Synthetic Aperture Focusing Technique Imaging Research Articles

Ultrasonic Pulse-Echo Signals for Quantitative Assessment of Reinforced Concrete Anomalies

This paper presents a study to accurately evaluate defects in concrete decks using ultrasonic pulse-echo signals. A reinforced concrete deck with void defects was designed and evaluated for validation, and a commercial ultrasonic pulse-echo (UPE) device was used to obtain the 2D images of the void defect inside the deck. The UPE image is based on the ultrasonic shear-wave test method and an extended synthetic aperture focusing technique (SAFT). To enhance the accuracy of the defect location in the SAFT imaging, the recorded A-scan data from UPE was analyzed using an advanced denoising approach and defect echo peak extraction, which are based on empirical modal decomposition, Hurst exponent characterization, and Hilbert envelope estimation. The results demonstrated that the location and depth of the void defect in the deck can be accurately assessed by using the developed approach. The new method provides quantitative information of the anomalies inside the deck, which can be used to calibrate the qualitative images of UPC devices with the SAFT.

Defect detection and imaging using electromagnetic acoustic transducer with butterfly coil.

Electromagnetic ultrasonic detection technology utilizes the electromagnetic coupling method to generate and receive ultrasonic waves without a couplant, which is suitable for rapid detection. However, the detection can be affected by the spatial distribution of the acoustic field and the polarization direction of the shear wave, which can result in suboptimal detection performance. The acoustic field directivity of the shear wave generated by the butterfly coil electromagnetic acoustic transducer was measured using the transmission method. The data indicate that the acoustic pressure amplitude of the shear wave is maximized along the axis of the acoustic field, thereby meeting the requirements of synthetic aperture focusing technique imaging. We used the reflection method to detect the through-hole defects and investigated the effect of shear wave polarization direction. By comparing the experimental data and imaging results, it can be concluded that higher echo amplitudes are obtained when the polarization direction of the shear wave is perpendicular to the axis of the through-hole defects. Based on the explosive reflection model, the frequency domain phase shift migration (PSM) method converts the time-domain signal to the frequency domain for processing and uses a phase-shift factor for layer-by-layer imaging. We used the PSM method to process the experimental data, which not only produced high-resolution images but also had a high computational speed.

Image quality improvement in multi-mode laser ultrasound imaging using reconstructed synthetic aperture focusing technique

The synthetic aperture focusing technique (SAFT) is used as a laser ultrasonic imaging algorithm for non-destructive internal defect testing, and multi-mode time domain SAFT (MMT-SAFT) imaging is developed to enhance the imaging capability of defect detection. However MMT-SAFT algorithm has the problem of artifacts caused by the laser ultrasound (LU) modes waves. Thus to solve this problem, an automatic imaging technique based on MMT-SAFT called Reconstructed synthetic aperture focusing technique (Re-SAFT) is proposed. This technique consists of two steps: a preliminary MMT-SAFT is firstly performed to locate the defect position tentatively and futher separate LU modes wave signals using the defect-reflected signal; then a secondary reconstructed MMT-SAFT based on the priori defect location information is achieved for artifacts removal. In addition, directivity patterns of LU modes wave have been used in Re-SAFT to achieve more accurate defect imaging in the second step. Results indicate that by using Re-SAFT, the typical artifact caused by laser ultrasonic waves can be removed and the imaging quality can be significantly improved from 19.67 to 33.85 dB. Further, the consumed time of Re-SAFT is economized by optimizing the imaging pixel size without compromising the image quality. Therefore, Re-SAFT has potential applications in industrial laser ultrasonic testing.

A modified synthetic aperture focusing algorithm used for transmission detection based on laser ultrasonics method

Compared with traditional ultrasonic nondestructive testing technology, the laser ultrasonics method (LU) has the advantages of high precision, non-contact, wide-band response, suitable for non-destructive testing (NDT) of materials having complex shapes or working in extreme environments such as the additive manufacturing (AM) process, etc. However, for a purely optical LU system, it is still limited by the poor reflection of rough/absorption surface of the material. In order to increase the signal-to-noise ratio (SNR) of the signal detected on the rough surface of a sample, and to overcome the limitation of the resolution restricted by half wavelength, a modified Synthetic Aperture Focusing Technique (SAFT) that can be used for internal defect detection of the material is proposed. The modified SAFT is based on LU centering transmission detection and it improves the resolution of the detection system by using the characteristics of ultrasonic propagation and its mode conversion. It is firstly studied in theory with a numerical model based on finite element method (FEM), and then it is applied in experiments to reconstruct the defect according to the signals detected by LU method. In our experiment, the artificial defects buried in an AM sample with diameter from 0.2 mm to 0.75 mm are successfully characterized by the SAFT imaging of ultrasonic signals. The results prove that the position of the defect in 3D and its size can be successfully obtained by the proposed SAFT.

