Time Of Flight Diffraction Research Articles

Reduction of Layered Dead Zone in Time-of-Flight Diffraction (TOFD) for Pipeline with Spectrum Analysis Method

Reduction of Layered Dead Zone in Time-of-Flight Diffraction (TOFD) for Pipeline with Spectrum Analysis Method

Applicability Analysis of Inspection and Monitoring Technologies in Wind Turbine Towers

Wind turbines are one of the key systems in wind energy development. The wind tower supporting the whole wind turbine is a towering structure, which has been affected by installation, transportation, environment, and other factors. Furthermore, it is prone to experience other quality problems that would be difficult to detect for wind turbine towers. Therefore, the key to maintain the wind tower structure and to ensure the normal operation of a wind turbine is to carry out comprehensive and detailed detection and monitoring studies during its service stage. This paper sorted out several common quality problems including structural damage, deformation, flange bolts loosening, and corrosion of wind tower and relevant research on the detection and monitoring of these quality problems. In addition, some nondestructive testing technologies are introduced, including the ultrasonic phased array, time of flight diffraction, magnetic memory, acoustic emission, fiber Bragg grating and piezoelectric impedance, and applications in wind turbine towers.

Towards an Alternative to Time of Flight Diffraction Using Instantaneous Phase Coherence Imaging for Characterization of Crack-Like Defects.

Time of flight diffraction (TOFD) is considered a reliable non-destructive testing method for the inspection of welds using a pair of single-element probes. On the other hand, ultrasonic phased array imaging has been continuously developed over the last couple of decades, and now features powerful algorithms, such as the total focusing method (TFM) and its multi-view approach to rendering detailed images of inspected parts. This article focuses on a different implementation of the TFM algorithm, relying on the coherent summation of the instantaneous signal phase. This approach presents a wide range of benefits, such as removing the need for calibration, and is highly sensitive to defect tips. This study compares the sizing and localization capabilities of the proposed method with the well-known TOFD. Both instantaneous phase algorithm and TOFD do not take advantage of the signal amplitude. Experimental tests were performed on a ¾″-thick steel sample with crack-like defects at different angles. Phase-based imaging techniques showed similar characterization capabilities as the standard TOFD method. However, the proposed method adds the benefit of generating an easy-to-interpret image that can help in localizing the defect. These results pave the way for a new characterization approach, especially in the field of automated ultrasonic testing (AUT).

Quantitative detection of shallow subsurface cracks in pipeline with time-of-flight diffraction technique

When the ultrasonic time-of-flight diffraction (TOFD) technique is applied to the inspection of pipeline, the propagation direction of the direct longitudinal wave is not parallel to the pipe surface, leading to the increment of the depth of dead zone and the measurement errors of shallow subsurface cracks. In this paper, the differences between the dead zones in flat plate and pipeline are compared theoretically. Subsequently, the method based on the mode-converted waves in TOFD B-scan image, which has been used for reducing the dead zone in flat plate, is developed to restrict the dead zone in pipeline and to realize the accurate measurement of shallow subsurface cracks. The simulated and experimental results indicated that the depth of dead zone in the carbon steel pipeline with a radius of 148 mm was reduced from 6.3 mm to 3.0 mm and that the measurement errors of crack length and angle were no more than 0.27 mm and 1.57°, respectively. The depth of the dead zone in pipeline and the measurement errors before modification both increase with the reduction of the ratio of pipe radius to probe center spacing (PCS). The modified method has universality for the detection of the shallow subsurface cracks in pipeline with ultrasonic TOFD technique.

A wavelet analysis-based matching pursuit algorithm for an accurate ultrasonic TOFD measurement

The time-of-flight diffraction (TOFD) technique is used as an important non-destructive testing method in weld integrity evaluation and failure analysis. However, an accurate measurement of the time-of-flight (TOF) has proven to be difficult due to the low time resolution of the measured signal. Conventional deconvolution techniques have been used to improve the time resolution of the signal but are not effective for ultrasonic TOFD signals because the frequency contents of the signals are non-static in space-frequency distribution. To overcome this problem, a method is proposed in this paper that estimates the TOF in two steps. In the first step, the measured signal is decomposed into a series of narrowband signals using a wavelet transform and an atom dictionary is adaptively established according to the characteristics of a selected narrowband signal. In the second step, matching pursuit (MP) is used to derive a sparse representation of the selected narrowband signal. A steel specimen with artificial defects is prepared, experiments are carried out and the results confirm the efficacy of the proposed algorithm.

