There are many non-destructive testing techniques. They’re not all suited to every type of application. For example, electromagnetic testing techniques don’t work on most composite materials, while X-rays and computed tomography are perfect. Similarly, ultrasounds are very versatile, but require direct contact and appropriate coupling with the surface under test, which is not always possible.
One of the most common, low-cost solution (and one of the oldest) to non-destructive testing challenges is liquid penetrant testing (PT), also known as liquid penetrant inspection (LPI) and dye penetrant inspection (DPI). The technique leverages capillary action—the ability of a liquid to flow into narrow spaces without the assistance of, or even in opposition to, external forces like gravity—to detect surface-breaking defects.
After applying the penetrant and letting it dwell for a certain period of time, the excess is removed and a developer is applied. The developer draws the penetrant from surface-breaking defects where it seeped, revealing their presence.
The earliest instance of a surface inspection technique resembling penetrant testing involved rubbing carbon black (a by-product of the incomplete combustion of heavy petroleum products) on glazed pottery. The carbon black would accumulate in surface cracks, making them visible when the excess black was wiped off.
By the end of the 19th century, the railway industry was widely using the oil and whiting method. It involved applying a light oil on the part under test. The oil—just as in modern penetrant testing—was allowed to dwell into surface-breaking defects, and then whitewash was applied on the surface to make the defects visible (by providing a high-contrast background and drawing the oil from the surface-breaking defects). Until the 1920s, this testing method was very popular because it yielded visible results that could be understood by anyone.
Aircraft inspection, propelled by the advances during World War II, was the engine behind the evolution of liquid penetrant testing.
It is generally accepted that liquid penetrant testing has the following advantages:
- Sensitive to small surface discontinuities
- Few material limitations—works on metallic, non-metallic, magnetic, non-magnetic, conductive, and non-conductive materials
- Works on complex geometric shapes
- Visual, real-world results
- Liquid penetrant testing materials are very portable
- Liquid penetrant testing materials are individually very affordable
Disadvantages of LPI
However, liquid penetrant testing is not without several disadvantages:
- Only sensitive to surface-breaking defects
- Extensive, time-consuming pre-cleaning critical—surface contaminants can mask defects
- Only works on relatively non-porous surface materials
- Direct access to the surface under test necessary
- Multi-process testing procedure
- No depth sizing
- No recordable data available for progress monitoring
- Time-consuming post-cleaning also necessary
- Environmental concerns—may require costly handling and disposing of chemicals
- User dependent
But, perhaps the greatest disadvantage is that over time and despite lower costs (less training, cheaper materials), LPI is little more than a screening tool; you can locate defects and measure their length, but using this technique, it’s impossible to determine the severity of its depth or to monitor the progress of defects over time. It also relegates the technique to a pass/fail evaluation, which can lead to discarding healthy parts and retaining unhealthy parts—which can both prove costly.
Therefore, overall, despite the immediate attractiveness of this low-cost solution, it possesses several downsides that must be looked at long and hard before dismissing more advanced and potentially more expensive inspection solutions, whether you perform your own inspections or contract them out.
Eddy Current Testing (ECT)
For companies looking to achieve the most bang for their buck, eddy current testing (ECT) on conductive materials can potentially save time and money. Eddy current testing systems have a higher initial cost, but they generate no chemical waste, require little to no surface preparation, and are faster than LPI in typical applications, despite being user dependent.
Eddy Current Array (ECA)
This offshoot of eddy current testing improves on the technology by using multiplexed arrays of coils arranged in rows (instead of one or two coils), which allows covering a larger area in a single scan pass. Some advantages of ECA over ECT are obvious:
- The wider coverage leads to significantly faster scans.
- The larger ECA probes considerably lower operator dependence—eddy current array probes offer better data than manual raster scans.
- ECA offers better detection capabilities, as well as accurate defect positioning because the inspection data can be encoded, and—perhaps most importantly—sizing.
- The simpler ECA scan patterns make analysis markedly simpler, faster, and beyond doubt.
- Data can be recorded, making defect progress monitoring possible.
Finally, perhaps eddy current array’s (and ECT’s) greatest advantage over dye-penetrant testing, is ECA’s ability to be flexible or shaped to specifications, making hard-to-reach areas and complex geometries easier to inspect—something that’s even difficult for PT, at times.
Of course, ECA systems are more expensive than ECT solutions and LPI, but, in some situations, they can prove 3 to 10 times faster, depending on the initial surface conditions, the presence of coatings, and so on. ECA also offers critical data, which allow making better decisions when it comes to key assets.