Inspection of threaded components is vital to ensure the reliability of bolted connections. Usually such inspection would entail i) the removal of the fastener, ii) measurement of the tolerances using GO/NO-GO gauges, and iii) non-destructive testing (NDT) of the fastener, after they have been retightened a certain number of times or after a specific period in operation. In the absence of NDT bolts are often replaced when overhauling components. Although inspection with GO/NO-GO gauges requires the bolts to be removed, it is possible to detect cracks in large bolts while they are still in service with the judicious use of ultrasonic methodologies. However, in most cases the bolts generally need to be removed to facilitate inspection, which makes inspection difficult and time consuming.

In a typical bolted joint, the nut is loaded in compression, which reduces the propensity for the initiation of fatigue cracks, and hence the requirement for crack detection. However, in the case of components with internal threads loaded in tension, fatigue cracking can initiate and propagate from the root of the thread, or the base of the hole. Internal threads are also susceptible to corrosion damage – especially in marine or corrosive environments and where the threaded hole is orientated vertically upward. Inspection of these threads, and especially in deep blind holes, is challenging unless the threads are big enough to allow visual inspection, or the use of purpose made ultrasonic probes/jigs. In most cases the threads are simply re-tapped, which highlights and partially removes corrosion and gross deformation, but is insufficient to ensure the integrity of the thread. Other options to address the issue include tapping the hole oversized or the use of Helicoils to refurbish the thread. However, it is normally difficult to ascertain whether such measures are in fact necessary and these measures typically cannot be used on multiple occasions, which effectively limits the life of the component which can have significant cost implications.

But how does one ensure that the condition of the threaded blind holes are indeed fit for purpose, especially if the components have been in use 20/30 years? Go/No-go gauges could be to measure thread tolerances and hence condition of the threads. These are useful tests, as they highlight whether or not the thread is within the upper and lower limits of the tolerances. However, positive results from these tests should not be considered as definitive proof that the threads are still fit for purpose, as they cannot test for corrosion damage, cracks, and/or some forms of wear or deformation that would result in the thread not being fit for purpose. Furthermore, NO-GO gauges are designed to STOP at the first thread which is within tolerance and as such only indicates that this thread (which in the case of a uniformly machined undamaged thread would be the first thread) is ‘within tolerance’ thereby preventing assessment of the other threads which may have been damaged in service. In addition, as different gauges are used for each tolerance class, GO/NO-GO gauges can only be used when the specified/original thread tolerance is known, which is often not the case.

Boroscope inspections are also possible, but the results are difficult, making once off assessments and ongoing trending of damage difficult. Boroscope cameras can be used in conjunction with dye penetrant NDT, or mag particle inspections, but uniform coating of the threads (and cleaning between stages of testing) with the dye and developer is difficult. New techniques using boroscope cameras have been developed to measure thread spacing/tolerances however these are i) expensive, and ii) generally limited to inspections in laboratory conditions which is generally not an option for large components. Inspection using 3D imaging techniques are also being developed, however probes, and technology which could be used for deep internal threads is still highly specialised/in development.

Advanced NDT testing including conventional Ultrasonic Testing (UT), Phased Array Ultrasonics (PAU), Time of Flight Diffraction (TOFD) and Eddy Current techniques are possible and in theory they can be used without removal of the components, which is a significant benefit. However, these techniques need sophisticated equipment operated by qualified technicians with a good understanding of the technique and hence is relatively expensive. Furthermore, they can generally only be used on relatively large diameter fasteners, quantification of degradation is difficult and grain size variation in cast materials can complicate the use of ultrasonics in large cast components.

Most of the issues with the assessment of threaded holes, stem from restricted access that can be addressed by creating replicas of the threads in their current state and condition. Once made, stud-like replicas can be removed from the threaded hole and inspected using visual inspection methods or be scanned either in 3 dimensions, or by shadowgraph techniques. The results from scans allow quantitative assessment with respect to specified tolerances.

Although replicas can be made with many deformable moulding compounds, accurate assessment requires the replicating compounds to i) be non-shrinking, ii) be dimensionally stable, iii) retain their shape after being deformed, iv) be able to replicate to micron accuracy, v) allow the replica to be removed from the thread without adhesion/leaving residue on the threads, and vi) be sufficiently pliable to be pulled out of the threads (rather than having to be screwed out of the threads which would damage the replica). Furthermore, the compound needs be able to be dispensed in a manner that allows it to be used on site and used on holes with various orientations.

Trials undertaken on M42 x 160mm deep threaded blind holes using a number of silicones, replicating compounds, resins and polyurethane products highlighted that, although a few resins and silicones could be employed, Plastiform has a number of silicone-based products that could be used directly with high levels of accuracy and repeatability. Other silicone products were partially successful, but were found to be difficult to dispense and were plagued by bubbles which made analysis of the replicas difficult (especially in the case of translucent products). Replicas made with harder resins injected into 3D printed internal moulds were difficult to remove from the threads without damaging the impression. Cleaning of the thread after replication using resins was difficult especially in regions where release agents were not adequately applied. These difficulties were found to add significant complexities to the assessment.

The products imported from Plastiform are self-releasing, have up to micron accuracy, are dimensionally stable and are fast curing. The replica/impressions produced are solid in colour and can easily be manipulated and inspected (visually, and with scanning techniques). Plastiform offer a variety of products that can be selected to meet the requirements of the job, with fluid silicones of various hardnesses (hardness ranges from 20-80 Shore A), and ‘putty’ and ‘pasty’ products where fluid products (used for internal threads/voids may not be suitable (i.e. on vertically orientated components, or highly localised impressions).

Since our initial trials, Origen has used replicating techniques on a number of occasions and have achieved good success using Plastiform, with whom Origen now has a distribution agreement, to assess the level of degradation on M42 x 160mm and M56 x 290mm deep threaded blind holes. We have developed procedures required to allow the work to be undertaken efficiently and have been able to use these replicas to the document the integrity of the threaded holes in components used in the marine industry with a high level of confidence. Although the technique is not perfect it allows the condition of the threaded holes to be quantified with a relatively high degree of repeatability and accuracy.

The message is simple – the integrity of thread in components with threaded internal bores needs to be assessed during refurbishment and this can be achieved successfully using a combination of replicating and 3D measurement techniques.