PAUT vs TOFD on Blade Laminates: Complement, Not Rival

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

TL;DR Strongest at: PAUT planar separations and dry laminate, TOFD porosity clusters and volumetric voids. Geometry: PAUT a plane through the thickness, TOFD scatter through the volume. Blade case: PAUT spar cap delamination, kissing bonds, TOFD porosity against the 3 mm cluster limit. Deployment: PAUT 64-element array, 40-70° sector, TOFD paired transmitter and receiver, same carriage.

PAUT and TOFD are not competing methods on a blade, they are two answers to two defect geometries. PAUT images planar separations that lie parallel to the plies, TOFD reads volumetric features whatever their orientation. Picking one and dropping the other leaves a class of defects unmeasured.

The dimensions that decide

Dimension	PAUT	TOFD
Strongest at	planar separations and dry laminate	porosity clusters and volumetric voids
Geometry	a plane through the thickness	scatter through the volume
Blade case	spar cap delamination, kissing bonds	porosity against the 3 mm cluster limit
Deployment	64-element array, 40-70° sector	paired transmitter and receiver, same carriage

Which way the call goes

The reasons the two travel together are the two defect classes and their two limits. Table G.1 caps delamination at 6 mm and a porosity cluster at 3 mm, so a pass that reads planes but not volumes can certify a cap that carries a cluster near the tight limit, and a pass that reads volumes but not planes can miss a flat separation that a light tap would not reveal. A 256-channel system runs both mechanisms in one pass and overlays the amplitude map with the depth data, so the accept decision has the geometry it needs for each class rather than for the one the method happens to be good at.

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

Which is better, PAUT or TOFD?

Neither, they answer different questions. PAUT images planar separations parallel to the plies, and TOFD reads volumetric features whatever their orientation, so a blade pass runs both rather than choosing one.

Why does a blade need TOFD if PAUT is running?

Because porosity is volumetric and does not sit in a plane. TOFD reads the scatter a PAUT sector can understate, and the porosity limit of 3 mm is tight enough that understating it matters.

What does PAUT add where TOFD is already running?

Because TOFD is not aimed at planar separations. A delamination lying parallel to the plies is what PAUT images, and it is what the 6 mm delamination limit is applied to.

Can both run at once?

Yes, on a single carriage and a 256-channel system. The two mechanisms are collected in the same pass and the C-scan overlays the PAUT amplitude map with the TOFD depth data, so one scan answers both limits.

Bottom line: PAUT and TOFD do not compete, they cover two defect classes and their two Table G.1 limits. Run both on one carriage, overlay amplitude with depth, and let a single pass answer the 6 mm delamination and the 3 mm porosity limits together.