Gaussian Mixture Model-Based Classification of Corrosion Severity in Concrete Structures Using Ultrasonic Imaging

Corrosion in reinforced concrete (RC) structures is associated with a reduction of the rebar diameter, loss of interfacial bond, cracking, and eventual spalling and probable collapse of the structure. The negative effects of corrosion on structural safety, durability, and longevity imposes significant costs on the national economy. Therefore, planned non-destructive testing (NDT) of concrete structures is essential to enhance the safety and economic sustainability of infrastructure. Previous work by the research group has established the capability of the ultrasonic Synthetic Aperture Focusing Technique (SAFT) as a tool for detection of rebar corrosion. This work extends the previous research towards application of statistical learning for ascertaining the corrosion severity through analysis of SAFT images of the rebar. Using features extracted from images, a Gaussian mixture model (GMM) is implemented to classify the severity of corrosion damage to the rebar. The results from the research positively demonstrate the potential of the proposed technique as an enabler for decisions pertaining to maintenance and timely repair of concrete infrastructural assets.

Ultrasonic measurements of grouted tendon ducts in the concrete structure using Elop Insight Scanner

Infrastructures such as bridges, tunnels, dams, retaining walls, docks, parking garages and buildings are constructed from reinforced concrete. These structures deteriorate due to cyclic temperature and loading variations, aging, as well as physical causes such as accidents and aggressive chemical attack from the environment. Post tension tendon ducts commonly used in bridges are filled with grout and steel tendons. The voids in the grout inside the tendon ducts can occur due various reasons such as improper filling, material mixing, blockage due to tendons etc. The water can penetrate in these voids inside the tendon ducts which can initiate corrosion. It is therefore vital to detect voids/ unfilled or defective portions of the tendon ducts at an early stage of the construction phase to ensure the strength and safety of the structure. Different types of NDT (Non-Destructive Testing) methods are frequently used for monitoring such infrastructures during regular operation to prevent failure, increase life span and improve safety. One of these NDT techniques, ultrasonic testing, is relatively simple in operation and gives more accurate description of air pockets, voids, cracks, and delaminations in the concrete structures compared to other NDT methods. ELOP has developed a rolling ultrasonic scanner COBRI (COncrete and BRidge Inspection) together with a complete software solution for asset management of structures for faster and better inspection and monitoring of the internal condition of the concrete. It will provide asset owners with information about when, where and how to inspect, maintain and more accurately assess damages and overall condition of their structures. The COBRI scanner contains an array of transducers assembled in two separate rollers used in pitch-catch mode. It uses a patented elastomer solution to provide dry coupling to the concrete surface during scanner movement, without the need for any added fluid couplant. The system uses SAFT (Synthetic Aperture Focusing Technique) algorithm for image reconstruction to increase image quality and signal to noise ratio. It will provide users with improved tomographic (B, C and D-scan) image quality and 3D imaging. In the current work, we have used the COBRI scanner to explore its ability to image grouting defects inside tendon ducts used in concrete structures. The concrete specimen used in the evaluation has two tendon ducts positioned at 8 cm from the top surface with dimensions of 70 cm in length, 55 cm in width and 25 cm in thickness with average aggregate size of 22 mm. One of the tendon ducts is completely empty. The second one is half-length fully filled with grouting along the prestressed steel and other half only three quarters filled, leaving an air cavity on the top. The scanner is rolled approximately 50 cm over the concrete surface and the tomographic (B, C and D-scan) SAFT images are generated at different locations of the concrete specimen together with a 3D volume image. The results showed that the COBRI scanner is able to differentiate the filled and partially filled tendon ducts in the concrete structure. As COBRI is a rolling ultrasound scanner, it will help the bridge owners to scan and assess the grouting defects in the tendon ducts much more efficiently than other scanners available in the market.

