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第8頁(yè)
遼寧科技大學(xué)本科生畢業(yè)設(shè)計(jì)
NDT of Welds: state of the art
R.J. Ditchburn , S.K. Burke and C.M. Scala
Ultrasonics
Ultrasonics was introduced as an NOT technique for weld inspection in the 1960’s .Since then, the technique has undergone extensive development and gained increasing acceptance. Consequently, ultrasonics is now the major technique used for validation of welded structures in many in-service inspection applications, eg in off-shore structures. In nuclear and pressure vessel industries and in a range of naval applications.
The emergence of ultrasonics as a preferred technique over X-radiography in these in-service inspections is due both to inherent limitations in radiography and to actual benefits in applying ultrasonics. As described above, radiography is excellent for identifying volumetric defects but is limited in its ability to detect or size planar defect, such as cracks, which are likely to be the more serious defects type. Ultrasonic waves are scattered by planar and volumetric defects, making the ultrasonic technique useful for detecting and sizing both types of defects. Even closed cracks are detectable by ultrasonic. Provided that appropriate procedures are used. Ultrasonics also readily gives depth information concerning a defect, whereas for X-rays, specialized and expensive techniques such as computer tomography are needed to obtain such information. Ultrasonics also offers benefits over radiography in terms of cost savings through increased productivity. Finally, in the 1990’s the increasing concerns about radiation safely are a severe disincentive to the continued use of X-radiography.
In the last few decades, ultrasonics has developed from a purely manual technique to a manual technique with computer-assisted processing to the use of automatic scanners and more recently to the development of fully automated systems incorporating multiple piezoelectric studies on the use of this range of increasingly sophisticated systems for defect detection have formed a major factor in establishing the credibility of ultrasonics for weld inspection. Studies such as the Programme for Inspection of Steel Components (PISC) ultrasonic inspection in the nuclear and pressure vessel industries. Outside the PISC studies, useful work has been carried out to determine reliable procedures for inspecting specific weld geometries including single-V and double-V welds and also compared the reliability of radiography vs. ultrasonic inspection Overall, the results of the reliability studies indicate that the probability of detecting a defect with ultrasonics increases with the degree of sophistication of the system. According to Lebowitz and DeNale the results also indicate that manual ultrasonic procedures, can be expected to reject an equal or greater percentage of the discontinuities present than will radiography.
Ultrasonic validation of welded structures requires not only reliable defect detection but also sufficiently accurate defect location and sizing to allow acceptance/rejection criteria to be correctly implemented Manual ultrasonic systems usually rely on the use of amplitude dependent techniques for defect sizing. Techniques commonly used are the 20dB drop (shown in Figure la), the 6dB drop, or comparison with the amplitude form a drilled hole. However these techniques are known to be inaccurate. The inaccuracies are caused not only by the effects of defect shape, orientation and location, but also by attenuation, coupling, resolution and equipment characteristics. The incorporation of computer-assisted processing into ultrasonic systems has allowed the easy implementation of potentially better methods for defect detection and sizing such as time-of-flight-diffraction (TIFD) (see Figure 1b),eg in PISCⅡ the addition of TOFD to standard procedures gave nearly perfect results in terms of required rejection rate for-defects. Important advances in defect sizing have also been made possible by the incorporation in automated ultrasonic systems of ultrasonics imaging based on Synthetic aperture focusing (SAFT) and variants such as SUPERSAFT.
