Ultrasonic vs Eddy Current Testing: Which and When
Ultrasonic and eddy current testing are both instrumented NDT methods, but they detect different things. This guide explains when each is the right choice.
Ultrasonic testing (UT) and eddy current testing (ET) are the two instrumented NDT methods most often confused by owners, partly because both require specialist equipment and trained operators. In practice they answer very different questions. Understanding the distinction helps owners follow an inspection report and ask sensible questions about which method was applied where.
How each method works
Ultrasonic testing sends high-frequency sound waves into the component and interprets the returning echoes. Because sound travels through the full thickness of the material, UT can reveal sub-surface flaws and measure remaining wall thickness. It is the method of choice for internal defects and for corrosion that attacks a member from the inside.
Eddy current testing induces electrical currents in a conductive material and senses the disturbance caused by a crack. It is a surface and near-surface method, but its distinctive strength is the ability to detect cracking through thin coatings and around fastener holes without disassembly or stripping paint.
Strengths, limits and typical applications
UT is unmatched for thickness measurement and for interrogating thick welds and shafts, but it needs good surface coupling and skilled interpretation, and it is less convenient over complex or coated geometries. ET is fast over fastener regions and coated surfaces and works on non-magnetic conductors such as aluminium, but it is limited to surface and near-surface defects and needs careful calibration.
On a typical EWP or crane, UT is used to verify boom and chassis wall thickness where internal corrosion is suspected and to examine thick load-bearing welds, while ET is used to inspect bolt holes and coated surfaces for fatigue cracking without dismantling the joint.
- UT: sub-surface flaws and remaining wall thickness
- ET: surface cracks through thin coatings, around fastener holes
- UT needs coupling and skilled interpretation
- ET works on aluminium and other non-magnetic conductors
How the engineer decides
The choice is driven by the question being asked. If the concern is loss of section from corrosion, or a defect buried within thick steel, ultrasonic testing is selected. If the concern is fatigue cracking at coated surfaces or fastener holes that cannot easily be dismantled, eddy current is the better fit. Often both are used on the same machine at different locations.
Neither method replaces surface techniques such as magnetic particle or dye penetrant inspection for exposed welds. A competent inspection blends methods so that each critical component is examined by the technique best suited to detect its most likely failure mode.
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