Ultrasonic inspection of curved structures with a hemispherical-omnidirectional ultrasonic probe via linear scan SAFT imaging

A reliable non-destructive testing technique that enables effective detection and quantification of defects within safety-critical curved structures is highly demanded both in manufacture and service. Conventional Ultrasonic Testing (UT) methods usually adopt planar ultrasonic probes and it is preferable to adjust the probe axis direction perpendicular to the curved surface, because both the transmitting and receiving sensitivities in the probe axis direction are the highest. This paper proposes a new UT method using a hemispherical-omnidirectional ultrasonic probe. This probe has an omnidirectional directivity pattern and can cover curved surfaces with normal incident ultrasounds at different scan positions without the necessity of adjusting the probe attitude and position to follow the curved surface profiles. A surface reconstruction algorithm is firstly proposed to estimate curved surface profiles, which is based on a fast-imaging algorithm for ultra-wideband radar called the Shape Estimation Algorithm based on the Boundary scattering transform and Extraction of Directly scattered waves (SEABED). This method only uses surface reflection echoes, and thus can inspect curved structures without knowing the surface profiles a prior. Further, the Synthetic Aperture Focusing Technique (SAFT) is implemented to generate a focused image of internal defects by performing delay-and-sum beamforming on defect echoes based on the reconstructed surface profile. The proposed UT method is eventually experimentally verified on a curved plexiglass specimen with 29.4 mm surface curvature radius containing three Ø 3.5 mm Side Drilled Holes (SDHs) at different depths. Results show that compared with the planar ultrasonic probe, the reconstructed surface profile via SEABED is much wider and accurate with a relative error ≤ 0.47%. All SDHs are not only clearly positioned with relative errors ≤ 7.43%, but also can be accurately quantified from the curvature radius of SDH indications in the SAFT images with relative errors ≤ 8.60%. This work proves that the UT method based on the hemispherical-omnidirectional ultrasonic probe can emerge as a promising technique to inspect curved structures with high efficiency and accuracy, which has great potential in practical applications.

Nondestructive evaluation of rebar corrosion in concrete structures using ultrasonics and laser-based sensing

ABSTRACT Corrosion is a serious affliction for reinforced concrete (RC) structures and the development of advanced inspection techniques is crucial for maintaining serviceability and durability. In this study, a novel proof-of-concept is presented to monitor corrosion in the concrete by employing contact-based ultrasonic excitation and non-contact laser-based sensing. Accelerated corrosion of rebars in RC slabs is implemented, and ultrasonic scanning is performed on specimens at various corrosion levels. The excitation source is a contact ultrasonic transducer, which enables better penetration of the waves into the concrete medium. For the reception, a Doppler laser vibrometry based sensing is used, which expedites the inspection by circumventing the requirement of coupling. The Synthetic Aperture Focusing Technique (SAFT) is employed for subsurface image reconstruction. Rebar images obtained by hybrid sensing technique, are qualitatively similar to those obtained via traditional contact-based source and receiver system. A histogram-based qualitative analysis is presented to correlate the characteristics of the rebar images to the progress of corrosion. A planar SAFT imaging algorithm is presented to demonstrate the rebar on a horizontal plane for better visualisation. The proposed technique is significantly faster in comparison to the contact-based approach and, therefore, has the potential of being applied for inspection of larger concrete infrastructure.

An optimized laser ultrasonic synthetic aperture imaging method using differential technique

Laser ultrasonic synthetic aperture focusing technique (SAFT) is susceptible to multiple modes of ultrasonic generated by laser pulse. In this work, we propose an optimized synthetic aperture imaging method based on the differential technique, resulting in the effective detection of internal defects. The laser ultrasonic SAFT finite element model is established, and the models are calculated in the case of existing internal defect or not. Thereby, the original ultrasonic data containing the defect reflected waves and the differential data are obtained, and the internal defects are reconstructed by the delay superposition algorithm. The results show that the defect imaging is submerged in the high-amplitude background noise superimposed by the surface acoustic wave; when only the original data are used. However, the optimized SAFT imaging based on the differential technique can not only reduce the interference of other mode waves on the imaging area significantly, but also retain the defect reflected waves effectively. Moreover, the imaging and precise locating of multiple defects are realized, which pave the way for enhancing the internal defects detection ability of laser ultrasonic.

Noncontact testing and imaging of internal defects with a new Laser-ultrasonic SAFT method

In this paper, completely noncontact and nondestructive testing and imaging of internal defects in thick metal with a modified combination of laser ultrasonic testing (LUT) and the synthetic aperture focusing technique (SAFT) is studied. Strong ultrasonic plane wave generated by a high-power and unfocused laser source irradiate on a sample surface with a constraint and transparent layer is proposed for the new laser-ultrasonic SAFT. In order to enhance the imaging accuracy, the algorithm based on the signal correlation is applied for the laser-ultrasonic SAFT imaging. The detection capability and imaging performance of defects is verified by an experiment validation.

