Non Destructive Testing Part 1: A practical perspective

In this modern age of design and manufacture, an essential complementary corollary to fabrication is the assessment of the component or structure, to ensure that it is ‘fit for purpose’. It is one thing to design the engineering system, and quite another to ensure it is made to the specifications envisaged, and, critically, that it is free of defects. This is conventionally assessed by means of the appropriate Non Destructive Testing (NDT) process and methodology. There are many NDT testing methods, some of which are very sophisticated, and this current Tech Tip provides a practical perspective to such NDT assessments, based on our experience in South African industry.

The field of NDT is extensive and it is important to understand what is required and what is practically achievable, to ensure the right NDT technique is used and correctly interpreted especially given the currently financial constraints associated with the COVID pandemic.  Proper understanding of the requirements and limits will help prevent funds from being wasted by endeavouring to answer unnecessary questions, but at the same time ensure that the NDT undertaken is truly relevant.  It is also important that NDT inspections are undertaken at the appropriate time, and not unnecessarily repeated too frequently.  Similarly, the NDT inspection should be undertaken to the right code in order to identify the relevant defect sizes, what type of defect being assessed (e.g. globular, porosity, crack-like, or corrosion), and what the probable limits of flaw detection for each of the available NDT techniques are.  The emphasis here is very much one of i) being appropriate (so not to inspect for sub millimetre sized defects (at vast cost) when this is not needed), ii) the importance of searching for defects using the correct technique, and iii) being aware of the limits of flaw detection for each NDT methodology.  In addition, one needs to understand the limits of defect sizes which would be acceptable (as determined, for example, from codes or a stress-based fracture mechanics assessment methodology).  It is often necessary, not only to detect defects, but also establish their size, shape and location within the component.

Before discussing specific pertinent and practical aspects of NDT testing (which will be handled in future Tech Tips in this series), it is important to consider some of the key aspects of NDT which include:

The NDT technique chosen must be appropriate for the task at hand, both in terms of its ability to detect the defect(s) and that the size range of investigation is appropriate/applicable.  This is most appropriately addressed by examples.

In one case, a series of small quench cracks developed in a clutch plate, which was inspected using Eddy Current techniques (a form of Electromagnetic NDT detection (ET)).  The cracks themselves were small, but surface breaking and the eddy current method employed could reliably detect such surface cracks of length about 1.1mm or longer, with definite and clear (100%) repeatability.  However, when the cracks were between 0.5mm and 1.0mm long, detection depended on the operator’s skill and experience, which led to a lower probability of detection.  When the defects were less than about 0.4mm long, they could not be detected at all using the eddy current and no cracks were reported below this size limit, EVEN THOUGH they existed, as confirmed by higher magnification physical observation.  Thus, in this case, the particular eddy current NDT technique led to a high probability of detection above about 1.1mm, but no detection below about 0.4mm, even though the defects were still there (but below the level of detection).  This example illustrates the importance of understanding the limits of detection for a particular NDT technique.

Similarly, we are aware of a number of cases where X-ray radiography was used in an attempt to detect fatigue cracks.  Radiography is a powerful and sensitive technique/tool and very suitable for the detection of ‘globular’ type defects, such as weld porosity, inclusions or similar type volumetric defects where there are clear voids.  However, It is generally not suitable for the detection of fatigue cracks, because fatigue cracks are typically very tight and narrow (of the order of a few microns wide) and not readily detectable by radiography, which is effectively identifying significant density differences (or voids of ‘missing’ material).  Fatigue cracks, at least initially, are so sharp and tight that there is not any ‘missing material’ and hence they are not detected by radiography.  In one case where X-ray techniques were used in an attempt to detect internal fatigue cracks developing in a pipeline, the pipeline was given an apparent ‘clean bill of health’, which was manifestly not the case.  In this case, careful ultrasonic testing would have been more appropriate.

Surface versus internal defects: 
Flaw or defect detection is often driven by the fabrication process and used as a way of ensuring that the component has been correctly made to meet specification.  There are indeed times when a complete and thorough examination of the component is important, but perhaps more often (at least in a South African industrial context), it is sufficient to know that the component is ‘fit for purpose’.  In more sophisticated applications (such as pressure vessels, the aircraft industry, or nuclear power applications), more rigorous flaw characterisation is appropriate, but for many local conventional engineering applications a more general and less sophisticated assessment is often appropriate.  In these latter cases, the assessment often refers to surface flaws (i.e. surface breaking defects), where such techniques as visual inspection, dye penetrant, eddy current or magnetic particle inspection may be sufficient to characterise manufacturing acceptability.  For internal defect detection, it is often more important to determine: i) the flaw type (globular or crack like defects); ii) where the flaw is located in relation through the thickness, and particularly iii) details of flaw sizes, as this is fundamental in assessing fitness for purpose and potential for fatigue, particularly from a fracture mechanics perspective.  In such cases, ultrasonic methods (and its variants) and X-ray techniques are best, particularly when the size of the defect is important.

The aspect of obtaining reliable results from NDT testing is very important.  In certain cases, for example from pulse-echo ultrasonic testing, flaw sizes are reported as a certain size, with a probable scatter (e.g. 9mm long ± 3mm), which is entirely consistent with the Gaussian distribution of the sound echo detection.  Thus the flaw might be assessed as either 6mm long or 12mm long, which can lead to very different ‘fitness for purpose’ interpretations.  Time of Flight Diffraction (TOFD) is a much more precise ultrasonic flaw detection method, wherein the scatter error is much smaller, say typically 9mm ±0.5mm (actual flaw size between 8.5mm and 9.5mm).   Also under the heading/subject of reliability is the question of ‘false positives’, where flaws may be reported as being present (and lead to possible component rejection), when more careful examination may reveal that the flaws were not there at all.  Similarly, ‘false negatives’ can lead to potentially dangerous situations in engineering application under load.  The confidence of reporting NDT flaw size results thus becomes of prime importance, with reliability and flaw detection confidence being key issues.

The next tip in this series will consider the various NDT techniques commonly used in general engineering in South Africa.