The development of reliable procedures for the application of ultrasonics to weld inspection had required an understanding of the interaction waves with the various types of weld defects, of wave propagation in complicated geometries, of particular problems caused by inspecting for defects close to the surface of a structure of the effects of cladding and other micro structural influences influences on wave propagation. While wave propagation in ferritic and light-alloy welds Is relatively uncomplicated, the microstructures of austenitic welds have caused special concerns. These materials strongly attenuate ultrasonic waves, cause high background noise due to scattering from the large grains present, and result in skewing of the ultrasonic beam unless the propagation is along principal crystallographic
(a) 20 dB Drop Technique (b) TOFD Technique
Figure 1 Ultrasonic defect sizing (a) The 20dB intensity drop technique. Where the transducer is positioned at points is used along with probe calibration characteristics to estimate the defect length.(b) Time-of-flight-diffraction (TOFD) technique. A two probe technique used to determine crack size and location utilizing the diffracted waves from the tips of the defect.
axes. Thus, much recent research has been directed toward the development of specialized ultrasonic techniques to deal with these complications. Considerable progress has already been made, especially under PISCⅡ and Ⅲ where detailed has been undertaken. In the future, these models should allow more accurate estimation of location and sizing errors for specific defects, and provide the basis for improved codes for inspection of austenitic steels and weld steels.
In today’s world, there is an increasing need to minimize the cost of weld inspection. The advent of automated scanners, the use of multiple probes and computer-assisted processing in modern ultrasonic systems have reduced costs by increasing both the speed and reliability of inspection. On the negative side, equipment, and calibration costs are higher with automated equipment. Also, the costs in actually interpreting ultrasonic data could rise due to the recent advances in ultrasonic systems, since all types of defects and even very small defects can be detected, whether or not the defects are critical. The solution to this problem would be improvements in the automated application of acceptance/rejection criteria on defect criticality. Hence, considerable effort is now being directed towards the development of neural networks to be used in ultrasonic systems to classify defect type, size and location, and resulting conformance with a particular inspection code. Very promising results have already been obtained in several laboratories in studies both on simulated weld defects, where a 100% correct classification rate was achieved in defect type, and on real weld defects where success rates of the order of 90% were achieved using a variety od methods. Preliminary work has also been made in the automated application of acceptance/rejection codes via neural networks.
Clearly neural networks will only prove successful if they can be trialed on representative data. However, representative data can prove expensive to acquire. For example, the PISC programme is currently costed at ＄200M, and it seems unlikely that this type of effort will be duplicated in other industries in the near future. An alternative approach would be to trial the networks using data generated from robust mathematical models of the interaction of ultrasonic waves with weld defects. The development of such models is a continuing PISC objective under PISCⅢ。 Until these mathematical models are more complete, an emphasis staff seems necessary and technically qualified staff seems necessary for ultrasonic weld validation.
For the future, many challenges remain in optimizing ultrasonic inspection of welds. Substantial improvements are possible in the application of neural networks has only just started. Various options exist for the improved generation and detection of ultrasonic in welding applications, eg by the use of phased arrays, laser techniques (as described below) and other specialist probes. A number of additional factors need to be considered in the ultrasonic reliability area, eg residual stress, the effect of higher frequencies, more extensive consideration of real rather than simulate defects.
Greater consideration also needs to be given to the overall cost effectiveness of inspection. One of the elements in maximizing cost-effectiveness is the selection of the most appropriate techniques for a given inspection, including the possible use of more than one technique to validate different parts of a welded structure. For example, magnetic particle testing is already used in conjunction with ultrasonics for rapid and cost-effective detection of surface cracks in welds. New electromagnetic methods could also have a role to play here (as discussed in the following section). Finally, there are clearly challenges in implementing advances in ultrasonic inspection technology into the codes for weld validation.
In-production weld inspection
On-line monitoring and control of the welding process has the potential to improve weld quality and increase productivity in automated welding. Weld monitoring and control can be achieve by the integration of real-time nondestructive evaluation techniques with the welding process. In-production weld inspection can improve weld quality and may provide a significant cost reduction. The welding parameters can usually be adjusted to prevent defects do occur the flaws can be found and repaired before they are covered by subsequent welding passes, leading to a decrease in the level of post-weld inspection and repair.
