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 being conducted before we reached a stage where now TOFD has become an essential tool in the Ultrasonic toolbox.
Training Courses at Sagnik NDE
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.
Use of animated presentation material together with video projection of instrument displays enhances the visual aids required to enable students to absorb the volume of information. Live data can be processed and scrutinised as a classroom exercise stimulating participation from all students and encouraging vigilant observations to correctly identify relevant flaws on each scan. Use of live video images projected onto a large screen also facilitates easier group training into the correct use of different types of TOFD equipment and related software. Software can be explained by the process of demonstration both for set-up prior to data acquisition and analysis post data acquisition.
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 visualization of the constraints of flaw detectability 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 back wall removal, synthetic aperture focusing 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 2 students are required to produce written techniques for data acquisition. 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.
Candidates must hold appropriate UTL2 prior to this course