefield imaging and fatigue crack evaluation tests on an uncoated steel plate with emissivity of 0.8 and a welded T-shape joint with emissivity of 0.7. Test results confirm that thermal wavefield images are effectively captured even when surface-reflected background noises and laser-generated thermal waves coexist, and surface-breaking cracks are successfully evaluated without any special surface treatment.

Eddy-current non-destructive testing system for the determination of crack orientation

Abstract: Detecting theorientationofdeeporientedcracksisamajorchallengeinthedevelopmentofEddy- Current (EC)Non-DestructiveTesting(NDT).Infact,thedetectionsensitivityofEC-NDTdependsonthe interactionbetweenthecracklengthdirectionandtheEC flowinginthematerials.Inconventional EC-NDT systems,theinducedcurrentsareprimarilygeneratedalongasingledirectioninthetested sample. Thispaperpresentsanewexcitationmethodforgeneratingapseudo-rotatingacmagnetic field and, consequently,pseudo-rotatingeddycurrents.Thismethodsignificantly improvesthedetectionof deep cracksofanyorientation.ThedetectedsignalduetothecrackismeasuredbyanImprovedGiant Magneto-ResistanceMagnetometer(IGMRM).Themagnetic flux densitysignatureofthecrackisstudied using a3D finite elementmodel.Numericalandexperimentalresultsdemonstratethepotentialandthe efficiency ofthismethod.

Accurate depth measurement of small surface-breaking cracks using an ultrasonic array post-processing technique

Abstract: In this paper, the half-skip configuration of the Total Focusing Method (TFM) is used to image and size surface-breaking cracks. The TFM is an ultrasonic array post-processing technique which is used to synthetically focus at every image point in a target region. This paper considers the case of inspecting for cracks which have initiated from the far surface of a parallel-sided sample using an array on the near surface. Typically, only direct ray paths between the array and image points are included in the TFM algorithm and therefore the image obtained for this case consists only of root and tip indications; no specular reflection from the crack faces is captured. The tip indication often has such a poor signal-to-noise ratio that reliable crack depth measurement is challenging. With the Half-Skip TFM, instead of using directly-scattered signals, the image is formed using ultrasonic ray paths corresponding to the ultrasound that has reflected off the back surface and has then undergone specular reflection from the crack face back to the array. The technique is applied to experimental and simulated array data and is shown to measure the depth of small cracks (depth

An innovative method for measuring pavement dielectric constant using the extended CMP method with two air-coupled GPR systems

Abstract: Ground penetrating radar (GPR) is a common non-destructive tool for pavement layer thickness measurement. The accuracy of such measurement is directly affected by the accurate prediction of the dielectric constant of pavement material using GPR. In this study, an innovative extended common mid-point (XCMP) method using two air-coupled GPR systems was developed to improve the accuracy of the traditional method for pavement material dielectric constant estimation. The equations and algorithm for calculating the dielectric constant of pavement material were developed, and then the accuracy of this new method was validated successfully by the data collected from a custom-designed test site.

Time domain reflectometry, ground penetrating radar and electrical resistivity tomography: A comparative analysis of alternative approaches for leak detection in underground pipes

Abstract: In this work, three different techniques, namely time domain reflectometry (TDR), ground penetrating radar (GPR) and electrical resistivity tomography (ERT) were experimentally tested for water leak detection in underground pipes. Each technique was employed in three experimental conditions (one laboratory or two field experiments), thus covering a limited but significant set of possible practical scenarios. Results show that each of these techniques may represent a useful alternative/addition to the others. Starting from considerations on the obtained experimental results, a thorough analysis on the advantages and drawbacks of the possible adoption of these techniques for leak detection in underground pipes is provided.

Detection of surface crack defects on ferrite magnetic tile

Abstract: A new approach is proposed for automatically detecting crack defects with dark colors and low contrasts in magnetic tile images using the fast discrete curvelet transform (FDCT) and texture analysis. In this methodology the original images were first decomposed and reconstructed based on the FDCT. Then the thresholds of decomposition coefficients were calculated by texture feature measurements. With these thresholds the surface textures in the images can be eliminated. Finally by extracting contours from the reconstructed images, the expected images without textures but with crack defects contours were obtained. Experimental results show that the proposed method could eliminate the contours of the textures, and extract from the image cracks longer than 0.8 mm.