In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

Laser Ultrasonic Technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding and it’s effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is prototype product (1064nm, 20mJ/pulse, 1.5ns, 20Hz), was used to generate ultrasonic, and a laser interferometer (532nm, 1W) was used to detect the surface micro vibration caused by arrival of ultrasonic wave and was scanned using a mechanical stage. After acquiring the B-scope, which is composed of a generation-point and multi detection-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during Gas Metal Arc (GMA) welding process in the mid-pass of multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

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.

In-situ measurement of weld quality during MAG welding using laser ultrasonic

ABSTRACT Laser ultrasonic technique (LUT) is applicable for measurement under high temperature condition because of the non-contact and remoteness. In this study, LUT was applied to detection the welding defect such as solidification crack and/or lack of penetration at single bevel groove during Gas Metal Arc (GMA) welding process. Nanosecond pulse laser (1064 nm, 60 mJ/pulse 100 Hz) was used to generate ultrasonic, and was scanned using a galvanometer mirror. Nd:YAG laser interferometer (532 nm, 1 W) was used for detection the surface micro vibration caused by arrival of ultrasonic wave. After acquiring the B-scope, which is composed of multi transmit- and receive-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in-process measurement was evaluated to confirm the presence or absence of welding defect indication. As a result, it was possible to indicate the solidification crack and the lack of penetration in the joint even during the welding process. Furthermore, it was able to indicate the penetration depth change (2 mm difference) due to the welding condition change during process. These results suggest that in-process measurement using LUT is very effective technique for detecting weld defects.

A SAFT Method for the Detection of Void Defect inside a Ballastless Track Structure Using Ultrasonic Array Sensors.

High-precision ultrasound imaging of void defects is critical for the performance and safety assessment of ballastless track structures. The sound propagation velocity of each layer in the ballastless track structure is quite different. However, the traditional concrete Synthetic Aperture Focusing Technique (SAFT) ultrasound imaging method is based on the assumption that the concrete has a single constant shear wave velocity. Thus, it is not a suitable method for the ultrasonic imaging of multilayer structures. In this paper, a Multilayer SAFT high-precision ultrasound imaging method is proposed. It is based on the ray-tracing technique and uses the Fermat principle to find the refraction point that minimizes the delay of the acoustic wave propagation path at the interface of the discrete layers. Then, the acoustic wave propagation path is segmented by the position of the refraction point, and the propagation delay of the ultrasonic wave is obtained segment by segment. Thus, the propagation delay of the ultrasonic wave is obtained one by one, so that the propagation delay of the ultrasonic wave in the multilayer structure can be accurately obtained. Finally, the focused image is obtained according to the SAFT imaging algorithm. The finite element simulation and experimental results show that the Multilayer SAFT imaging method can accurately track the propagation path of the ultrasonic wave in ballastless track structures, as well as accurately calculate the propagation delay of the ultrasonic wave and the lengths of void defects. The high accuracy of the Multilayer SAFT imaging represents a significant improvement compared to traditional SAFT imaging.

Three-dimensional SAFT imaging for anisotropic materials using photoacoustic microscopy

A pulsed laser illuminates a target zone that causes rapid thermoelastic expansion, generating broadband high-frequency ultrasonic wave (photoacoustic wave, PA wave). We developed a PA microscopy (PAM) with a confocal area of laser and ultrasonic wave for applications in nondestructive testing (NDT). The synthetic aperture focusing technique (SAFT) is applied in the PAM for the three-dimensional (3D) imaging of interior flaws. Here, we report proof-of-concept experiments for the NDT of a subsurface flaw in a thin laminar material. Graphical abstract (a) shows a specimen of carbon-fiber-reinforced plastic (CFRP) with an artificial delamination. Here, it should be noted that the group velocity varies directionally due to the strong anisotropy of the CFRP specimen (see Graphical abstract (b)). By considering the group velocity distribution in the SAFT, the shape and location of the subsurface delamination were accurately estimated as shown in Graphical abstract (c). Coating the surface of the CFRP specimen with a light-absorbent material improved the amplitude of the PA wave. This finding showed that the signal-to-noise ratio of the waves scattered from the flaws can be improved.

An Efficient Ultrasonic SAFT Imaging for Pulse-Echo Immersion Testing

An ultrasonic synthetic aperture focusing technique (SAFT) using a root mean square (RMS) velocity model is proposed for pulse-echo immersion testing to improve the computational efficiency. Considering the immersion ultrasonic testing of a steel block as an example, three kinds of imaging were studied (B-Scan, SAFT imaging based on ray tracing technology and RMS velocity). The experimental results show that two kinds of SAFT imaging have almost the same imaging performance, while the efficiency of RMS velocity SAFT imaging is almost 25 times greater than the SAFT based on Snell's law.

