Single-Element vs Phased Array on a Blade Spar Cap

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

TL;DR Beam control: Single-Element UT fixed, the probe is moved, Phased Array UT steered and focused electronically. Cap coverage: Single-Element UT 20 parallel passes over a 600 mm cap, Phased Array UT one longitudinal pass. Scan time: Single-Element UT about five times longer, Phased Array UT 500 mm/s, matching takt. Defect reach: Single-Element UT one angle at a time, Phased Array UT a 40-70° sector in one pass.

A single-element probe and a phased array both send ultrasound into a cap. The difference is what each can do between the probe and the defect. A phased array steers and focuses the beam electronically, so the mechanical raster scanning that makes single-element work slow disappears.

The dimensions that decide

Dimension	Single-Element UT	Phased Array UT
Beam control	fixed, the probe is moved	steered and focused electronically
Cap coverage	20 parallel passes over a 600 mm cap	one longitudinal pass
Scan time	about five times longer	500 mm/s, matching takt
Defect reach	one angle at a time	a 40-70° sector in one pass

Which way the call goes

The pass count is the whole argument. A 600 mm spar cap takes one longitudinal pass on a 64-element linear array and 20 parallel passes on a single-element probe, and that is a factor of about five on time before setup is counted. On a blade line that runs a shell per mould per day, five times the scan time does not fit, so the single-element route cannot deliver the 100 % coverage Section 11 asks for at production speed. The array also adds the sector, so a 40-70° sweep images steeply inclined separations a fixed-angle probe would pass over.

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.

Read the figures as sourced, not as marketing. Each one sits in the documentation we already publish on this site.

Frequently Asked Questions

How many passes does a spar cap take on each probe?

One longitudinal pass on a 64-element linear array, and about 20 parallel passes on a single-element probe over the same 600 mm cap. That is a factor of roughly five on scan time.

Why does the pass count matter so much?

Because a blade line runs on takt time. Five times the scan time does not fit a shell-per-mould-per-day cadence, so 100 % coverage of the primary bond lines cannot be delivered at production speed on a single-element probe.

What does the sector scan add?

Angle. A phased array sweeps 40-70° in one pass, so a separation inclined steeply to the surface is imaged, where a fixed-angle single-element probe would pass over it.

Is a single-element probe useless on a blade?

Not useless, limited. It works on thin access-restricted spots where an array will not fit, but it cannot deliver the coverage and the speed the primary structure needs, so it supplements rather than replaces the array.

Bottom line: A phased array steers the beam electronically and a single-element probe must be walked, which is one pass against 20 on a 600 mm cap. That factor of five is why the array carries a blade line, and why 100 % coverage fits inside takt time only on the array.