Side-drilled Hole Research Articles

Ultrasonic high frequency guided waves for testing of dissimilar metal welds

Abstract Ultrasonic testing of nickel-based dissimilar metal welds (DMWs) in nuclear power plants is always challenging due to its internal structures. Indeed, ultrasonic bulk waves can be severely scattered and attenuated because of coarse grains in DMWs and consequent difficulties such as splitting and skewing can be observed due to the anisotropic and inhomogeneous structure of the weld pool. Ultrasonic high frequency guided waves (HFGW) has the potential to reduce and overcome these issues and can offer a good compromise between the volume coverage and defect sensitivity for testing of DMWs. This work presents defect detection in DMW joints made by 12 mm thickness P91 and alloy 800 plates using HFGW. The generation and propagation characteristics of HFGW on both P91 and alloy 800 have been extensively studied by using 2D plane strain finite element modelling (FEM). For both experimental measurements and FEM, the input parameters such as the excitation frequency, incident angle and scan distance have been optimized with the help of Dispersion curves. It has been observed that the amplitude response of HFGW is more than two times on the P91 plate when compared to the alloy 800 plate. The interaction of HFGW with artificial defects made on the DMW and at the weld interfaces have been tested. It has been noticed that the HFGW can able to detect defect size down to ~ 10% of 12 mm wall thickness and 2 mm diameter side-drilled holes reliably from both sides of P91 and alloy 800.

Damage detection of thick plates using trailing pulses at large frequency-thickness products

Trailing pulses, which are Lamb waves generated at large frequency-thickness (fd) products, act like a list of longitudinal pulses with constant time interval. These longitudinal pulses with short wave length are a promising tool for detection of damage in thick plates. However, the waveform complexity of trailing pulses brings challenges. In this paper, a damage detection method for thick plates using trailing pulses at large frequency-thickness products is introduced. Firstly, the characteristics of trailing pulses, including slightly change of velocity and pulse splitting, are discussed from the aspects of ray tracing and approximate analytical solution. Then, a blind pulse compression method for trailing pulses is interpreted, which doesn’t need the prior knowledge of material properties. A compression item is designed for trailing pulses. Finally, an experiment is implemented on a thick aluminum plate with three Side Drilled Holes (SDHs), in which all the SDHs are recognized with accurate position estimation.

A reverse time migration-based multistep angular spectrum approach for ultrasonic imaging of specimens with irregular surfaces

We develop a new ultrasonic imaging framework for non-destructive testing of an immersed specimen featuring an irregular top surface and demonstrate its capability of accurately depicting the lower surfaces of multiple damages hidden in the specimen. Central to the framework is a multistep angular spectrum approach (ASA), via which the forward propagation wavefields of wave sources and backward propagation wavefields of the received wave signals are calculated. Upon applying a zero-lag cross-correlation imaging condition of reverse time migration (RTM) to the obtained forward and backward wavefields, the image of the specimen with an irregular surface can be reconstructed, in which hidden damages, if any and regardless of quantity, are visualized. The effectiveness and accuracy of the framework are examined using numerical simulation, followed with experiment, in both of which multiple side-drilled holes, at different locations in aluminum blocks with various irregular surfaces, are characterized. Results have proven that multiple damages in a specimen with an irregular surface can be individually localized, and the lower surface of each damage can further be imaged accurately, thanks to the RTM-based algorithm in which multiple wave reflections from the specimen bottom are taken into wavefield extrapolation. The proposed imaging approach presents higher computational efficiency, compared to conventional RTM, and enhanced imaging contrast over prevailing total focusing methods.

Experimental validation of a phased array probe model in ultrasonic inspection

New manufacturing technologies such as additive manufacturing facilitate flexible and complex designs and production of components. However, these new techniques should not compromise the safety aspect, which imposes higher demands on the integrity insurance and inspection methods. Phased array ultrasonic testing (PAUT) provides advanced inspection and evaluation processes, whereas qualification is still needed when applied together with new manufacturing techniques. Numerical modeling, as one of the potential qualification methods, has been developed for decades and should be validated before practical applications. This paper presents an experimental validation work of the phased array probe model implemented in a software, simSUNDT, by comparing the maximum echo amplitudes between the physical experiments and simulations. Two test specimens with side-drilled holes (SDHs) and different materials are considered for validation and practical purposes. An experimental platform with a mechanized gantry system, which enables stabilized inspection procedure, is built and applied during the validation work. Good correlations can be seen from the comparisons and this model is concluded as an acceptable alternative to the corresponding experimental work. The relation between depth and beam angle is also noticed and investigated, which is essential to guarantee an accurate inspection.

