Showing papers on "Time-of-flight diffraction ultrasonics published in 2002"

Time of flight diffraction imaging for double-probe technique

Abstract: Due to rapid progress in microelectronics and computer technologies, the system evolving from analog to digital, and a programmable and flexible synthetic aperture focusing technique (SAFT) for the single-probe pulse-echo imaging technique of ultrasonic nondestructive testing (NDT) becomes feasible. The double-probe reflection technique usually is used to detect the nonhorizontal flaws in the ultrasonic NDT. Because there is an offset between the transmitter and receiver, the position and size of the flaw cannot be directly read from the image. Therefore, a digital signal processing (DSP) imaging method is proposed to process the ultrasonic image obtained by double-probe reflection technique. In the imaging, the signal is redistributed on an ellipsoid with the transmitter and receiver positions as focuses, and the traveltime sum for the echo from the ellipsoid to the focuses as the traveltime of signal. After redistributing all the signals, the useful signals can be constructively added in some point in which the reflected point is; otherwise, the signals will be destructively added. Therefore, the image resolution of the flaw can be improved and the position and size of the flaw can be estimated directly from the processed image. Based on the experimental results, the steep flaw (45/spl deg/) cannot be detected by the pulse echo technique but can be detected by the double-probe method, and the double-probe B-scan image of 30/spl deg/ tilted crack is clearer than the pulse echo B-scan image.

Crack defect detection and localization using genetic-based inverse voting Hough transform

Abstract: We propose a genetic-based inverse voting Hough transform (GBIVHT) method to detect buried crack defects in engineering structures. The method is applied to B-scan images obtained according to the ultrasonic time of flight diffraction technique. In these image representations of the ultrasound data, crack defects are characterized by multiple arcs of diffraction that can be approximated by a parabolic model. Thus, the crack defect detection problem in non-destructive inspection of engineering structures is transformed into a parabola detection and localization on B-scan images. In the proposed GBIVHT method, the local peak detection problem of conventional HT is converted into a parameter optimization problem that operates directly on the B-scan images. The optimization task is done using the well-known genetic algorithms. Our main goals are an accurate detection of the parabolas while circumventing the computational complexity and huge storage problem tied to conventional HT.

Tofd inspection of v-groove butt welds on the hull of a container ship with a magnetically adhering wall climbing robot

Abstract: This paper presents the results of Time Of Flight Diffraction (TOFD) ultrasonic inspection of long welds on the hull of a container cargo ship. The TOFD probes are deployed by a low cost miniature climbing robot that adheres to the steel hull by using rare earth permanent magnets. 100% detection of defects sized larger than 0.6mm is obtained in V groove butt welds by a single pass scan. A scan and B scan results obtained by the inspection robot detect weld defects such as porosity, lack of root penetration and internal cracks.

Qualification of final closure for disposal container II - applicability of TOFD and phased array technique for overpack welding

Abstract: With a focus on carbon steel, which is one of the candidate materials for the disposal container used in the geological disposal of high-level radioactive waste in Japan, the defect detection capabilities were examined regarding engineering defects of the TOFD technique, an ultrasonic testing method, and the phased array TOFD technique as non-destructive test techniques for the inspection of the weld of a carbon steel overpack. Regarding the TOFD technique, a measurement was conducted concerning the influence of the crossing angle of the ultrasonic beams on the capability of detect flaws, for examining the detection characteristics of the technique in relation to the lid structure of an overpack, and it was pointed out that it is appropriate to consider the lower tip of slit as the reference flaw. Based on the measurements and calculations regarding sound pressure distribution, projections about the scope covered by one test session were made and the optimum testing conditions were examined. Regarding the phased array TOFP technique, the detectability and quantification characteristics were investigated, and comparisons with those of the TOFD technique and the phased array UT technique were made. From the viewpoint of securing long-term corrosion resistance for an overpack, the ways of thinking for ensuring the quality and long-term integrity of the final sealing area of a disposal container were examined. This study stresses that identifying and defining the defects that are harmful to corrosion allowance is important as well as achieving improvements in the welding and testing techniques, and that the question to solve in particular from now on is how to establish effective means to detect defects on the weld surface and the near surface and how to approach the level of tolerance concerning the defects on and near the surface. (orig.)

