PAUT for Wind Turbine Blade Spar Caps
PAUT for spar cap inspection: defect classification, IEC 61400-5 acceptance criteria, and how one pass resolves porosity, delamination and dry laminate.
RAS Thermal Co., Ltd.
NDT Applications
The spar cap is the load-bearing backbone of a wind turbine blade. If it fails in service, the blade fails. If it passes QC with undetected defects, the liability sits on the manufacturer’s books for 20 years. PAUT is the only method that can inspect a 60 m spar cap in production time with the sensitivity IEC 61400-5 demands.
TLDR: Phased array ultrasonic testing delivers full volumetric coverage of spar caps at 500 mm/s. Combined with TOFD, it resolves porosity (< 3 mm), delamination (> 6 mm) and dry laminate in a single pass. IEC 61400-5 Table G.1 defines the acceptance criteria, automated PAUT makes them enforceable.
Why Spar Caps Demand Volumetric UT
Spar caps are the thickest laminate sections in a blade, up to 80 mm of unidirectional glass or carbon fibre. Defects that are invisible at the surface, internal wrinkles, resin-starved layers, kissing bonds at ply drops, become critical under the 10⁸ fatigue cycles a blade sees in its lifetime.
Visual inspection and tap testing cannot detect these. Thermography sees near-surface anomalies but cannot resolve through-thickness defects in thick carbon laminates. Only volumetric ultrasonic inspection gives full-depth coverage.
The PAUT Advantage
A phased array probe with 64 elements can steer and focus the ultrasonic beam electronically, generating a sector scan that covers 40-70° through the spar cap laminate in one pass. This eliminates the mechanical raster scanning that makes conventional UT so slow.
At 500 mm/s, a linear PAUT array covers the full width of a 600 mm spar cap in a single longitudinal pass. The same coverage with a single-element probe would require 20 parallel passes and take 5 times longer.
Dual-Modality: PAUT + TOFD
PAUT excels at detecting planar defects parallel to the laminate, delamination and dry laminate. Time-of-Flight Diffraction (TOFD) is complementary: it detects volumetric defects like porosity clusters and resin-starved zones regardless of orientation.
Running both modalities simultaneously on a 256-channel acquisition system gives you both detection mechanisms in one scan pass. The C-scan map overlays PAUT amplitude data with TOFD depth data, producing a complete picture of spar cap integrity.
IEC 61400-5 Acceptance Criteria (Simplified)
IEC 61400-5 Annex G defines the following maximum allowable indications in spar cap laminates:
| Defect Type | Acceptance Limit | Detection Method |
|---|---|---|
| Porosity cluster | ≤ 3 mm diameter, < 3 % area | TOFD + PAUT |
| Delamination | ≤ 6 mm in any direction | PAUT sector scan |
| Dry laminate / resin starved | Not permitted | PAUT amplitude drop |
| Wrinkle / fibre misalignment | ≤ 3° from nominal | PAUT + mechanical scanning |
| Foreign inclusion | Not permitted | PAUT + TOFD |
Automated PAUT enforces these criteria objectively, the software flags every indication that exceeds the threshold, removing operator judgement from the accept/reject decision.
Inline Integration
At a blade plant producing one shell per mould per day, the inspection window is measured in minutes, not hours. The MIS-100 gantry integrates PAUT scanning into the moulding station workflow:
- Post-infusion: PAUT scan of the spar cap within 30 minutes of infusion completion
- Pre-bonding: TOFD scan of bond-line preparation surfaces
- Post-assembly: Full-volume scan of the completed shell before painting
Results from all three stations feed into a single digital record per blade serial number, traceable from lay-up to field service.
Frequently Asked Questions
Why does a spar cap need volumetric UT rather than a surface check?
Internal wrinkles, resin-starved layers and kissing bonds at ply drops sit under an 80 mm laminate and show nothing at the surface. Those defects grow under the 10⁸ fatigue cycles a blade sees in service.
How fast can a phased array probe cover a spar cap?
A 64-element array steers a 40-70° sector through the laminate in one pass at 500 mm/s, so a 600 mm wide spar cap is covered in a single longitudinal traverse.
What does adding TOFD buy over PAUT alone?
PAUT reads planar defects parallel to the laminate. TOFD picks up volumetric porosity clusters and resin-starved zones regardless of orientation, so both mechanisms land in one pass on the 256-channel front end.
Which acceptance limits does IEC 61400-5 set for spar caps?
Table G.1 caps porosity clusters at 3 mm diameter and delamination at 6 mm in any direction, and it permits no dry laminate and no foreign inclusion.
Where does PAUT scanning sit in the moulding workflow?
Three points. A post-infusion scan of the spar cap within 30 minutes of infusion, a TOFD scan of the bond-line preparation surfaces, and a full-volume scan of the completed shell before painting.
Bottom line: If your spar cap QC still relies on manual UT or visual inspection alone, you are accepting defect escape rates that automated PAUT would catch. The capital cost of a gantry system is recovered in the first major warranty claim it prevents.
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