Estimates of Probability of Detection and Sizing of Flaws in Ultrasonic Time of Flight Diffraction Inspections for Complex Geometry Components With Grooved Surfaces

Abstract In a reliability assessment of ultrasonic time-of-flight diffraction (TOFD) inspection, probability of detection (POD) and sizing (POS) curves are developed. Experiments are performed on a complex geometry specimen with the grooved inspection surface simulating the gland seal area of a steam turbine rotor. In the reliability experiment, it is assumed and confirmed that the distribution of signal responses is normal. The effects of probe center spacing on detection and sizing are observed. The PODs developed here have a decreasing trend with flaw size which is in contrary to the generally observed increasing trend in conventional ultrasonic amplitude-based flaw sizing techniques. The reason for this decreasing POD with crack height is explained in the present study. The curves developed in this work are specific to the geometry and dimensions of the specimen with the set of notches and the probes used in the experiment. Hence, these curves can only be used under similar conditions. In TOFD inspection of similar type of complex shaped structures, e.g., turbine, the POD and POS curves developed here can be used in taking an appropriate engineering decision with respect to run, repair, or replace.

Studying Applicability of TOFD Technique to Inspection of Welded Joints in Polyethylene Pipes

Issues of quality control of welded joints in hot-tool butt-welded polyethylene pipelines are considered. It is indicated that the main scope of control currently falls on inspections based on the parameters of welding and on selective mechanical testing of welded joints. Physical nondestructive post-welding testing methods are used little, which is generally not typical for the practice of testing welded joints. The reason for this is the general problem of ultrasonic methods, viz., the low detectability of contracted defects, partially transmitting ultrasound. To ensure the detectability of such defects, we evaluate the possibility of increasing the sensitivity of ultrasonic testing by using the ultrasonic time-of-flight-diffraction (TOFD) technique. It is proposed to use the tip of vertical drillings to simulate defects weakly scattering ultrasound. It is shown that when tuning is performed based on such targets, the inspection sensitivity can be increased by at least 6 dB, i.e., two times or more. In this case, the drilling tip can simulate the upper and lower edges of a planar defect vertically oriented in the section of the weld. The results of experimental studies aimed at revealing multidirectional drills with a diameter of 1 mm in welded seams of polyethylene pipes, including pipes with an additional removable outer layer, are presented. The expediency of gathering statistical data on the identification of real defects in welded butt joints of polyethylene pipes using the considered testing method is shown.

Defect detection using the phased-array laser ultrasonic crack diffraction enhancement method

The ultrasonic time-of-flight diffraction (TOFD) method has drawbacks such as blind areas on the upper and lower surfaces during detection, a low signal-to-noise ratio (SNR) for ultrasonic diffraction signals, and difficulty achieving accurate quantitative analysis. A phased-array laser ultrasonic method using a fiber picosecond laser and an ultra-high-speed rotating mirror has been developed for crack detection to address these problems. Moreover, the four factors that affect the performance of the phased-array laser ultrasonic method were analyzed using the finite element method, and the change rule of the diffraction signal is analyzed using two calculated indexes: SNR and node displacement. Furthermore, an experimental verification was conducted, and the numerical simulation is consistent with the experimental results. Based on the experimental and simulation results, the weights of the four factors were calculated using the entropy evaluation method.