Comparison between surface impulse ground penetrating radar signals and ultrasonic time-of-flight diffraction signals

Abstract: Surface impulse ground-penetrating radar (GPR) and ultrasonic time-of-flight diffraction (TOFD) are recent innovations in the respective geophysical remote sensing and non-destructive testing industries. Both techniques have proved highly versatile and valuable applications. This paper provides a brief description of the time-of-flight diffraction (TOFD) signals and the surface impulse ground penetrating radar (GPR) signals. The similarities between the two techniques are highlighted and it is shown how processing techniques developed for GPR data processing may be adapted for use with TOFD data.

Relations between crack opening behavior and crack tip diffraction of longitudinal wave

Abstract: TOFD method has attracted attention as the most accurate crack depth measurement technique in industrial inspection field. Since this method utilizes the crack tip ultrasonic diffraction echo and the amplitude of this echo is weak, enhancement of S/N ratio of received signal is required for accurate and reliable measurement. The most harmful defect for industrial structures is a crack and a crack closure sometimes causes failure in nondestructive crack detection by TOFD method. However, quantitative behavior of crack tip diffraction echo depending on crack closure for longitudinal wave used in TOFD method have not been investigated yet. In this paper, we prepared 7075-T6 aluminum alloy specimens with a penetrating surface fatigue crack by three point bending test. During the fatigue test, maximum applied load Kmax was reduced gradually according to the crack extension to control the maximum stress intensity factor to be constant. Using the specimen with a closed fatigue crack, crack tip opening displacement (CTOD) was controlled by loading within Kmax. The amplitude of the crack tip diffraction echo of 5 MHz ultrasonic longitudinal wave depending on CTOD was measured. Using the obtained relation as a calibration curve, the minimum CTOD required for stable TOFD measurement of fatigue crack was estimated to be 0.1 micrometers .© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Development of Simplified Ultrasonic CT System for Imaging of Weld Metal and It's Comparison with TOFD Method

Abstract: In this paper, as a new measurement method to estimate the structure change of weld metal, the simplified ultrasonic CT system, which uses the information of three directions, that is, 90°, +45° and -45° about inspection plane is developed. Use of simplified ultrasonic CT system has two merits: Firstly, the measurement time is very short comparing with general CT. Secondly, it can detect sensitively very infinitesimal defect in vertical or slant direction about inspection plane because the obtained image is not C scan image but CT image calculated from three directions. From these merits, this method can be considered as a very effective method for the evaluation of material condition. In order to compare the performance of simplified ultrasonic CT, the CT image obtained from several specimens with several simple defects was compared with the D scan image obtained by TOFD (Time of Flight Diffraction) method. We can see simple defects more clearly by new proposed method. Experimental results on several kinds of specimen, having welded joint by electron beam welding, welded joint by electron beam welding and fatigue crack showed that the obtained C scan or CT image has better resolution than the D scan image by TOFD method and shows similar image to actual shape.

Ultrasonic control of weld joints combining time-of-flight diffraction and inclined longitudinal wave techniques using at least three transducers

Abstract: The ultrasonic control of weld joints (1) assembling two abutted components (2, 3) consists of displacing, using a time-of-flight diffraction technique, in the longitudinal or circumferential direction along the weld joint at least one couple formed from a first transducer (5) and a second transducer (6), one transmitting and the other receiving ultrasonic waves. The transducers are laterally positioned on either side of the weld joint and comprising piezoelectric ceramics or crystals, in a manner to be able to detect all defects in the joint located at a thickness of at least 5 mm; displacing along the weld joint, using the inclined longitudinal wave technique, at least a third transducer, so as to detect any defect in the joint located at a thickness ranging between 0.5 and 15 mm. An Independent claim is also included for putting this method of ultrasonic control into service.

