Dry Laminate and Resin Starvation: Detection and Fix

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

TL;DR Acceptance: dry laminate and resin starvation are not permitted at any size per Table G.1. Signature: rising attenuation and fading back-wall amplitude, not a discrete reflector. Method: PAUT amplitude channel for the drop, TOFD to confirm no separate volumetric cluster. Calibration: amplitude referenced to a 6 mm squared flat-bottom hole at 20 mm depth before the run.

The scan shows a region of low back-wall amplitude with no discrete reflector, in a cap that looks normal from the outside. Dry laminate is resin-starved fibre, so there is no adhesive to reflect from and no void to scatter. What remains is a region where the ultrasonic attenuation climbs and the back-wall echo fades. That is a different signature from porosity and from a flat separation, and it is the amplitude drop that marks it. IEC 61400-5 Table G.1 does not permit dry laminate at any size, so the moment the signature appears the disposition changes from accept to rework.

Common root causes

Root cause	What you see	Where it bites
Resin shortfall in a thick stack	a low-amplitude region with no discrete reflector and no through-thickness loss	thick unidirectional cap buildups
Vacuum loss during infusion	a broad region of fading back-wall echo across a panel	large laminates under bagged infusion
Premature gel in a slow wet-out	resin-starved fibre left where the front set early	wide caps and cold shop conditions
Local dry patches at ply drops	an amplitude drop confined to a ply transition	ply drop-offs and cap edges

What the numbers allow

The disposition is absolute. Table G.1 lists dry laminate and resin starvation as not permitted, so there is no size below which it is acceptable. The scan detects it by attenuation and back-wall amplitude rather than by reflector geometry, which is why the amplitude channel has to be calibrated, not just the time-of-flight. A 2 dB gain error matters here in the other direction, because it can hide the very drop that defines the defect. Reference the drop to a 6 mm squared flat-bottom hole at 20 mm depth and a 2 dB error stops being invisible, because the gain is set against a known reflector rather than a neighbouring region.

Acceptance: dry laminate and resin starvation are not permitted at any size per Table G.1
Signature: rising attenuation and fading back-wall amplitude, not a discrete reflector
Method: PAUT amplitude channel for the drop, TOFD to confirm no separate volumetric cluster
Calibration: amplitude referenced to a 6 mm squared flat-bottom hole at 20 mm depth before the run

How to confirm it

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

Calibrate the amplitude channel. Reference the gain to a known flat-bottom hole before the run, because the defect is defined by an amplitude drop and an uncalibrated channel hides it.
Map the back-wall echo. Sweep the cap and watch the back-wall amplitude across the whole width, since a fading echo is the primary marker of resin starvation.
Rule out a planar reflector. Use the planar channel to confirm there is no discrete separation, because a flat reflector also drops the back-wall echo but carries its own signature.
Bound the affected area. Outline the low-amplitude region and size it, because the disposition is rework regardless of size and the outline drives the repair scope.
Record against the serial. Log the region and its coordinates to the blade serial so the rework instruction reaches the station and the process can be corrected at source.

Prevention

Control wet-out against the gel time, and hold vacuum integrity through the whole infusion so the resin front never sets ahead of the fibre. Then treat the amplitude channel as a process meter, because a dry region is a report about the infusion line, not a surprise about one blade.

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

Frequently Asked Questions

Is any amount of dry laminate acceptable?

No. IEC 61400-5 Table G.1 lists dry laminate and resin starvation as not permitted, so there is no size at which the disposition becomes accept. The scan confirms the signature and the region goes to rework.

How is dry laminate detected if there is no void?

By attenuation and back-wall amplitude rather than by a discrete reflector. There is no adhesive to reflect and no void to scatter, so what the scan records is a fading back-wall echo across the region.

Why does gain calibration matter more here?

Because the defect is defined by an amplitude drop. A 2 dB gain error can hide the drop that defines it, so the run is referenced to a 6 mm squared flat-bottom hole at 20 mm depth before the scan starts.

What is the repair scope for a dry region?

It follows the outline the scan records. Because the disposition is rework at any size, the value of the scan is bounding the region and giving the repair team its coordinates before the shell is closed.

Bottom line: Dry laminate has no reflector to find, only a fading back-wall echo, and Table G.1 does not permit it at any size. Calibrate the amplitude channel, map the echo across the cap, and bound the region so the rework scope is set from the scan rather than from a guess.