Air-coupled detection of nonlinear Rayleigh surface waves in concrete—Application to microcracking detection

Abstract: This paper proposes a new technique for the application of the second harmonic generation (SHG) in Rayleigh surface waves to nondestructively quantify microstructural changes and microcracks in heterogeneous cement-based materials (mortar and concrete). The effect of shrinkage reducing admixture (SRA), as it influences microcrack formation in these materials, is considered as an example. A 50 kHz wedge transmitter and a 100 kHz air-coupled receiver are implemented for the generation and detection of nonlinear Rayleigh waves; the non-contact receiver has the advantage of eliminating the inconsistencies associated with coupling and can be used for inspections of in-service concrete components. The experimental results show that the SRA plays a crucial role in reducing shrinkage microcracks and the addition of the SRA to cement-based materials influences the material nonlinearity. These results demonstrate that the proposed SHG technique is capable of evaluating the relative nonlinearity parameter, βre and can be a reliable method to characterize microstructural changes in cement-based materials.

Small crack detection in cementitious materials using nonlinear coda wave modulation

Abstract: This paper presents an ultrasonic method, based on the nonlinear acoustic mixing of coda waves with lower-frequency swept pump waves, for providing an efficient global detection of small cracks in cementitious materials. By simultaneously comparing, forboth uncracked and cracked mortars, the ultrasonic velocity variations and decorrelation coefficients between the unperturbed and perturbed signals with pump amplitude, this method makes it possible to accurately detect cracks with widths of around 20 μm in correlation with velocity variations of approximately 0.01%. The potential influence of certain material parameters such as microscopic damage is also discussed.

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

Abstract: 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.

A new sensing skin for qualitative damage detection in concrete elements: Rapid difference imaging with electrical resistance tomography

Abstract: In this paper we investigate whether a thin layer of electrically conductive materials that is painted to the surface of concrete elements can be used as sensing skin to detect and locate cracking and damage in the concrete substrate. Cracking of the concrete results in the rupture of the sensing skin, thus locally increasing its electrical resistivity. We monitor the local change in the electrical resistivity of the sensing skin using electrical resistance tomography. In this work, we utilize difference imaging scheme. Experiments on polymeric substrates as well as on concrete substrates are performed. The results indicate that the developed sensing skin can be successfully used to detect and locate cracking and damage on concrete and potentially other nonconductive substrates.

An approach to reduce lift-off noise in pulsed eddy current nondestructive technology

Abstract: The pulsed eddy current (PEC) technique, as an emerging technique of the eddy current technique, has been used in engineering, such as aircrafts, oil/gas pipelines, nuclear steam pipes and high-speed rails, due to its richer information in time domain and frequency domain. However, the lift-off noise, introduced by varying coating thicknesses, irregular sample surface or movement of transducers, has a serious influence on the accuracy of the detection for the defects in these key structures. It greatly limits the application of PEC in quantitative nondestructive testing. In order to reduce the effect of the lift-off, the lift-off effect is analyzed theoretically and experimentally; based on the investigation of the relationship between the peak value of the difference signal and the lift-off, an approach to reduce the lift-off noise for detection the defect depth or width is proposed. In this approach, the defect depth and width are determined by the slope of the linear curve of the peak value of the difference signal and the lift-off. The proposed approach is verified by experiment and the results indicate that it can highly reduce the lift-off noise in the PEC technique. Therefore, it can be applied in characterization of the surface defects in sample with non-ferrous material.

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

Abstract: 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.

Integrated health assessment strategy using NDT for reinforced concrete bridges

Abstract: This paper presents the results of the application of several Non-Destructive Techniques (NDT) on the Pentagon Road Bridge, in Chatham, Kent, England. This provides crucial structural information such as rebar position, moisture ingress and deflection. A FEM (Finite Element Model) of the bridge was created using data from the visual inspection and was parameterised and calibrated using information gather from the NDT results. The results of all of the above are compared in order to identify the portion of the bridge which had undergone the greatest amount of deterioration. This process forms the basis for the integrated bridge health mechanism proposed by this paper.

Guided wave tomography of pipes with high-order helical modes

Abstract: The transmission of guided ultrasonic waves across corrosion or erosion damage encodes information about the defect depth. Tomography maps the depth profile from multiple transmission experiments performed under different insonification angles by solving the so-called inverse problem; the accuracy of the depth estimation being dependent on the range of angles available for the inversion. Practical application of tomography to tubular structures, such as pipes and bends, requires the use of two ring arrays of ultrasonic transducers at the two ends of the pipe section to be inspected. However, such a configuration leads to an insufficient angular coverage when considering the signals that travel along the shortest temporal path between a pair of transducers. This paper introduces a general inversion method that extends the range of insonification angles by exploiting the information carried by the signals that wrap around the pipe multiple times before reaching the receive array, thus resulting in superior image resolution and increased depth estimation accuracy. In addition, to address typical thermal fluctuations encountered during continuous monitoring, a strategy that combines a temperature compensation scheme with the intrinsic thermal stability of electromagnetic acoustic transducers (EMATs) is developed and tested with full-scale experiments performed on a schedule of 40, 8″ diameter steel pipe instrumented with two ring arrays of EMAT transducers. It is shown that for an irregularly shaped defect the proposed inversion method yields maximum depth estimations that are as accurate as single point ultrasonic thickness gaging measurements and over a wide temperature range up to 175 °C. The results indicate that advanced inversion schemes in combination with EMAT transduction offer great potential for continuously monitoring the progression of corrosion or erosion damage in the oil and gas industry.