Elastic-wave-based synthetic aperture focusing technique imaging system for detecting voids inside concrete structures

Elastic-wave-based synthetic aperture focusing technique imaging system for detecting voids inside concrete structures

Investigation of synthetic aperture methods in ultrasound surface imaging using elementary surface types

Synthetic aperture imaging methods have been employed widely in recent research in non-destructive testing (NDT), but uptake has been more limited in medical ultrasound imaging. Typically offering superior focussing power over more traditional phased array methods, these techniques have been employed in NDT applications to locate and characterise small defects within large samples, but have rarely been used to image surfaces. A desire to ultimately employ ultrasonic surface imaging for bone surface geometry measurement prior to surgical intervention motivates this research, and results are presented for initial laboratory trials of a surface reconstruction technique based on global thresholding of ultrasonic 3D point cloud data. In this study, representative geometry artefacts were imaged in the laboratory using two synthetic aperture techniques; the Total Focusing Method (TFM) and the Synthetic Aperture Focusing Technique (SAFT) employing full and narrow synthetic apertures, respectively.Three high precision metallic samples of known geometries (cuboid, sphere and cylinder) which featured a range of elementary surface primitives were imaged using a 5MHz, 128 element 1D phased array employing both SAFT and TFM approaches. The array was manipulated around the samples using a precision robotic positioning system, allowing for repeatable ultrasound derived 3D surface point clouds to be created. A global thresholding technique was then developed that allowed the extraction of the surface profiles, and these were compared with the known geometry samples to provide a quantitative measure of error of 3D surface reconstruction. The mean errors achieved with optimised SAFT imaging for the cuboidal, spherical and cylindrical samples were 1.3mm, 2.9mm and 2.0mm respectively, while those for TFM imaging were 3.7mm, 3.0mm and 3.1mm, respectively. These results were contrary to expectations given the higher information content associated with the TFM images. However, it was established that the reduced error associated with the SAFT technique was associated with significant reductions in side lobe levels of approximately 24dB in comparison to TFM imaging, although this came at the expense of reduced resolution and coverage.

Improvement of SAFT by Implementation of Approximate Wave Solution in Fluid-Solid Two-Phase Media: A Case Study on Imaging of Aluminum Rod with Side-Drilled Holes

In ultrasonic testing, Synthetic Aperture Focusing Technique (SAFT) is a signal processing technique to produce flaw images by superposing of A-scan waveforms. With rapid development in computing technology, recently, SAFT has become easier in data acquisition and faster in computation. However, additional signal processing techniques may be integrated into SAFT to get ultrasound image with higher resolution. Wave theory is one of the most important keys in SAFT improvement since it can lead to better understanding of ultrasonic wave phenomena. In this paper, the approximate wave solution (AWS) in fluid-solid two-phase media is used to compute the beam radiations inside target of inspection. Multiplying A-scan waveforms by the ultrasonic beam radiations obtained by AWS, wave diffraction characteristics which are spatial functions between transducer and target are incorporated into the SAFT. The performance of the improved SAFT, so-called AWS–SAFT, is experimentally tested in ultrasonic imaging of a side-drilled hole (SDH) in an immersed rod. It is shown that AWS–SAFT can improve a flaw image, especially in that a flaw image with narrower side lobes is obtained, and artifacts due to L-mode are eliminated in T-mode SAFT images. Eventually, an efficient way in using AWS–SAFT to obtain an accurate ultrasound image of an immersed rod with SDHs is proposed and tested.

Using the synthetic aperture focusing technique with velocity responses to detect the defects inside reinforced concrete structures

In civil engineering, imaging can be a useful non-destructive testing (NDT) tool for revealing defects in a concrete structure. To date, there is no effective method for imaging the defects inside in situ concrete structures. In this study, a new elastic wave-based NDT method has been developed and then applied to reveal the defects inside concrete specimens using an imaging method. This method integrates the point-source/point-receiver method with the synthetic aperture focusing technique (SAFT) to achieve a scanning effect when using a phase array system. This method operates with a large functioning depth because the transient elastic wave method possesses a high-energy feature, whereas the traditional ultrasonic method does not. As an alternative to using displacement responses, this method uses velocity responses to construct the SAFT image. Based on the numerical simulation and the experimental results, using the proposed method allowed a high image quality to be produced and successfully revealed defects underneath a layer of rebars. The results of this study indicate the great potential for using the elastic wave-based imaging method to inspect defects in situ concrete structures.