Showing papers on "Time-of-flight diffraction ultrasonics published in 2021"

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

Abstract: 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.

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

Abstract: 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).

Time of flight diffraction for rough planar defects

Abstract: Ultrasonic non-destructive evaluation techniques, such as time-of-flight diffraction (ToFD) for which the arrival times of waves diffracted from crack tips are analysed to locate and size defects, are well understood for smooth defects. In environments where extreme changes in temperature and pressure occur, the damage that may arise is often non-uniform and more difficult to characterise when designing and qualifying an inspection. This article investigates the implementation of ToFD methods for sizing rough defects using a purely theoretical approach. High-fidelity finite element modelling and stochastic Monte Carlo methods are used to provide physical and statistical insights for the dependence on both incident beam angle and degree of roughness for the case of planar defects. Optimal incident angles for ultrasonic ToFD techniques were determined in the 1980s but largely based on theoretical and experimental investigations for smooth defects. However, rough defects produce tip-diffracted signatures that are more complicated than for their smooth counterparts, largely due to multiple scattering effects related to mode conversion and propagation of surface waves along the rough surface. It is shown that roughness may cause larger diffraction amplitude values at different angles, which leads to increased uncertainty when sizing, with illustrative examples and physical interpretations provided. Comparisons of amplitudes for smooth and rough defects of the same size are also demonstrated. The ToFD method, using envelope peak detection and autocorrelation approaches, is implemented to estimate the size of rough cracks, and the effects of roughness on the accuracy of this sizing are investigated with statistical analysis.

TOFD Echo Signal Processing to Achieve Superresolution

Abstract: It is proposed to use the method of maximum entropy (ME) and the method for constructing the AR model of the TOFD echo spectrum together with the method of splitting the spectrum to increase its resolution and, consequently, information content. The effectiveness of the approach proposed is demonstrated in model experiments. As a result, the resolution of TOFD echo signals has increased by at least two times, and the noise level has decreased by 6 dB. The higher resolution of TOFD echoes allows for phase analysis of echoes and conclusions about reflector type.

Enhancement of Time Resolution in Ultrasonic Time-of-Flight Diffraction Technique With Frequency-Domain Sparsity-Decomposability Inversion (FDSDI) Method

Abstract: The lack of time resolution restricts the quantitative detection of shallow subsurface defects with ultrasonic time-of-flight diffraction (TOFD) technique due to the superposition between lateral wave and diffracted waves from upper and lower tips. In this article, the frequency-domain sparsity-decomposability inversion (FDSDI) method was proposed to enhance the time resolution in TOFD based on the sparsity and decomposability of the ultrasonic reflection sequence. An optimization problem was formulated in the frequency domain by combining ${l}_{{1}}$ - and ${l}_{{2}}$ -norm constraints. The simulation was performed with a carbon steel model containing a series of shallow subsurface cracks at the depths of 2.0, 2.5, 3.0, 3.5, and 4.0 mm. The relative measurement errors of defect depths and heights were no more than 6.57%, and the depth of the dead zone was reduced by 70%. Subsequently, the feasibility of the FDSDI method was experimentally verified on a carbon steel specimen with an artificial defect. The defect depth and height were calculated with relative errors within 6.0%. Finally, the detection capacity of the FDSDI method was discussed, and the effects of frequency bandwidth, regularization parameter, and noise on inversion results were analyzed by experiments. It is concluded that the FDSDI method decouples the multiple overlapped signals and significantly improves the time resolution to quantify the small defects in the dead zone.

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

Abstract: When ultrasonic time-of-flight diffraction (TOFD) B-scan is implemented along the circumferential direction of pipeline, the ray path of direct longitudinal wave (DLW) is not parallel to the curved pipeline surface, inducing the layered dead zone. In this paper, the spectrum analysis method based on Fourier transform is employed to establish the relationship between flaw depth and harmonic frequency interval. On this basis, the relative position between flaw tip and DLW is determined combining with the characteristics of the tip-diffracted waves in B-scan image, realizing the quantitative detection of the defects with different depths. Simulated and experimental results show that the range of dead zone in pipeline is reduced by 40% with spectrum analysis method, and the relative quantitative errors of flaw depths are within 11%. Finally, the formation mechanism of extreme values for the tip-diffracted waves in B-scan image is discussed by theoretical analysis.

Determining the Coordinates of Reflectors in a Plane Perpendicular to Welded Joint Using Echo Signals Measured by Transducers in the TOFD Scheme

Abstract: The TOFD method, widely used in ultrasonic flaw detection, makes it possible to distinguish a crack from a volume reflector by the phase of echo signals and to determine its height with high accuracy. However, the TOFD method without piezoelectric transducers scanning across the welded joint does not allow determining the offset of the reflector from the weld center, which is very important when evaluating the test results. The scanning devices used for this have a complex design, their price is higher than that of one-dimensional scanning devices, and, most importantly, the testing time increases considerably. If we use echo signals reflected from the bottom of the test object, taking into account the change in the wave type, then the combined reflector image can be obtained from the set of partial images reconstructed by the digital antenna focusing (DFA) method. Using the echo signals measured in the combined mode for each piezoelectric transducer, then it is possible to estimate the displacement of the reflector across the welded joint with an accuracy of $$ \pm 1.5$$ mm. Numerical and model experiments have confirmed the efficiency of the proposed approach.

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.