Application of ultrasonic TOFD method for welds of LNG storage tanks

Abstract: 9% Ni steels are used for LNG storage tanks and nickel-based materials are used for weld joints of 9% Ni steels. Therefore, detecting flaws by ultrasound is more difficult than in carbon or low alloy steel welds because of development of dendrites in the austenitic weld metal. Detectability of flaws in nickel-based weldments by the longitudinal angle beam method and creeping wave method were compared. Moreover, signal processing method, which uses time-frequency analysis, was applied to waveforms detected by the ultrasonic TOFD (Time of Flight Diffraction) method to remove noise from waveforms and enhance signal to noise ratio of the waveforms.

Characterisation and sizing of weld defects using phased array technology

Abstract: Ultrasonic Phased Array technology has now been in industrial use for a number of years mainly within the power industry and in recent years has developed into a production tool for automated inspection within the pipe inspection industry. This paper expands on the use of phased array technology and presents examples where the technology has been successfully used for the critical characterisation and sizing of complex defects. These include the unambiguous characterisation of porosity, slag, lack of fusion and rough cracks. It is believed that correct defect characterisation allows correct choice of defect sizing techniques. In conclusion it is believed that by using this technology appropriately, phased array inspection can achieve defect sizing accuracy similar to that of TOFD together with de reliable defect detection available with automated pulse echo inspection (A) For the covering abstract see ITRD E118270.

A Comparative Study of Pulse-echo Method and TOFD Method for the Defect in the Welding Joint of the STS304 Stainless Steel

Abstract: For the evaluation of a flaw in a welded stainless steel by the Time of Flight Diffraction(TOFD) Method, we have made the reference specimen for experimentation. As a result, we could analyze a specific character and the inner state of the structure in a welded stainless steel and we came to the conclusion as followed. (1) For analyze the structure state of a section in a welded stainless steel through the optical microscope, we could have analyzed the generated shape and the location of a flaw and the inner parts of the structure state through the microscope of eighty magnification and two hundred magnification about the soundness, the heat affected zone(HAZ) and the welded part. (2) Through the comparison with the shape and the size of a flaw in the welded part about the conventional ultrasonic test and the TOFD Method, we could make an observation the special character of the TOFD Method and principles. (3) We analyzed and collected the merit of the TOFD Method on the basis of the experimental result by the shape and the size of a flaw in the inner welded parts. So, we made up a base that we could use as a basic data for a similar flaw like that. Through the study as mentioned above, we could make an observation the flaw detective method and principles used in the TOFD Method.

Extending the synthetic aperture focusing algorithm to deal with flat and curved features in NDT

Abstract: The synthetic aperture focusing (SAF) technique allows exploration of ultrasonic array imaging performance with minimal hardware and it can provide excellent imaging results for point defects, such as fatigue crack tips, commonly found in non-destructive testing (NDT). However, it performs less well when imaging defects which are flat or curved with radii large relative to the ultrasonic wavelength. Here, we report an enhanced time-domain algorithm which can deal with flat and curved defects and which we have termed "synthetic aperture correlation". It is based on processing signals from multiple overlapping subsets of elements from an entire array. The algorithm has roots in SAF, matched filtering, beam steering and biomedical ultrasonic imaging. The data collection process is the same as for SAF and the new algorithm operates entirely in reception, off-line. It has been tested using a 96-element linear array operating at 6 MHz on aluminum test objects. We present simulated and experimental results and compare these with B-scans showing timer-of-flight diffraction (TOFD) characteristics and with conventional SAF. We highlight the advantages of the new algorithm, such as its enhanced capability to deal with noise, and drawbacks such-as increased computational expense.