Advanced & Mechanised NDT - Eddy Current

Currently, several nondestructive examination (NDE) techniques are used to inspect welds for defects: magnetic particle inspection (MP), liquid penetrant inspection (LP), ultrasonic testing (UT), X-ray testing (RT), and eddy current testing (ET).

All have their advantages and disadvantages:

• Magnetic particle is straightforward and relatively easy to use but is not good for welds that have coatings or wet surfaces (surfaces must be dried first)
• Ultrasonic testing is good for finding subsurface defects
• Dye penetrant is good for surface cracks, but it also requires a clean and dry surface. (Non-coated)
• X-ray is good for subsurface defects, but the radiation hazard requires additional safety considerations
• Eddy current is good for detecting surface-breaking defects, can detect these defects through fairly thick coatings (up to 2 mm), and can be used on wet surfaces (even underwater), but several scans of an individual weld must be performed to ensure a defect is not missed

Eddy Current is best used for the detection of surface-breaking cracks, its most practical applications relate to the in-service inspection of welded structures that are subject to a cyclical loading that can lead to fatigue crack propagation in critical welded areas.

Practical Applications:

• Offshore structures - By far the widest use of eddy current weld inspection occurs in the offshore industry
• Bridges, cranes, traffic signals - All of these structures are subject to cyclical loading on their welded structures, as well as weather-related loading and weight loading in cranes and bridges
• Ships - Ships are subject to bending and torsional moments in day-to-day use
• Submarines - Eddy current is often used in the inspection of new welds and for the detection of fatigue cracks in welded joints
• Amusement park rides - Another relatively recent application is the inspection of amusement park rides

Incotest (INsulated COmponent TEST) is an unique corrosion survey method that allows ferrous objects to be surveyed without the need to make contact with the surface. This means that measurements can be performed on objects covered with insulation, asbestos, fire proofing, concrete or coating. High expense for removal of this coverage can be avoided. It can significantly reduce the associated inspection costs, in some cases as much as 75%.

Using pulse eddy current method for measuring the remaining average wall thickness under insulation of low alloy carbon steel/ferromagnetic object and analyzing their decay and is the most reliable method for corrosion detection.

This is an excellent technique to be applied on objects that are insulated and also can be applied on objects that are difficult or extreme cost accessible. The range of application is enormous and INCOTEST contributes in significantly reducing inspection cost. This pulse eddy current technology is an excellent on-stream tool for positioning further inspection works during shut down or periodic maintenance works.

This system does not require cleaning or removal of concrete/coating/insulation. In most cases scaffolding is not necessary and this results in huge savings in comparison to traditional inspection preparation cost.

Eddy Current Tube Inspection:

EC offers inspection in non–carbon content (non-ferrous) tubes, including non-ferrous finned tubes. Local defects as well as overall wall-loss can be detected and quantified. EC can detect both internal and external defects and can distinguish between them. Cracks can be detected depending on their size and orientation. Normally only a basic cleaning of the tubes is sufficient. EC is notably the fastest of the available tools. Approximately 450 tubes with a standard 6 meter length can be done on a first inspection day. Speed can go up to as many as 700 tubes per day on additional days of inspection. Advantages:

• Fast (450-700 tubes per day)
• Overall wall-loss and local defects can be detected
• High sensitivity to small defects (diam. holes/pits > 0.5 mm)
• Accurate sizing of defects possible

• Only on thin or slightly permeable materials
• Gradual defects could be under classified