Finite Element Analysis Model on Ultrasonic Phased Array Technique for Material Defect Time of Flight Diffraction Detection

In this study, the finite element method (FEM) for phased array technology in ultrasonic time of flight diffraction (TOFD) for the defect detection of two-dimensional (2-D) geometric materials was researched. The phased array technology generated the FEM model for the TOFD signal. We have established the finite element model by the FEM software ANSYS based on the ultrasonic mechanism about the defects and the phased array transducer. A plane strain elements have simulated the reflected signal of the defect. We can compare the error ratio between simulation and experiment by using the theoretical calculation value as the benchmark, and find the feasibility of the FEM detection.

Research on Scanning Path of Automatic Phased Array Ultrasonic Testing of Pipe Welds with Complex Geometry

The automatic phased array ultrasonic testing method has been developed for inspecting the pipe welds with complex geometry. Applying time of flight diffraction (TOFD) technique has two problems: one is the difficulty in recognition of head wave under complex structure; the other is the limited scanning space cannot provide the needed span for pitch-catch mode. To overcome these two problems, an array probe can be placed on the surface of base metal of the weld by the direct and double traverse techniques. In order to design the optimal scanning path for high probability of detection and inspection rate, the model of beam path on tilt steps was established to quantitatively analyze the effects of tilt steps on beam coverage and mode conversion. Experiments were conducted to validate the proposed rule of planning the optimal scanning path. All artificial defects in the mock-up block were detected on the scanning side of which exists the optimal scanning path. While for the scanning side without the optimal scanning path, a combination of scanning is needed to avoid missing inspection.

Quantitative Detection of Shallow Subsurface Defects by Using Mode-Converted Waves in Time-of-Flight Diffraction Technique

An easy-to-implement method based on mode-converted waves was proposed for quantitatively detecting shallow subsurface defects using the ultrasonic time-of-flight diffraction (TOFD) technique. First, an isotropic model with a shallow subsurface defect was established to analyze the ray path of the mode-converted wave in TOFD B-scan image. The mode-converted wave with the shortest travel time and highest amplitude was identified to calculate the flaw depth via Snell's law. Subsequently, the quantitative expression of flaw depth was corrected by introducing the travel time of mode-converted wave, improving the measurement accuracy of the flaw height. Simulated results showed that the depth of the dead zone in carbon steel was reduced from 5.5 to 2.4 mm. The measurement error of shallow subsurface cracks was no more than 0.02 mm when the crack height was greater than or equal to 0.7 mm. Finally, three shallow subsurface defects were experimentally detected with the mode-converted waves in B-scan image. The quantitative errors of the measured heights were within 8.5% of the actual values, verifying the validity of the proposed method. This method is appropriate for the quantitative detection of shallow subsurface defects in thick-walled components.

Automatic Recognition of Weld Defects in TOFD D-Scan Images Based on Faster R-CNN

Abstract The type recognition of a weld defect in an ultrasonic Time of Flight Diffraction (TOFD) image is lack of efficiency, stability, and reliability that is due to the limitation of experience and professional knowledge of the inspector. In this study, characteristics of a weld defect of TOFD D-scan images were analyzed, and the Faster Region-based Convolutional Neural Networks (Faster R-CNN) was adopted for autorecognition of the defect’s type. In the course of the training process, the proposed box configuration was optimized to improve training and recognition efficiency, and the imaging samples were expanded for training, adjusting, and verifying the Faster R-CNN. Eventually, the recognition effect and influence factors of misrecognition were analyzed. Research results show that a D-scan image of the weld defect is closely related to the defect’s shape, which can be used to decide the type a weld defect’s type. Automatic recognition of the defect’s type based on the Faster R-CNN possesses the advantages of high recognition rate, robustness, and antijamming ability, which can achieve a recognition accuracy of 80–97 % for the weld defect’s type. In addition, the analysis of misrecognition shows that it is necessary to denoise the D-scan images before autorecognition because, in the light of the noise, stripes are liable to be misrecognized as porosity and cracks in the recognition experiments.