Performance Evaluation of Austenitic Stainless Steel Weld by Ultrasonic Phased Array Inspection Based on Probability of Detection

To evaluation capability of ultrasonic phased array (PHA) in industrial austenitic stainless steel (ASS) weld quantitatively, the 304 ASS butted weld specimen with various depth and side-drilled holes (SDH) was drilled to analyze reliability of PHA testing. Based on phased array probes, the ultrasonic waveform data are processed by in traditional dynamic depth focusing and total focusing method (TFM). The welding zone defects imaging results are compared. The probability of detection (POD) curves of ultrasonic response data were used for estimation the detection capability and reliability of ultrasonic PHA.

Joint Estimation of Direction-Dependent Velocity and Damage Location of CFRP

AbstractUltrasonic phased array (UPA) provides a powerful tool for nondestructive testing (NDT) of carbon fiber-reinforced plastic (CFRP). By the aid of full matrix capture (FMC) technique, the optimum resolution of anisotropic CFRP inspection could be achieved by the total focusing method (TFM). The directional dependence of ultrasonic velocity is one of the biggest challenges due to the anisotropy of CFRP. The objective of this research is to establish a joint method to estimate direction-dependent velocity and damage location of CFRP. To obtain group velocity without prior knowledge of neither theoretical calculation nor experimental determination, a limited angle range of the anisotropic velocity is first obtained by backwall reflection method (BRM), which is then extended by analyzing the relation between the time delay of backwall and side drilled hole (SDH) reflection. The effectiveness of the proposed method is experimentally demonstrated with UPA imaging of SDH in composite laminates.

Structured channel metamaterials for deep sub-wavelength resolution in guided ultrasonics

Experimental results on deep subwavelength resolution of defects are presented for the first time in the context of guided ultrasonic wave inspection of defects, using novel “structured channel” metamaterials. An Aluminum bar with side-drilled holes is used as a test sample, interrogated by the fundamental bar-guided symmetric mode. Simulations were conducted to optimize dimensional parameters of the metamaterial structure. Experiments using metamaterials fabricated accordingly demonstrate a resolution down to 1/72 of the operating wavelength, potentially bringing the resolution of guided wave inspection to the same range as that of bulk ultrasonics. This work has much promise for remote inspection in industry and biomedicine.

A Fast Interface Reconstruction Method for Frequency-Domain Synthetic Aperture Focusing Technique Imaging of Two-Layered Systems with Non-planar Interface Based on Virtual Points Measuring

Frequency-domain synthetic aperture focusing technique (SAFT), such as non-stationary phase shift migration (NSPSM), plays an essential role in the ultrasonic imaging of two-layered systems with non-planar interfaces. However, with NSPSM fast imaging under blind test conditions is hard to realize due to a lack of interface information in practice. To overcome this difficulty, a fast interface reconstruction method for NSPSM imaging of non-planar two-layered systems is proposed based on virtual points measuring. In blind testing conditions, a series of virtual points (VP) were created at the non-planar interface of the two-layered system by the time-of-flight (TOF) of the first incoming echo. Then, the interface for the two-layered system was quickly established by the interpolation of the VP. Finally, the NSPSM imaging for the two-layered system with non-planar interfaces was realized by the borderline which is used to distinguish the velocities of the coupling medium and tested part in the same extrapolating depth. The imaging results showed that the position of side-drilled holes was located correctly and the efficiency of the algorithm was significantly improved. Hence, the VP-NSPSM method is expected to achieve fast or even real-time imaging of two-layered systems with non-planar interfaces in practical NDT applications.

