Shear Web Bond Delamination: Detection Before Assembly

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

TL;DR Scope: primary bond lines inspected 100 % volumetrically per IEC 61400-5 Section 11. Threshold: 6 mm indication, the same volumetric floor as the cap. Companion method: TOFD for fillet depth alongside PAUT for planar separation. Scale: 100 m blade carries over 1,200 linear metres of bond line.

The web to shell interface reads clean by eye, yet the volumetric scan shows separations along the adhesive fillet. The shear web carries the aerodynamic load out of the skin and into the spar, so its bond line is a structural joint, not a panel. A separation in the fillet is a stress concentration that grows with every one of the 10^8 fatigue cycles a blade sees. Scan the interface while the shell is still open and the fix is a re-bond. Find it after assembly and the fix is a cut-out. Same defect, two very different days.

Common root causes

Root cause	What you see	Where it bites
Incomplete adhesive wetting	the fillet holds voids where the adhesive bridged rather than bonded	web-to-shell fillet and web-to-cap joint
Surface contamination before bonding	the bond forms over dust or release agent and later separates	pre-bond surfaces after handling
Fillet profile shortfall	the joint is geometrically thin where the drawing calls for a full fillet	web ends and transitions
Trapped air during closure	the separation sits at the interface with no surface sign at all	closed shells after assembly

What the numbers allow

The interface is scanned on the open shell as a matter of sequence. Section 11 of IEC 61400-5 calls for 100 % volumetric inspection of primary bond lines, and the shear web to shell bond is one of them. Threshold is 6 mm as for the cap, and the practical companion is TOFD, which reads the fillet depth rather than only its foil. On a 100 m blade the interface runs for a large share of the 1,200 linear metres of bond line, so the pass has to be fast as well as sensitive.

Scope: primary bond lines inspected 100 % volumetrically per IEC 61400-5 Section 11
Threshold: 6 mm indication, the same volumetric floor as the cap
Companion method: TOFD for fillet depth alongside PAUT for planar separation
Scale: 100 m blade carries over 1,200 linear metres of bond line

How to confirm it

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

Scan before the shell closes. Confirm the inspection happens on the open shell, because the access to the fillet disappears the moment the two halves are brought together.
Set the threshold for the zone. Dial in the 6 mm volumetric floor from Section 11 and confirm the reference block covers the fillet geometry you are about to scan.
Orient normal to the interface. Keep the probe normal to the adhesive interface so the beam meets the bond square on, because a tilted pass reads the adhesive thickness as noise.
Add TOFD for fillet depth. Bring in TOFD to measure how deep the fillet actually runs, since planar PAUT alone will not tell you whether the joint is geometrically thin.
Log the voids against the serial. Write every void to the blade serial with its coordinates so the rework instruction reaches the operator while the shell is still accessible.

Prevention

Treat the fillet as a structural joint in the work instruction, not as adhesive that happens to be there. Control the pre-bond surface, hold the fillet profile to the drawing, and scan on the open shell so a void becomes a re-bond rather than a cut-out. Sequence is the cheapest fix in the whole programme.

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

Frequently Asked Questions

Why scan the shear web bond before assembly?

Because access to the fillet goes away once the shell closes. On the open shell a void is a re-bond, and after assembly it is a cut-out, so the same defect lands as two completely different repair bills.

What is the acceptance limit for a bond-line void?

IEC 61400-5 Section 11 sets a volumetric floor of 6 mm for the primary bond lines, the shear web to shell joint included. Anything at or above that floor is recorded against the blade serial.

Is PAUT enough for a bond line, or is TOFD needed?

Run both. PAUT images the planar separation along the interface, and TOFD reads the fillet depth, so the one-pass pair tells you both whether the bond separated and whether the joint is geometrically thin.

How much bond line does a large blade carry?

A 100 m blade carries over 1,200 linear metres of adhesive bond line across shear webs, spar caps and trailing-edge joints, which is why the interface scan has to move at production speed rather than at hand-scan speed.

Bottom line: The web to shell bond is a structural joint scanned on the open shell, where a void costs a re-bond rather than a cut-out. Set the Section 11 floor, keep the probe normal to the interface, and let TOFD confirm the fillet depth in the same pass. Sequence decides the repair bill.