TOFD technique application to examine welded joints with non-equal wall thickness of the vertical steel tanks

During construction, reconstruction and technical inspection of vertical steel tanks (VST), all wall butt weld joints are subjected to ultrasonic testing to detect internal defects or flaws. The considerable length of examined welds (it can exceed 3,000 m) necessitates the development and implementation of mechanized and automated inspection systems that can increase the rate of testing and documentation of results. Systems of mechanized and automated ultrasonic echo-testing do not support the detection of all defect types and reliable determination of their geometric parameters. The Time-of-Flight Diffraction (TOFD) technique makes it possible to estimate the height of the defect regardless of its type and orientation. However, the application of TOFD technique for welds with non-equal wall thickness is complicated by the fact that the standard software of flaw detectors (‘depth calculator’) does not consider this non-uniformity. The authors have developed the testing procedure to overcome this limitation and to determine the depth of defects in the welds with non-equal wall thickness by TOFD technique using the standard ‘depth calculator’. An experimental verification of this technology confirmed the correctness of the calculations. The inspection scheme and calculation of the depth of discontinuity boundaries using the TOFD technique were tested using the example of the welded joint of the walls with non-equal thickness of the 10,000-m3 tank under construction. The thickness of the walls was 12 and 14 mm; 8 and 10 mm. The inspection of the real facility has confirmed good detection of defects and high reliability of the results.

Discrete Wavelet Transform based Denoising of TOFD Signals of Austenitic Stainless Steel Weld at Elevated Temperature

Abstract—Time of flight Diffraction technique is an advanced ultrasonic NDE methods, adopted in weld integrity testing. During the Time of flight Diffraction (TOFD) inspection of the stainless steel welds at high temperature, grain scattering noise is developed. In Time of flight Diffraction testing, the quality of the signal plays a dominant role in characterizing the defects. Hence, signal denoising is an essential prerequisite for the successful application of Time of flight Diffraction testing. In this work, one austenitic stainless steel weldment was artificially induced with slag defect. At high temperatures, the Time of flight Diffraction testing has been conducted on the weld piece and the resultant signals are applied over the proposed algorithms. Various combinations of wavelets, decomposition levels with different thresholding levels are applied to select an optimum denoising method. The evaluation of wavelet based denoising is achieved by calculating the Signal to Noise Ratio (SNR). Results show that the noises can be suppressed well and Signal to Noise Ratio is improved.

Design and Validation of an Articulated Sensor Carrier to Improve the Automatic Pipeline Inspection.

Pipeline inspection gauges (PIGs) carry out automatic pipeline inspection with nondestructive testing (NDT) technologies like ultrasound, magnetic flux leakage, and eddy current. The ultrasonic straight beam allows technicians to determine the wall thickness of the pipeline through the time of flight diffraction (TOFD), providing the pipeline reconstruction and allowing the detection of several defects like dents or corrosion. If the pipeline is of a long distance, then the inspection process is automatic, and the fluid pressure pushes the PIG through the pipeline system. In this case, the PIG velocity and its axial alignment with the pipeline cannot be controlled. The PIG geometry, the pipeline deformations, and the girth welds cause a continuous chattering when the PIG is running, removing the transducers perpendicularity with the inspection points, which means that some echoes cannot be received. To reduce this problem, we propose a novel method to design a sensor carrier that takes into account the angularity and distance effects to acquire the straight beam echoes. The main advantage of our sensor carrier is that it can be used in concave and convex pipeline sections through geometric adjustments, which ensure that it is in contact with the inner pipe wall. Our improvement of the method is the characterization of the misalignment between the internal wall of the pipeline and the transducer. Later, we analyzed the conditions of the automatic pipeline inspection, the existing recommendations in state-of-the-art technology, and the different mechanical scenarios that may occur. For the mechanical design, we developed all the equations and rules. At the signal processing level, we set a fixed gain in the filtering step to obtain the echoes in a defined distance range without saturating the acquisition channels. For the validation, we compared through the mean squared error (MSE) our sensor carrier in a straight pipe section and a pipe elbow of steel versus other sensor carrier configurations. Finally, we present the design parameters for the development of the sensor carrier for different pipeline diameters.

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.