Calibration of ultrasonic hardware for enhanced total focusing method imaging

Experimental variation from ultrasonic hardware is one source of uncertainty in measured ultrasonic data. This uncertainty leads to a reduction in the accuracy of images generated from these data. In this paper, a quick, easy-to-use and robust methodology is proposed to reduce this uncertainty in images generated using the total focusing method (TFM). Using a 128-element linear phased array, multiple full matrix capture (FMC) datasets of a planar reflection are used to characterise the experimental variation associated with each element index in the aperture. Following this, a methodology to decouple the time-domain error associated with transmission and reception at each element index is presented. These time-domain errors are then introduced into a simulated array model used to generate the two-way pressure profile from the array. The side-lobe-to-main-lobe energy ratio (SMER) and beam offset are used to quantify the impact of these measured time-domain errors on the pressure profile. This analysis shows that the SMER is raised by more than 6 dB and the beam is offset by more than 1 mm from its programmed focal position. This calibration methodology is then demonstrated using a steel non-destructive testing (NDT) sample with three side-drilled holes (SDHs). The time delay errors from transmission and reception are introduced into the time-of-flight (TOF) calculation for each ray path in the TFM. This results in an enhancement in the accuracy of defect localisation in the TFM image.

An Inverse Approach for Ultrasonic Imaging From Full Matrix Capture Data: Application to Resolution Enhancement in NDT.

In the context of nondestructive testing (NDT), this article proposes an inverse problem approach for the reconstruction of high-resolution ultrasonic images from full matrix capture (FMC) data sets. We build a linear model that links the FMC data, i.e., the signals collected from all transmitter-receiver pairs of an ultrasonic array, to the discretized reflectivity map of the inspected object. In particular, this model includes the ultrasonic waveform corresponding to the response of transducers. Despite a large amount of data, the inversion problem is ill-posed. Therefore, a regularization strategy is proposed, where the reconstructed image is defined as the minimizer of a penalized least-squares cost function. A mixed penalization function is considered, which simultaneously enhances the sparsity of the image (in NDT, the reflectivity map is mostly zero except at the flaw locations) and its spatial smoothness (flaws may have some spatial extension). The proposed method is shown to outperform two well-known imaging methods: the total focusing method (TFM) and Excitelet. Numerical simulations with two close reflectors show that the proposed method improves the resolution limit defined by the Rayleigh criterion by a factor of four. Such high-resolution imaging capability is confirmed by experimental results obtained with side-drilled holes in an aluminum sample.

Ultrasonic multi-frequency time-reversal-based imaging of extended targets

In this paper, two multi-frequency time-reversal-based imaging methods, time reversal with multiple signal classification (TR-MUSIC) and its phase-coherent form (PC-MUSIC), are explored for application to the non-destructive testing (NDT) imaging of extended target machined in solids. The size of which is on the order of the ultrasonic wavelength or larger. The principle for multi-frequency TR-based imaging method is presented, and the performance is estimated in terms of point spread function (PSF). Both TR-MUSIC and PC-MUSIC are tested with experimental ultrasonic array data acquired using the full matrix capture (FMC) process. Two experiments in metal solids with different kind of defect, side-drilled hole (SDH) as well as electrical discharge machining (EDM) lines that can be considered as extended targets, were implemented. Here, the EDM lines have different positional relationships with array. For SDH, it is shown that both TR-MUSIC and PC-MUSIC can locate the position, but fail to distinguish the shape. For EDM lines, TR-MUSIC can only detect the defect, but PC-MUSIC can assess the length based on the imaging results under some dimension of signal subspace. In addition, the effect of dimension of signal subspace on the imaging methods is also investigated, and it is shown that TR-MUSIC is sensitive to the dimension when compared with PC-MUSIC.

Phased Array Detection Parameters for Electron Beam Welds in CFETR Vacuum Vessel Port Stubs

The China Fusion Engineering Testing Reactor (CFETR) vacuum vessel (VV) port stub was formed by electron beam welding (EBW). In this article, the phased array ultrasonic testing (PAUT) method was introduced for testing the quality of the weld. First, the acoustic field simulation and analysis of the EBW of 316L austenitic stainless steel were carried out with different frequency probes, then the microstructure of electron beam welds were observed by microscopic metallographic to analyze the influence of the type and influence of probe, finally fabricating the reference test blocks according to the NB/T47013-2015 standard to conducting the PAUT experiment. The test results show that the type of probe has less influence on the signal-to-noise ratio (SNR) and localization of the detection of large-thickness austenitic stainless steel electron beam welds under low-frequency conditions. The SNR of EBW detection was gradually decreased as the frequency of the probe increases under the same type of probe. In the direction of depth of the EBW, with the increase in the depth, the weld microstructure size becomes finer, so the sector scan image of $\phi ~1$ -mm side drilled hole (SDH) near the upper weld was severely distorted, while the sector scan image of $\phi ~1$ -mm SDH near the lower EBW was relatively good.

