Ultrasonic vs Thermography for Blade Defect Detection

Author: RAS Thermal Co., Ltd. · Updated 2026-10-07

TL;DR Returns: Thermography a thermal contrast, no scale, Ultrasonic Testing thickness and depth at plus or minus 0.1 mm. Strongest at: Thermography hidden coating disbond and near-surface separation, Ultrasonic Testing through-thickness defects and bond-line voids. Blade case: Thermography coating disbond a drone would miss, Ultrasonic Testing spar cap delamination, erosion depth. Limit: Thermography a 1.5 mm and a 4 mm scar look alike, Ultrasonic Testing needs couplant and surface access.

Thermography and ultrasonic testing both find defects a camera cannot, and they part company on what they return. Thermography reads a contrast across a surface, ultrasonic testing reads a depth. A blade programme uses the first to find where to look and the second to decide what to do.

The dimensions that decide

Dimension	Thermography	Ultrasonic Testing
Returns	a thermal contrast, no scale	thickness and depth at plus or minus 0.1 mm
Strongest at	hidden coating disbond and near-surface separation	through-thickness defects and bond-line voids
Blade case	coating disbond a drone would miss	spar cap delamination, erosion depth
Limit	a 1.5 mm and a 4 mm scar look alike	needs couplant and surface access

Which way the call goes

The limit of thermography is that a contrast carries no scale, so a 1.5 mm scar and a 4 mm scar look the same on the thermal feed. That single fact is what the repair decision runs into, because the split between a cosmetic touch-up and a structural grind and fill sits inside that range. Ultrasonic gauging returns remaining wall to plus or minus 0.1 mm with a 5 MHz dual-element transducer, so where the depth decides the repair the ultrasonic pass is the one that can. Thermography earns its place by finding the hidden separation cheaply across a whole blade, then handing the marked zones to the method that can size them.

How to choose between them

Run these checks in order. By Step 5 the choice follows from the geometry and the threshold rather than from habit.

Fix the acceptance threshold. Start from the threshold the zone carries, not from the method you happen to own. IEC 61400-5 Section 11 asks for volumetric detection of indications 6 mm and larger in the laminate plane, and DNV-ST-0126 tightens the leading edge to 3 mm, so the threshold is set before the probe is chosen.
Read the geometry off the drawing. Measure the thickness and the curvature the probe has to travel. A spar cap up to 80 mm of unidirectional laminate and a root transition of changing radius are two different scan problems, and the radius decides how the array is coupled.
Choose the modality pair. Pick modules by defect geometry, not by habit. PAUT images planar separations that lie parallel to the plies, TOFD catches volumetric clusters whatever their orientation, and running both on a 256-channel system puts the two mechanisms in one pass.
Automate the scan path. Put the probe on an encoder-driven carriage and let the path, not the technician, set coverage. A 600 mm spar cap falls to one longitudinal pass at 500 mm/s on a linear array, where a single-element probe needs 20 parallel passes and runs about five times longer.
Lock the evidence chain. Write the coverage map, the daily calibration and the indication log to the blade serial while the scan is running. It is the difference between a 3-day type-certification audit and a 3-hour document review, and it is what ISO 9712 qualification is there to sign.

These are the working numbers from our own machine and campaign records, restated rather than re-pitched.

Frequently Asked Questions

Can thermography replace ultrasonic testing on a blade?

No. Thermography returns a contrast with no scale, so it cannot separate 1.5 mm of loss from 4 mm, which is exactly the range a cosmetic or structural decision sits in. It locates, and ultrasonic testing sizes.

What is thermography best at on a blade?

Hidden coating disbond and near-surface separation. It reads the change a subsurface air gap makes to thermal decay, so it finds a separation under an intact surface that a visual pass would miss.

What does ultrasonic testing add?

A depth. A 5 MHz dual-element transducer maps remaining wall thickness to plus or minus 0.1 mm, so the repair decision gets a number rather than a region, and 100 % bond-line coverage comes from the same pass.

Which comes first in a campaign?

Thermography, cheaply across the blade, then ultrasonic testing on the marked zones. The thermal map decides where to look and the ultrasonic pass decides what to do, so the two run in sequence rather than in competition.

Bottom line: Thermography returns a contrast and ultrasonic testing returns a depth, so use the first to find hidden separation and the second to size it. Keep thermography for locating and let the plus or minus 0.1 mm thickness number make the repair call.