# IEC 61400-5 NDT Compliance Guide

> IEC 61400-5 Annex G sets blade manufacturing NDT requirements: which ultrasonic methods satisfy it, how compliance is documented, what auditors check.


**IEC 61400-5 is the governing standard for wind turbine blade design and manufacturing. Annex G defines the NDT requirements. If you manufacture blades for the European market, you comply with it, or you do not sell blades.**

> **TLDR:** IEC 61400-5 requires volumetric NDT of all critical bond lines and spar caps. PAUT + TOFD is the only practical method combination that satisfies the standard at production speed. Documentation must include scan coverage maps, calibration records and acceptance criteria traceable to the standard's Table G.1.

## What IEC 61400-5 Actually Requires

The standard divides blade inspection into three categories:

1. **Manufacturing QC (Section 11):** 100 % volumetric inspection of primary bond lines (spar cap-to-shell, shear web-to-shell) and spar cap laminates before the blade leaves the factory.
2. **Type Testing (Section 12):** Full-scale structural testing of a prototype blade with NDT before and after static and fatigue testing.
3. **In-Service Inspection (Annex H):** Periodic inspection requirements for operating blades, less prescriptive but references the same NDT methods.

The key phrase in Section 11 is *"volumetric NDT method capable of detecting indications ≥ 6 mm in the plane of the laminate."* This rules out visual inspection and tap testing. It requires either ultrasonic testing or radiography, and radiography is impractical on blades thicker than 60 mm of glass fibre.

## Which NDT Methods Satisfy the Standard?

| Method | Satisfies IEC 61400-5? | Practical at Production Speed? |
|--------|------------------------|-------------------------------|
| Visual inspection | No, surface only | Yes but insufficient |
| Tap testing | No, not volumetric | Yes but insufficient |
| Conventional UT (single-element) | Yes, nominally | No, too slow for 100 % coverage |
| **Phased Array UT (PAUT)** | **Yes** | **Yes, 500 mm/s with full coverage** |
| TOFD | Yes (complementary) | Yes, combined with PAUT |
| Radiography (X-ray/γ) | Yes | No, safety exclusion zone kills production flow |
| Thermography | No, near-surface only | Yes but insufficient depth |
| Shearography | No, near-surface only | Yes but insufficient depth |

PAUT is the only method that simultaneously satisfies the volumetric requirement and the production speed requirement.

## The Documentation Chain

During a type certification audit, the notified body (DNV, TÜV, Lloyd's Register) will check:

1. **Scan coverage maps**, Proof that 100 % of the required zones were scanned, with no gaps
2. **Calibration records**, Daily calibration on reference blocks traceable to national standards
3. **Indication log**, Every indication above the recording threshold, with its C-scan image, coordinates and classification
4. **Acceptance criteria disposition**, For each indication, a documented decision (accept / repair / concession) referencing Table G.1
5. **Operator qualification**, ISO 9712 Level II or III certification for every technician who acquired or interpreted data

An automated NDT system that generates these records automatically, keyed to blade serial number, turns a 3-day audit into a 3-hour document review.

## The Bond-Line Specific Challenge

IEC 61400-5 is particularly strict on adhesive bond lines. The shear web-to-shell bond carries the full aerodynamic load from the blade skin into the structural spar. A void in this bond line is a stress concentration that grows with every fatigue cycle.

Automated PAUT scanning along the bond line, with the probe oriented normal to the adhesive interface, detects voids ≥ 6 mm and kissing bonds where the adhesive has wetted but not bonded to one substrate. The latter requires careful gain calibration, a 2 dB error can mean the difference between detecting and missing a kissing bond.

## DNV-ST-0126: The Offshore Overlay

For offshore blades, DNV-ST-0126 adds additional requirements on top of IEC 61400-5:
- NDT after every major maintenance intervention
- Higher sensitivity for leading-edge inspection (3 mm indication threshold vs 6 mm)
- Corrosion-resistant NDT equipment certification (IP67 / NEMA 4X for offshore use)

## Practical Compliance Pathway

1. **Map your inspection zones** on the blade 3D model, spar caps, bond lines, root transition, trailing edge
2. **Select your modality combination**, PAUT for planar defects + TOFD for volumetric defects
3. **Automate the scan**, A gantry or crawler with encoder feedback guarantees 100 % coverage
4. **Automate the documentation**, Inspection software that generates compliance records per blade serial
5. **Audit internally first**, Run a mock type-certification audit with your own quality team before DNV arrives