Good quality welds rely in the correct weld pool size geometry and position relative to the weld preparation. In-production automated weld monitoring systems usually have sensors providing information on the stare of the weld pool. Using this information and determining a relationship between the stare of the weld pool and at least one of the critical welding parameters (eg current, voltage, torch position and travel speed) the welding process can be adjusted by a feed-back loop from the process parameters to maintain the desired stable process state. This can be achieved with little operator intervention.
Figure 3 Schematic diagram depicting the principle of introduction monitoring of the welding process
The dynamic nature of welding means that data acquisition and processing, must be rapid enough to extract useful information before any major change occurs in the welding process. A two-step real-time radiographic analysis involving a fast search for defective regions followed by fine identification and location of defects has achieved this requirement. Real-time radiographic images have been used in the control of arc welding conditions in butt-joint welds. A combined approach using real-time radiographic images of the weld immediately behind the pool was used for weld penetration and quality control.
In-production 371678 sensing has been used to determine the quality of both gas metal arc (GMA) and gas tungsten arc (GTA) welding processes. This technique allows detection of weld pool geometry and weld defects in real time. The technique evaluated the solidified weld metal behind the electrode. Two types of discontinuities were detected: incomplete sidewall penetration and porosity. These discontinuities were distinguished from sound welds using an expert-system technique with a success rate of 92%. Unfortunately the expert-system algorithm was unable to discriminate between the discontinuity types as successfully. Porosity was identified correctly 70% of the time and incomplete sidewall fusion was correctly identified 63% of the time. But et al, have shown that CTA and resistance spot-welding can be monitored ultrasonically and are making progress in developing an in-production monitoring system for resistance spot-welding.
Using a piezoelectric transducer and complaint presents the possibility of contamination of the weld by the coupling medium, obviously impractical for production. To overcome this problem, a non-contact ultrasonic system has been developed. The system developed uses a pulsed Nd: YAG laser for ultrasound generation and an electromagnetic acoustic transducer (EMAT) for ultrasound reception.
Two other non-contact transducer are currently under investigation. The fist technique involves the simultaneous observation of the infrared (IR) and the ultraviolet (UV) radiation from the welding process using dual wavelength fiber-optic sensors. The radiation is produced both from the hot melt pool and from the plasma produced by the beam/vapour interaction. The technique has been successfully employed to indicate disturbances encountered in laser welding. The second technique uses in-production processing of video images. Encouraging results have been reported that provide weld joint area and bead centerline cooling rates in GMA welding. This information is then used by a fuzzy logic controller and an artificial neural network to modify process parameters.
The increasing demands of high production rates and greater weld quality at lower costs will necessitate the strengthening of the bond between the technologies of welding and nondestructive inspection. In achieving these goals, in-production monitoring of the welding process will increase in importance and may well become indispensable.
Conclusions and future work
In recent years some exciting developments have occurred in NDT techniques for weld inspection. Major advances have been made in several fields particularly in ultrasonics, electromagnetic methods for both crack sizing and residual stress measurement, and in on–line monitoring of the welding process.
Many of the advances in NDT techniques have been driven by today’s increasing pressures for cost-effective weld inspection. Cost effectiveness is linked to factory such as reliability, sensitivity speed and coverage of NDT techniques. The need for greater reliability, speed and coverage has resulted in the increasing use of automated ultrasonic systems for weld inspection particularly in the nuclear application. Rapid development has occurred in a range of non-contact NDT techniques which should improve speed of inspection in the future, as will the continuing development of neural networks for automated data processing.
Worldwide, the need exists to implement these advance in NDT technology in weld validation codes. Clearly there is no value in replacing an acceptable inspection method for the sake of technological sophistication alone. However, substantial improvements are possible by the incorporation of advanced concepts such as Time-of-Flight-Diffraction in weld inspection.
In conclusion, many challenges remain in NDT of welds, particularly in minimizing inspection costs without prejudicing structural integrity. These challenges are best met by close cooperation between welding engineers and NDT experts, so that best practice is achieved based on a knowledge of not only NDT but also weld manufacture, fracture mechanics and structural mechanics.