NDT Technique

NDT Technique Vs. Fracture Mechanics

The use of fracture mechanics concepts places a premium on the ability of NDI to detect small cracks and on the need to determine the practical reliability of a particular inspection process when that process is used to detect defects of a specific type and size. If the design is such that the critical crack size based on design loads is greater than the smallest defect that can be reliably detected, the inspection process can be used.


The difference between the critical size and the smallest detectable size is the factor of safety.Any measurement technique will result in experimental errors. Since this error affects the fracture mechanics calculations, there have been concerted efforts in NDT to reduce the errors in defect sizing. For example, with conventional ultrasonic techniques, the error may well exceed 5 mm. Clearly there is some considerable benefit if precision is improved and with the Time-of-flight-diffraction (TOFD) technique, the error is unlikely to exceed 1.5 mm so the classification of defects can be more exact.


Despite a small echo from the defect tip, an effective reduction in the size of the minimum detectable defect may be provided by the use of TOFD (12).It is certain that the greater majority of the defects considered potentially dangerous will, if monitored, turn out to grow less rapidly than assumed and many will not grow to failure in the design life of the structure. Monitoring might then result in savings in repair and shut-down times without impairing the essential safety of structures.Selection of NDT TechniqueOne of the primary considerations when selecting an NDT technique, whether it is capable of detecting the existing discontinuities with sufficiently high probability.


This establishes the threshold rejection criterion, or the decision point of whether the component is fit for service. Sophisticated techniques are required for finding tight surface cracks and internal discontinuities. Further, not all NDT techniques are physically capable of detecting all discontinuities. Each on has its own limitations. The capability of a method depends on the inherent limitations of the method, technique or procedure used. For example, ultrasonic beam inspection can not reliably detect discontinuities very near to the surface due to the erratic effects of the probe’s near field and ring down. Conversely, eddy current techniques could not be expected to reliably detect discontinuities more than a few millimetres below the inspection surface. On the other hand, magnetic techniques can detect both surface-breaking discontinuities and volumetric discontinuities provided they are of sufficient size. So, care is required while selecting a technique for a definite inspection task.


Though in many instances single NDT technique is sufficient to solve an inspection problem, more than one technique are also employed. No doubt, they provide additional information and are expected to enhance inspection reliability (13).It needs to distinguish between effectiveness and efficiency of inspection. An effective inspection is one which finds all the required defects with the required probability of success. On the other hand, an efficient inspection is one which is not only effective for defects concern but also avoids the unnecessary rejection of minor imperfections.


It can be therefore be deduced that inspection reliability includes both effectiveness and efficiency.The choice of inspection equipment is a function of several important considerations. The equipment must have sufficient capability yet be simple to calibrate, maintain and operate, it must withstand the field conditions of the inspection, it should allow ease of signal interpretation and recording, it should be portable (14).Indications obtained during NDI need to be interpreted and evaluated. Any indication that is found is called a discontinuity. As discussed earlier, discontinuities are not necessarily defects, but need to be identified and evaluated to decide whether the part is at or below specifications.


The following definitions would help in categorising the NDI data:


False: Indication not due to the testing procedure. It may be due to improper processing, incorrect procedure, also known as a ‘ghost’, an artefact, ‘spurious’ or ‘electrical interference’.


Nonrelevant: An indication which has no relation to a discontinuity that is considered a defect in the part being tested; a defect within acceptable tolerance levels.


Discontinuity: An interruption, intentional or unintentional in the configuration of the part.Indication: Observation of a discontinuity that requires interpretation, for example, cracks, inclusions, gas pockets.Interpretation: Determination whether an indication is relevant, Nonrelevant or false.Evaluation: Assessment of a relevant indication to determine whether specifications of the serviceability of the part are met.Defect, flaw: One or several discontinuities that do not meet specifications.