Characterization of stress corrosion cracking in carbon steel using nonlinear Rayleigh surface waves

Abstract: This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in carbon steel. Cold rolled carbon steel is widely used for buried fuel pipelines; the environment surrounding these pipelines creates a mildly corrosive environment, which, in combination with an applied stress, can cause SCC. To ensure the safe operation of these structures, it is crucial that damage due to SCC be detected before their structural integrity is reduced by large cracks. In the early stages of SCC, microstructural changes such as dislocation formation and microcrack initiation occur, which have shown to considerably increase the acoustic nonlinearity of a material. These microstructural changes distort and generate higher harmonics in an initially monochromatic ultrasonic wave. This research considers four different levels of SCC induced in four separate 1018 steel specimens, a material which has a similar susceptibility to SCC as steel used for buried fuel pipelines. Then nonlinear ultrasonic measurements are performed before and after the SCC damage is induced. Nonlinear Rayleigh surface waves are utilized to detect the SCC damage that is concentrated near the material surface. The amplitudes of the fundamental and second harmonic waves are measured with contact wedge transducers at varying propagation distances to obtain the acoustic nonlinearity of the specimens as a function of SCC damage. The results show an increase in the measured acoustic nonlinearity in the early stages of SCC, indicating the feasibility of using this nonlinear ultrasonic method to detect the initiation of SCC in carbon steels.

Combined embedded and surface-bonded piezoelectric transducers for monitoring of concrete structures

Abstract: Piezoelectric lead zirconate titanate (PZT) transducers are increasingly used for monitoring various engineering structures. PZT transducers are used for monitoring structures based on the electro-mechanical impedance (EMI) for a single PZT and the wave propagation technique for multiple PZTs. In concrete structures, the EMI sensing region is small due to high damping of the concrete. Using the wave propagation technique with high actuation voltage, a larger area can be monitored. The smart aggregates (embedded PZT transducers) can be employed using the wave transmission technique to monitor very large areas with a reasonably low actuation signal. The combination of smart aggregates (using the wave transmission technique) with surface bonded PZTs (using the wave propagation technique) can provide an effective method to study both the local and overall conditions of the structure. In this work, a concrete beam with the dimensions of 220×40×20 cm was cast and nine PZT transducers were embedded or attached on the beam. The PZT readings were correlated with the damage on the structure. Combination of smart aggregates (using the wave transmission technique) with surface bonded PZTs (using the wave propagation technique) for SHM was studied. The results show that this combination provides an effective way to assess both the local and overall conditions of the structure.

Role of interlaminar interface on bulk conductivity and electrical anisotropy of CFRP laminates measured by eddy current method

Abstract: The purpose of this paper is to deal with the characterization of bulk electrical conductivity in multilayer carbon fiber reinforced polymer (CFRP) laminate by using eddy current method. The correlation between probe signals and conductivities in different directions of unidirectional CFRP is obtained with the aid of numerical code, which is based on finite element method in terms of magnetic vector potential. The effect of interlaminar interface on the bulk conductivity of CFRP laminate is investigated by visualizing the lift-off curves of probe signal in complex plane. It is found out that the cross-ply laminate has much better electrical conductance than unidirectional plate, and the bulk conductivity of CFRP laminate depends mainly on the number of current paths produced by the overlapped fibers at the interfaces between two angled layers. In addition the polar diagrams of T–R probe signals at multiple angular positions are depicted to characterize the anisotropy of CFRP laminate.

Micro-crack detection using a collinear wave mixing technique

Abstract: A collinear wave mixing technique was developed to detect micro-cracks in samples by measuring the mixing of two ultrasonic sinusoidal waves. The bispectrum was used to process the nonlinear response. Experiments were conducted to investigate the influence of excitation parameters, such as driving frequency and time delay, on intermodulation among ultrasonic waves and defects. Mixing components were tracked for varying frequency or time delay, and the nonlinear response was measured at fixed frequencies and time delay. The driving frequency was found to strongly affect micro-crack detection; the optimal driving frequency corresponded to maximum amplitudes of sum and difference frequency sidebands. The time-delay dependence of the amplitude of mixing components allowed the location of defects throughout a sample.

