Are There Alternatives To Liquid Penetrant Testing?
Liquid penetrant testing is one of the oldest, most versatile and most widely used Non-Destructive Testing (NDT) techniques.
It’s also one of the most economical and can be used on a wide variety of components including various metal alloys, plastics, glass, fired ceramics and composites. It is sensitive to small surface discontinuities (cracks as narrow as 150 nanometres can be detected) and it works on complex geometric shapes. Large areas and large volumes of parts and/or materials can be inspected quickly at low cost and the penetrant materials and associated equipment are very portable.
The technique works on the principle of capillary attraction which is the ability of a liquid to flow or absorb into a narrow space or passageway without requiring an external force such as gravity. The capillary action occurs when the adhesion of the material is stronger than the cohesive force between the liquid molecules.
The materials used in LPT make the results of this capillary action visible and capable of being interpreted – in other words, once the technician has drawn the penetrant from the surface under test, any areas where it has seeped will be visible and indicative of defects or discontinuities.
But like all NDT techniques, liquid penetrant testing is only suitable for certain applications and it has some downsides. It is really only a screening tool and won’t enable technicians to evaluate the severity of a surface defect or to monitor how that defect is progressing over time. Preparation can be time-consuming and it can be a messy process. Dry and contaminant-free conditions are essential which may require extensive pre-cleaning of the area, plus the inspector has to have direct access to the surface being inspected.
There are clearly many upsides of this technique (low-cost, ease of use etc), but there are also some downsides which require careful consideration. When making NDT decisions, it is crucial to recognised and evaluate the downsides of LPT testing before rejecting alternative inspection solutions (some of which are more expensive than LPT).
Eddy Current Testing (ECT) is a viable alternative to LPT which can potentially save an organisation time and money. Initial outlay on these systems is higher, but the longer-term savings in time, money, productivity and performance make them a very attractive proposition.
Advantages of ECT:
- Sensitive to surface defects
- Can detect through several layers
- Can detect through non-conductive surface coatings in excess of 5mm thick
- Little or no surface preparation required
- No chemical waste
An offshoot of ECT is Eddy Current Array (ECA), which improves on ECT technology by using larger probes and multiplexed arrays of coils arranged in rows. This allows much wider coverage with a single pass, resulting in significantly faster scans.
Further advantages of ECA include:
- Lower operator dependence
- Better data collection than manual scans
- Better detection capabilities
- Accurate defect positioning
- Defect sizing
- Recordable data enables the progress of defects to be monitored
- Data collection enables better decision-making
- Flexible systems can be shaped to specifications, making it easier to inspect hard-to-access areas and complex geometries
In terms of a comparison between ECA and LPT, the former is a clear winner on many fronts including performance, speed, accuracy, analysis, versatility, data collection and analysis. It is also a more expensive option, but widespread opinion is that the substantial benefits (both short and long-term) make it a worthwhile and valuable investment.
If you’re curious to explore the opportunities that Eddy Current Array technology can deliver for your operation, talk to the experts at Nexxis. We stock a range of high-performance equipment and solutions including GE’s portable Mentor EM, the Nortec 500 series and the Eddyfi Ectane2 which all significantly increase the probability of detection whilst improving inspection productivity and documentation.
Their team of industry professionals will evaluate your operation’s inspection requirements and your unique applications to work out the most appropriate, cost-effective and flexible NDT solution. Check out our extensive range of technical equipment at nexxis.com and make some advantageous changes to your asset condition monitoring and maintenance programme today.