Ultrasonic detection and characterization of delamination and rich resin in thick composites with waviness

A multi-frequency ultrasonic method was proposed to detect and characterize delamination and rich resin in thick composites with waviness. Addressing the challenges for ultrasonic inspection caused by waviness, multi-layer structure, and multiple defect types, ultrasound propagation in thick wavy composite was investigated through a dedicated numerical model built in OnScale®. Key features of ultrasonic testing and composite sample, including water immersion environment, fiber waviness, uneven inter-ply resin distribution, and side-drilled hole (SDH)-simulated delamination, were embraced in the model. Via this model, waviness and thick resin layers were found to cause disturbance and wave vector deviation to inter-ply reflection signal and thus introduce difficulties for delamination detection based just on the signal to noise ratio. In addition, reflections from inter-ply resin and SDH were revealed with different reliance on probe frequencies. Using a 5 MHz probe with time corrected gain, ultrasonic testing in wavy composite was performed experimentally. Based on the reliance of defect characterization on frequency, SDHs and rich resin were differentiated experimentally in the B-scan images with various filtering frequencies, demonstrating the effectiveness of the proposed method for detection and characterization of delamination and rich resin in thick wavy composite structures.

Using Pseudoorthogonal Signals to Reduce Noise Level in Ultrasound Testing of Highly Absorptive Materials

To increase the signal-to-noise ratio when testing materials with a high level of absorption, it has been proposed to use complex signals, formed on the basis of code sets employed in Code Division Multiple Access (CDMA) technology, as probing signals. The code sequences in the code sets have a low level of cross-correlation function and a delta-like autocorrelation function. Echo signals are recorded by the double-scanning method, in which the elements of an antenna array sequentially emit unique probing signals phase-shift keyed according to the Kasami code, with the echo signals being recorded in each radiation cycle concurrently by all the elements in the antenna array. Since the shape of the “side lobes” of the compressed echo signals is different for each shot, reconstructing the image of reflectors using digital focusing of the antenna (DFA) reduces the level of the “side lobes”. As a result, when different code sequences of length 15 are used for each antenna-array element, this level may prove to be lower than when one sequence of length 63 is used for all the elements. Provided that a unique code set is used in each position, reconstructing the DFA image when scanning with an antenna array should further reduce the noise level and the level of the “side lobes”. The effectiveness of the proposed approach was demonstrated in a model experiment on reconstructing the image of side-drilled holes in a Plexiglas™ SO-1 calibration block, when using Kasami codes of lengths 15 and 63.

Laser-Induced Full-Matrix Ultrasonic Imaging of Complex-Shaped Objects.

We present laser-induced full-matrix imaging of complex-shaped objects that combines the advantages of noncontact laser ultrasound inspection and high-contrast array imaging based on total focusing method (TFM). Full-matrix data were acquired by synthesizing a laser ultrasonic array through an alternate scanning of the generation and detection laser beams. Taking advantage of the simultaneous generation of all wave modes, surface Rayleigh waves were extracted for surface profiling, and longitudinal waves were utilized for imaging. To gain a better understanding, we analyzed the full-wave dynamics in generation, propagation, and scattering by using numerical simulations. TFM was adapted to accommodate complex surface and suppress interference from other waves, including surface waves and straightforward traveling bulk waves between emission and reception. Numerical and experimental studies on an aluminum sample with a sinusoidal surface and side-drilled holes were conducted, and results agreed well.

Modeling Flaw Pulse-Echo Signals in Cylindrical Components Using an Ultrasonic Line-Focused Transducer with Consideration of Wave Mode Conversion.

