Making TOFD inspection easier encourages bigger inspection tasks which generate larger quantities of data. This data must be analysed, preferably on site. Can this data analysis process be streamlined to encourage its use on site? In addition the inspection must be set up so as to maximise its effect. This can be done in advance of the inspection but it is not uncommon that site conditions require a revision to the procedure - can simulation aid this process? Finally there is the question of the aptness of the technique - how will it perform? - are alternatives better?
This paper describes some of the developments that have been carried out to ease decision making processes, to aid conformity with TOFD standards, to improve the speed of data analysis and to allow flaw growth monitoring.
It would normally be the intention to revise this document after a number of years, both to correct anomalies and to 'slim down' the text by concentrating on application requirements. However this was happily overtaken by the work, now already underway, to define a European standard for the technique. This document has already been circulated for comment within Europe and should be available quite soon as a Draft Standard.
Fig 1. Estimation of the Coverage Obtainable With a Defined TOFD Test Configuration.
Fig 2. Estimate of the Time Profile Expected from a 20mm Long Rectangular Crack. The Profile from a Pore is also Depicted.
These tools are kept under review and the range is expanded or an individual tool extended according to user feedback. Their main use is in planning inspections and in setting procedures but they form a particularly valuable aid if the inspection procedure has to be varied on site. In Figure 1 an example of the estimation of beam coverage is given. From this depiction decisions can be made to optimise the scan geometry or to increase the number of probes or scans. In Figure 2 an example of the time profile estimates for a rectangular crack and a pore are compared. Is this difference sufficient to allow the flaw to be detected or should the specification or test set up be changed?
PoD estimation has been attempted from the analysis of practical results but the modelling of the flaw detection task has rarely been attempted. Two main elements affect PoD; the technique and the human factor. Human reliability has been studied and quantitative data is emerging. It is not very dependent on the precise action being undertaken and thus can to an extent be separated from the PoD associated with the technique. In choosing between techniques it is the comparison of the technique PoD's which is of greatest relevance.
Within the NDT Centre software to estimate PoD has been produced for a number of techniques including the ultrasonic pulse echo and TOFD techniques. The performance of the techniques can thus be compared. For example, if flaws are growing normal to the back wall (corner reflectors), the only advantage of turning from pulse echo to TOFD is that detection and accurate sizing are possible in one measurement - fig 3. On the other hand, if the flaw angle varies, even by only up to 10 degrees from the vertical, the balance shifts towards the use of TOFD for flaw detection - fig 4.
Fig 3 and Fig 4
Fig 3. Estimates of PoD for TOFD and Pulse Echo Techniques. Flaws Assumed to be Open to Lower Surface and at an angle of 90 degrees. PE Calibration on 5 mm notch with Discrimination at 20% FSH. TOFD Calibration to BS7706.
Fig 4. Estimates of PoD for TOFD and Pulse Echo Techniques. Flaws Assumed to be Open to Lower Surface and at an angle between 80 and 100 degrees. PE Calibration on 5 mm notch with Discrimination at 20% FSH. TOFD Calibration to BS7706.
If software analysis is designed with the operator in mind we find that the priorities are:
Speed improvement is not simply a matter of improving hardware or techniques. Often it is the interaction with the software which is time consuming - changing menus, selecting tasks etc. Use of the mouse gives better interaction but, with many B-scans to analyse, this is also a process which the operator would like to limit. There are few things yet to rival the human brain and eye combination for picking out likely defects but ideally interaction would be limited to this. When the analysis is complete the result should be a simple representation of the flaws which can be left with the customer.
With these needs in mind software has been developed which is aimed at the rapid translation of data from the form of a TOFD B-scan to a simple presentation such as a cross-sectional chart or a tabulated list of flaw sizes.
Fig 5 and Fig 6
Fig 5. Example of a TOFD B-scan Showing Fitted Profiles
Fig 6. The Cross Section Derived from the B-scan Data in Fig 5
The synthetic aperture processing technique (SAFT) has become an adjunct of TOFD processing since it provides accurate estimates of flaw length; complementing TOFD which accurately determines flaw tip depth and flaw height. This process may take a few minutes and, to speed this up, an iterative processing technique has been developed which continues until the profile represents the best estimate of the true flaw shape. Despite the iteration the analysis is rapid and typically the analysis of an individual flaw profile is completed in less than 2 seconds on a 386 PC. The crux of the analysis is the estimation of flaw height. The errors likely in the estimate of the depth of flaw tips can be estimated, based on the scan parameters.The software is also capable of translating these into estimates of limits for flaw tip location . An optional display indicates the best estimate of the flaw tip position and also the upper and lower extremes for this based on experimental errors.
The software produces a best estimate of the size and shape of the flaw and presents this on a cross-section of the specimen in the form shown in fig. 6. The character of each flaw tip is indicated by a colour coding. The flaw profile can be stored in this processed form allowing rapid data retrieval. IN this way data may be compared with stored data from a previous examination, providing an ability to closely monitor the flaw for any growth. With suitable preparation for the scans, growth of less than 0.5mm would be identified by this means.
For more information see: TOFD in UTonline 09/97
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