Scour evaluation for foundation of a cable-stayed bridge based on ambient vibration measurements of superstructure

Abstract: This study develops a foundation scour evaluation method merely using the ambient vibration measurements taken from the superstructure of a cable-stayed bridge. Various modal frequencies of girder and those of the local pier are first identified for Kao-Ping-Hsi Bridge. The finite element model of this bridge is then constructed to perform the modal analysis with original design parameters. Combining the above results, this work further determines the best boundary support conditions for the model to fit the identified modal frequencies of bridge girder. According to this globally best fitted model, the optimal soil stiffness is first decided by fitting the critical bridge frequencies with a known deposit height at the pylon. Subsequently, the scour depth at a pier can be estimated by varying the depth of its supporting soil to fit the two sensitive frequencies of local pier modes. Finally, a direct measurement scheme is carried out to verify the estimated scour depth.

Near electrical resonance signal enhancement (NERSE) in eddy-current crack detection

Abstract: An investigation was performed into the effects of operating an absolute eddy-current testing (ECT) probe at frequencies close to its electrical resonance. A previously undocumented defect signal enhancement phenomenon, resulting from associated shifts in electrical resonant frequency, was observed and characterized. Experimental validation was performed on three notch defects on a typical aerospace superalloy, Titanium 6Al–4V. A conventional absolute ECT probe was operated by sweeping through a frequency range about the electrical resonance of the system (1 � 5 MHz). The phenomenon results in signal-to-noise ratio (SNR) peak enhancements by a factor of up to 3.7, at frequencies approaching resonance, compared to those measured at 1 MHz. The defect signal enhancement peaks are shown to be a result of resonant frequency shifts of the system due to the presence of defects within the material. A simple, operational approach for raising the sensitivity of conventional industrial eddycurrent testing is proposed, based on the principles of the observed near electrical resonance signal enhancement (NERSE) phenomenon. The simple procedural change of operating within the NERSE frequency band does not require complex probe design, data analysis or, necessarily, identical coils. Therefore, it is a valuable technique for improving sensitivity, which complements other ECT methods. & 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license

High temperature thickness measurements of stainless steel and low carbon steel using electromagnetic acoustic transducers

Abstract: Thickness measurements were made on steel pipe at high temperatures using a non-contact watercooled EMAT (electromagnetic acoustic transducer) and also a laser-EMAT system where a portable Nd:YAG laser with a fibre optic cable was used to generate ultrasound on a sample, and a water-cooled coil-wound EMAT used to detect ultrasound. The set-up was designed so that the laser was fired through a hole in the centre of the EMAT, and a low pass 5 MHz filter employed to reduce the plasma noise. Back wall reflections were clearly visible at temperatures up to 900 °C on stainless and ferromagnetic low carbon steel, enabling wall thickness to be measured, taking into account thermal expansion of the sample. The water-cooled EMAT system can measure wall thickness on ferromagnetic low carbon steel at temperatures up to the Curie point; here, ultrasound generation is dependent on the magnetic state of the steel.

Ultrasonic guided wave-based testing technique for inspection of multi-wire rope structures

Abstract: The aim of this paper is to investigate the propagation of ultrasonic guided waves (UGW) along multi-wire ropes with polymer cores and to determine whether it is possible to detect defects and to identify a defective strand inside the internal structure of a multi-wire rope. The modes of UGW that propagate along multi-wire ropes have been identified using modelling wherein dispersion curves are calculated using the semi-analytical finite element (SAFE) technique. The optimal excitation regions were estimated using 3D FE modelling. An ultrasonic testing technique to identify particular defective strands inside the internal structure of a multi-wire rope was developed and verified experimentally.

A feed-through ACFM probe with sensor array for pipe string cracks inspection

Abstract: Pipe string is highly susceptible to cracks caused by stress corrosion and fatigue damage in petroleum industry. The performance of existing magnetic flux leakage and magnetic particle inspection is inadequate to detect narrow axial cracks in pipe string. Results obtained from crack inspection simulations and experiments using a novel feed-through alternating current field measurement (ACFM) probe presented in this paper suggest that this probe can be adopted for an accurate, rapid and non-contacting detection of pipe string cracks. A structure of equal-spaced sensor array of this feed-through ACFM probe is applied to scan the full circumference of pipe string simultaneously in a single pass.

Improvement of sensitivity of eddy current sensors for nano-scale thickness measurement of Cu films

Abstract: Measurement of nano-scale copper film thickness is of great importance in the semiconductor industry. The eddy current method is used for the purpose due to its non-destructive and fast dynamic response features. In this paper, an equivalent circuit model is used to get the relationship between the measurement sensitivity and sensor parameters. It is found that the internal resistance of an eddy current sensor plays a primary role in the improvement of the measurement sensitivity beside of the Q factor of the sensor. A simple experimental setup is established and a series of Cu films with the thicknesses ranging from 20 nm to 350 nm are prepared as test samples. Test results indicate that the sensitivity of an optimized sensor made of a lower resistant multi-wire Cu line has better sensitivity than that wound with a higher resistant single Cu wire under large lift-off.