Investigations on flaw responses can benefit the nondestructive testing of cylinders using line-focused transducers. In this work, the system function, the wave beam model, and a flaw scattering model are combined to develop an ultrasonic measurement model for line-focused transducers to predict flaw responses in cylindrical components. The system function is characterized using reference signals by developing an acoustic transfer function for line-focused transducers, which works at different distances for both planar and curved surfaces. The wave beams in cylindrical components are modeled using a multi-Gaussian beam model, where the effects of wave mode conversion and curvatures of cylinders are considered. Simulation results of wave beams are provided to analyze their propagation behaviors. The proposed ultrasonic measurement model is certified from good agreement between the experimental and predicted signals of side-drilled holes. This work provides guidance for evaluating the detection ability of line-focused transducers in cylindrical component testing applications.

Enhanced ultrasonic detection of near-surface flaws using transverse-wave backscatter

Diffuse ultrasonic backscatter measurements have been shown to enhance the detection capability of sub-wavelength flaws when combined with extreme value statistics. However, for a normal-incidence immersion measurement, a “dead zone” created by the ring-down of the front-wall echo will hide near-surface flaws. In this article, a pulse-echo transverse wave backscatter measurement is used to detect near-surface flaws under high gain. The approach is validated using a magnesium specimen with side-drilled holes. The confidence bounds of the grain noise from this specimen are given by a transverse-to-transverse scattering model, which takes the grain size distribution and the hexagonal crystal symmetry into account. The upper bound is then treated as a time-dependent threshold for the C-scan. Experiments show that the developed method has good performance for detecting sub-wavelength, near-surface flaws, and can suppress both missed detections and false positives.

A model for multi-view ultrasonic array inspection of small two-dimensional defects.

The multi-view total focusing method (TFM) is an imaging algorithm for ultrasonic full matrix array data that exploits internal reflections and mode conversions in the inspected object to create multiple images, the views. Modelling the defect response in multi-view TFM is an essential first step in developing new detection and characterisation methods which exploit the information present in these views. This paper describes a ray-based forward model for small two-dimensional defects and compares its results against finite-element simulations and experimental data for the inspection of a side-drilled hole, a notch and a crack. A simpler version of this model, based on a single-frequency approximation, is derived and compared. A good agreement with the multi-frequency model and a speed-up of several orders of magnitude are achieved.

Dijkstra’s algorithm-based ray tracing method for total focusing method imaging of CFRP laminates

Accurate determination of ultrasonic travel time from a start point to an end point with known positions is fundamental to the application of total focusing method (TFM) to ultrasonic array imaging. However, this is particularly challenging for CFRP laminates whose wave propagation is complex due to material anisotropy and refractive ply interfaces. In this paper, a ray tracing method using Dijkstra’s path-finding algorithm is presented and used to calculate time delays of TFM. This modified TFM, termed ray tracing TFM, was demonstrated by post-processing full matrix capture (FMC) data from a 5.79-mm-thick multidirectional CFRP laminate containing three 1.5 mm diameter side-drilled holes (SDHs) at 3 mm depth. It shows that the ray tracing TFM enables the FMC signals to be accurately delayed and coherently summed to synthesize a focus at every image point. Consequently, peak amplitudes of the SDHs were increased by up to 49.74 dB and 2.69 dB with respect to those from paintbrush acquisition and the isotropic TFM with a constant velocity, respectively. Ray tracing results have also confirmed that use of a coupling layer is a simple way to minimize the anisotropic effect of CFRP and to reduce the structural noise from reflections at ply interfaces.

Enhancing ultrasonic time-of-flight diffraction measurement through an adaptive deconvolution method

Deconvolution is generally applied to improve the temporal resolution of ultrasonic signals. However, using this process in the time-of-flight diffraction (TOFD) measurement of small and shallow defects is challenging because TOFD signals are dispersive in space–frequency distribution. Particularly, determining the reference signal for deconvolution remains a critical barrier. To this end, an adaptive deconvolution method is proposed in this study. Using wavelet transform, we firstly decompose the TOFD signals into sub-band signals to standardise the space–frequency distribution. Then, sub-band signals with strong coherences are adaptively selected on the basis of coherence coefficient metric. Upon the opted sub-band signals, a lateral wave can be readily used as the reference signal, and TOFD signals can be reconstructed with established Wiener filtering and spectral extrapolation methods. The feasibility of the proposed method is validated with the TOFD measurement of a small side-drilled hole near the surface. Results show that the proposed method effectively separates overlapping TOFD signals and improves the axial resolution of a TOFD image.