Time of flight diffraction is an Ultrasonic technique developed originally to improve the sizing accuracy of flaws previously detected by other means. Early applications of the technique advanced from solely sizing flaws to applications relying on TOFD for principle detection means, this pioneering technology resulted in major flaws being missed due to poor application and a general lack of understanding by persons operating the equipment. As a result the cliché “give a dog a bad name” stuck for some time and many years passed with several industrial trials and validations 8,9 being conducted before we reached a stage where now TOFD has become an essential tool in the ultrasonic toolbox.

Technology Design Pocket-Scan for TOFD

As this technique is becoming more globally applied it is essential to maintain a base level of knowledge in order to educate practitioners as to the correct and incorrect use and interpretation of this test medium. To that end a working group was set up from industrial users of TOFD to establish a syllabus for unified training and a certification platform for level 1,2 and 3 personnel involved in application of TOFD. The working group consisted of individuals representing a wide range of major companies involved in the current application of TOFD. This working group expressed the need for a mechanism of training and certification that would not be equipment specific but rather underpin the basic principles and applications of the technique. It therefore had to cater for students working on a variety of their own company’s equipment during training and recommended that the employer had a responsibility to ensure adequate equipment familiarisation in addition to the minimum requirements stipulated in the training and certification documentation.

Lavender International NDT Ltd is a BINDT accredited training provider for TOFD and Lavender International is accredited to conduct SNT and EN473 TOFD exams for certification levels 1,2 and 3.

Training is based on sound theoretical material to technically justify principles that TOFD evolves from. The theory is carefully explained to enable the average NDT practitioner to comprehend all aspects. Far too frequently training establishments are guilty of blinding people with science which somewhat defeats the training objective. Therefore great care, patience and skill is required to hold a class of mixed ability in full attentiveness for the course duration and yet this is even more essential with the advanced complexity of contemporary technologies.

This is subsequently followed by individual data analysis sessions with students sited at workstations which are loaded with analysis software and a comprehensive array of scan files portraying a broad spectrum of pre-service and in service defects. Specimens include flat plate butt welds, sectional and complete pipe butt welds, tee butt welds and samples representing longitudinal seams in pipes to ensure the necessity for curved surface correction. These samples range from 10mm to 50mm thick with representative flaws of weld root erosion, corrosion, fatigue, re-heat cracking as well as a comprehensive array of welding production defects. In fact we are continually updating our sample inventory together with a library of scanned data to complement our comprehensive training package.

Successful applications of TOFD, as with any NDT method, rely upon the essential understanding of the methods weaknesses. Our belief is that unless you can appreciate the limitations of a method then how can you apply it?

Simple practical experiments are used to assist visualisation of the constraints of flaw detectablilty and sizing. This involves near and far surface sensitivity, resolution, timing errors & axial positioning of flaws relative to the probe arrangement. The practical session forms a fundamental foundation of the course to consolidate the theoretical philosophies and produce tangible evidence allowing reliable inspection data to be accrued from careful design of the inspection arrangement. Most students whether novices or experienced to TOFD find the course an eye opener, frequently commenting: “to think of all those inspections done in the past not considering these points”. Use of complementary pulse echo data collected simultaneously to the TOFD scans is a fundamental necessity to guarantee a high probability of detection.

Data acquisition is an element that all three NDT levels are required to perform. It is amazing how many theoreticians struggle to produce quality scan data! Getting involved with the messy end of TOFD is undoubtedly a skill to be honed by experience especially when manually moving a probe pair over the scan surface. This is compounded by application of multiple probe arrangements when attempting to collect all the weld scan data simultaneously. Use of pumped irrigation systems is naturally ideal for multiple probe configurations but does rather mess up the classrooms Wilton carpet! Frivolity aside, the principles of probe stability, coupling and manipulation learned manually can then be extended to automated circumstances allowing discussion to open regarding field applications and what is reasonable to expect from site data.

In turn the focus returns to limitations and capabilities of post test data processing tools, image enhancement, straightening, lateral and backwall removal, synthetic aperture focussing technique (SAFT) and rectifying erroneous file data that inadvertently generate false sizing information. A valuable lesson indeed is to comprehend why there are practical constraints on the collection of “the ideal scan”.

Level II students are required to produce written techniques for data acquisition where as level III students are required to produce a full inspection procedure. These elements involve the predictive coverage of beam geometry to assure an effective inspection. Hypothetical situations are given on paper to assess their management and understanding of transducer selection, choice of hardware parameters, selection of software parameters and the obligation to deploy complementary pulse-echo beams for supplementary information that cannot be obtained by TOFD.

Justifying the technique subsequently involves theoretical calculation and practical demonstration where representative material is available. This valuable lesson cultures a plane of thinking that blossoms into successful deployment of the TOFD method. No one said the TOFD course would be easy and learning can be a painful experience when all goes pear shaped but the satisfaction of achievement is all the more gratifying when ultimately students graduate into competent practitioners.

Example of data collected during the acquisition element of the exam.