Kissing Bond in a Blade Adhesive Joint: Why Gain Decides

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

TL;DR Signature: no bonded-line echo at any gain, an absence rather than a reflector. Control: gain calibrated to a reference, because a 2 dB error changes the outcome. Scope: primary bond lines inspected volumetrically per IEC 61400-5 Section 11. Mapping: encoder indexing the amplitude result onto the joint geometry per blade serial.

The joint passes a visual check and a low-gain scan, yet a calibrated high-gain pass shows a bonded-line signal that never forms. A kissing bond has adhesive wetted to both substrates but no load-transferring bond across the interface. It reflects almost nothing at low gain, so it reads as a sound joint. Raise the gain and the missing bond-line echo appears as an absence, not as a reflection. IEC 61400-5 asks for volumetric evidence on the primary bond lines, and a kissing bond is precisely the case that a low-gain pass will certify as clean.

Common root causes

Root cause	What you see	Where it bites
Adhesive wet out without co-bonding	a bond-line echo that fails to form at any gain, with no reflector to point at	shell to spar and web to shell joints
Release agent or contamination	localised gaps in the bonded line where wetting did not hold	pre-bond surfaces after handling
Cure cycle drift	a joint bonded over part of its length and merely wetted over the rest	long joints curing unevenly
Adhesive age or mix	bond strength low across a run without a visible void	extended pot life in a warm shop

What the numbers allow

The method is a calibrated amplitude pass on the interface. Section 11 wants volumetric evidence for the primary bond lines, and the practical warning from the field is blunt, a 2 dB gain error can be the difference between detecting and missing a kissing bond. So the gain is set on a reference, the bonded-line amplitude is read as an absolute, and the encoder maps the result onto the joint geometry. That is what turns an invisible joint defect into a sized one. A 2 dB gain error is smaller than the amplitude shift a missing bonded-line echo produces, so the reference is set before the run rather than assumed from a neighbouring region.

Signature: no bonded-line echo at any gain, an absence rather than a reflector
Control: gain calibrated to a reference, because a 2 dB error changes the outcome
Scope: primary bond lines inspected volumetrically per IEC 61400-5 Section 11
Mapping: encoder indexing the amplitude result onto the joint geometry per blade serial

How to confirm it

Work down this list in order. The first check that changes the result has found the cause.

Calibrate gain on a reference. Set the gain against a known reference block first, because the defect shows up as an amplitude anomaly and an uncalibrated pass simply will not see it.
Probe normal to the interface. Hold the probe square to the bond line so the bonded-line echo is available at all, since a tilted pass loses the very reflection the test depends on.
Read the bonded line as an absolute. Compare the bonded-line amplitude with the reference rather than with a neighbouring region, because a whole run can be weakly bonded and still look internally consistent.
Sweep the full joint length. Cover the joint end to end with encoder indexing, since a kissing bond can occupy part of a joint while the rest bonds fully.
Log the unbonded span. Record the length and position of any unbonded run against the blade serial so the rework can be scoped from the map rather than from a re-scan.

Prevention

Control the pre-bond surface and the adhesive’s open time so wet out and bonding happen in the same window. Then scan the joint on a calibrated amplitude pass, because the joints that fail this way are the ones that look sound. A calibrated gain is not a technicality here, it is the test.

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

Frequently Asked Questions

What is a kissing bond in a blade joint?

An adhesive joint that is wet out to both substrates but carries no bond across the interface. It reflects almost nothing at low gain, so it reads as a sound joint and only a calibrated high-gain pass exposes the missing bonded-line echo.

Why does a small gain error matter so much here?

Because field experience on blade joints puts it plainly, a 2 dB gain error can be the difference between detecting and missing a kissing bond. That is why the pass is referenced before the scan rather than left to a default.

Can a visual check catch a kissing bond?

No. The joint looks bonded and a low-gain scan agrees with the eye. The defect is an absence of a signal, not a visible flaw, so it only appears when the amplitude is read against a calibrated reference.

Does a kissing bond affect the whole joint?

Not usually. It can occupy part of a joint with the rest bonded fully, which is why the pass is run end to end with encoder indexing rather than spot checked at a few points.

Bottom line: A kissing bond is an absence, not a reflection, and it defeats the low-gain pass that would otherwise certify the joint. Reference the gain, probe square to the interface, read the bonded line as an absolute, and log the unbonded span so the repair is scoped from the map.