Thickness Gauging vs Flaw Detection on a Blade Surface

The dimensions that decide

Which way the call goes

The two questions surface at different points in a blade’s life. Erosion asks the gauge, because the decision between a cosmetic touch-up and a structural grind and fill sits between 0.5 mm and 3 mm of loss, and only a thickness returns that split. A cap or a bond line asks the flaw detector, because the answer is a defect against a Table G.1 limit rather than a remaining wall. A surface platform carries both, a 5 MHz dual-element gauge for thickness and an array for flaws, so a single deployment can answer both questions and the decision does not wait for a second visit. On a 5 MW offshore machine losing EUR 30,000-75,000 a year to erosion, that turnaround is not a detail.

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.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.

The numbers below are drawn from the equipment records and field campaigns this site already publishes, not from a generic catalogue.