Non-contacted Permanent Magnetic Absorbed Wall-climbing Robot for Ultrasonic Weld Inspection of Spherical Tank

A novel wall-climbing robot for the onsite weld inspecting of spherical tank with Time of Flight Diffraction (TOFD) method has been developed, in order to liberate operators from dangerous and heavy working environment. Patented non-contacted permanent magnetic absorbed technology was adopted to realize reliable and flexible all-position moving along the weld seam on the surface of spherical tank. To ensure stable coupling for TOFD test, a surface adaptive probe holder which can provide constant contact force, have been particularly designed. Equipped with a visual sensing based weld seam tracking unit and industrial PC station, the robot could perform automatic flaw detection remotely even in the darkness environment. Onsite ultrasonic weld inspections have been carried out on a 4000m3spherical tank with 28mm in thickness. It is verified that the robot could accomplish tasks in any position and the acquired TOFD images satisfy the requirements of engineering evaluation.

Detection of near surface rock fractures using ultrasonic diffraction techniques

Ultrasonic Time-of-Flight Diffraction (TOFD) techniques are useful methods for non-destructive evaluation of fracture characteristics. This study focuses on the reliability and accuracy of ultrasonic diffraction methods to estimate the depth of rock fractures. The study material includes three different rock types; andesite, basalt and ignimbrite. Four different ultrasonic techniques were performed on these intact rocks. Artificial near-surface fracture depths were created in the laboratory by sawing. The reliability and accuracy of each technique was assessed by comparison of the repeated measurements at different path lengths along the rock surface. The standard error associated with the predictive equations is very small and their reliability and accuracy seem to be high enough to be utilized in estimating the depth of rock fractures. The performances of these techniques were re-evaluated after filling the artificial fractures with another material to simulate natural infills.

Accurate ultrasonic measurement technique for crack sizing using envelope detection and differential evolution

In this paper, an ultrasonic non-destructive measurement technique is proposed to detect and size cracks in engineering materials. This measurement technique is based on Time of Flight Diffraction (TOFD) permitting the detection and location of cracks employing the compression waves diffraction phenomena. Then, the cracks are sized by the estimation of Time of Flight (TOF) of the diffracted waves. In the simulation and experimental investigations, the TOFs are estimated in two steps, the first step is the envelope extraction by Hilbert Transform (HT). The second step is the envelope estimation using a Matching Pursuit (MP) approach optimized by Differential Evolution (DE) in order to reach fast and accurate results. Consequently, the estimation results confirm the efficiency of the proposed measurement technique, which can be considered as an encouraging method for crack sizing in industrial materials.

The Maximum Entropy Method in Ultrasonic Non-Destructive Testing-Increasing the Resolution, Image Noise Reduction and Echo Acquisition Rate.

The use of linear methods, for example, the Combined Synthetic Aperture Focusing Technique (C–SAFT), does not allow one to obtain images with high resolution and low noise, especially structural noise in all cases. Non-linear methods should improve the quality of the reconstructed image. Several examples of the application of the maximum entropy (ME) method for ultrasonic echo processing in order to reconstruct the image of reflectors with Rayleigh super-resolution and a high signal-to-noise ratio are considered in the article. The use of the complex phase-shifted Barker code signal as a probe pulse and the compression of measured echoes by the ME method made it possible to increase the signal-to-noise ratio by more than 20 dB for the image of a flat-bottom hole with a diameter of 1 mm in a model experiment. A modification of the ME method for restoring the reflector image by the time-of-flight diffraction (TOFD) method is considered, taking into account the change of the echo signal shape, depending on the depth of the reflector. Using the ME method, 2.5D-images of models of dangling cracks in a pipeline with a diameter of 800 mm were obtained, which make it possible to determine their dimensions. In the object with structural noise, using the ME method, it was possible to increase the signal-to-noise ratio of the reflector image by more than 12 dB. To accelerate the acquisition of echoes in the dual scan mode, it is proposed to use code division multiple access (CDMA) technology based on simultaneous emission by all elements of the array of pseudo-orthogonal signals. The model experiment showed the effectiveness of applying the ME method.