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Spar Cap and Shear Web Bonding Inspection Solution for Wind Turbine Blades


Application-Specific, Low-Frequency Phased Array Probes and Holders for the Inspection of GFRM and CFRM Wind Turbine Blades

Application

Ultrasonic phased array solution for the inspection of spar cap and shear web bonding in wind turbine blades.

Background

Throughout its operating life, a wind blade is subjected to considerable lift forces. To ensure the essential shear strength of the blade assembly, the top and bottom blade shells are bonded together around a set of shear webs. The spar cap, the part of the shell that is bonded to the shear webs, is usually made of thick GFRM (glass fiber-reinforced materials) or CFRM (carbon fiber-reinforced materials) for additional structural solidity. The integrity of the wind blade is highly dependent on the quality of the bonding between the shear webs and spar caps.

Typical cross-section of a wind turbine blade
Typical cross section of a wind turbine blade

To verify the bonding integrity, Olympus has developed a new phased array inspection solution. The kit is fully compatible with OmniScan® MX2 and SX flaw detectors, as well as the FOCUS LT and FOCUS PX data acquisition instruments.

Problem

As the shear web and the spar cap are bonded together by a layer of adhesive of varying thickness, there are two interfaces that must be examined: (1) between the spar cap and the adhesive and (2) between the adhesive and the shear web.

In addition to the structural complexity of wind blades, the acoustically unfriendly nature of the assembly materials can be an inspection hurdle. Wind blade shells are typically fabricated using fiberglass, and the adhesive is made of epoxy. These materials attenuate the ultrasonic beam very rapidly, making ultrasonic examination extremely challenging.

As standard probes and holders are not adapted for wind blade inspections, we have developed an improved phased array solution that features an optimized probe and holder design.

While the OmniScan flaw detector is the instrument of choice for manual or semiautomated inspection during manufacturing or in-service inspection, the FOCUS PX acquisition instrument can be used as part of a customized automated inspection system in manufacturing.

Solution

This solution is based on a large aperture low-frequency phased array probe that is mounted on a holder. The holder can be fitted with an encoder for manual encoded inspection or mounted on a GLIDER scanner for semiautomated 2-axis mapping. The ultrasonic beam is multiplexed across the probe while it is moved on the blade.

There are two main probe holders: the semicontact design positions the probe surface close to the part surface, whereas the AQ25 design features a 25 mm Aqualene delay line.

Wind turbine blade test sample
Wind turbine blade test sample

The semicontact holder is ideal for inspecting thicker sections of the blade. Its high-energy ultrasonic beam penetrates deeper into the part without any repeated surface echo. The drawback is an increased dead zone close to the surface.

The Aqualene holder improves the resolution near the surface and is therefore more adapted for thinner components (up to 40 mm thick).

Both designs come in a flat or contoured variation. While the contoured model is ideal for scanning along the length of the blade, the flat model can be used to scan across the width.

Equipment

This phased-array solution is composed of the following items:

Item Number

Part Number

Description

Application Recommendation

PA Probes

Q3300971

0.5L64-96X22-I5-P-5-OM

0.5 MHz I5 type linear phased array probe, 64 elements, 96 × 22 mm total active aperture, 1.50 mm pitch, 22 mm elevation, 5 m long cable.

Greater penetration in very attenuative and/or thick materials.

Q3300970

1L64-96X22-I5-P-5-OM

1 MHz I5 type linear phased array probe, 64 elements, 96 × 22 mm total active aperture, 1.50 mm pitch, 22 mm elevation, 5 m long cable.

General purpose, better resolution.

Probe Holders

Q7201106

SI5-0L-WHC

Flat semicontact probe holder for I5 PA probe.

To scan across the length of the blade. Required on parts thicker than 40 mm.

Q7201114

SI5-0L-WHC-COD1978-4414MM

Curved semicontact probe holder for I5 PA probe.

To scan along the length of the blade. Required on parts thicker than 40 mm.

Q7201108

SI5-0L-AQ25

Flat Aqualene delay line probe holder for I5 PA probe.

To scan across the length of the blade. For increased near-surface resolution on parts up to 40 mm thick.

Q7201107

SI5-0L-AQ25-COD1978-4414MM

Curved Aqualene delay line probe holder for I5 PA probe.

To scan along the length of the blade. For increased near-surface resolution on parts up to 40 mm thick.

Encoding Systems

U8775296

ENC1-5-LM

Mini-Wheel encoder, 5 m long cable with LEMO® connector compatible with the OmniScan MX2 and SX flaw detectors.

Manual encoded inspection.

Q7750157

Y-PA-65x64-5Deg

Yoke to mount an SI5 probe holder to a GLIDER scanner.

Semiautomated encoded inspection with the GLIDER scanner.

Q7500034

Glider-72x24

Two-axis encoded scanner with a manually activated vacuum cup mounting system. 72-in. stroke on the fixed axis (X) and 24-in.stroke on the mobile axis (Y).

Semiautomated encoded inspection with the GLIDER scanner.

Aqualene holder
Aqualene holder



Semicontact holder
Semicontact holder
Phased array probe
Phased array probe

Results

Test #1: Thick spar cap volume inspection

Tests were performed on sectioned samples of wind turbine blades.

Tests were performed on sectioned samples of wind turbine blades.

The following figure shows the results obtained with the semicontact holder and the 1 MHz probe on a 50 mm thick sample. The reflectors are two 12.5 mm flat-bottom holes (FBH) positioned at 16 mm and 32 mm deep.

They simulate delamination of the skin. Both indications are easily detected in both the time-of-flight and amplitude C-scans.

C-scans

Test #2: Shear web bonding inspection

Tests were performed on a wind blade in manufacturing using a customized 2-axis encoded scanner that is similar to the GLIDER scanner. The data were acquired with an OmniScan® MX2 with a 1 MHz I5 PA probe and semicontact holder.

GLIDER scanner

C-scan

The C-scan is used to have a comprehensive view of the bonding of the two shear webs. The two blue lines represent the bonding interfaces of the shear webs with the spar cap. The ultrasound beam propagates in the shear webs, resulting in a lower amplitude for the return signal. The C-scan can also be used to measure the width of the bond using measurement cursors. In this test, the width was approximately 130 mm. The red areas represent where there is no bonding. There, we observe that the reflected signal from the spar cap back wall is strong.

C-scan

In this application, the thickness of the adhesive was great enough that both interfaces can be distinguished. Using the measurement cursors in the S-scan and A-scan views, the adhesive was determined to be 15 mm thick.

S-scan and A-scan views

To inspect large areas such as wind blades, the use of a 2-axis encoded scanner can be beneficial. The GLIDER scanner is now available in a format optimized for the wind blade application. The long axis of the GLIDER scanner, which features a total stoke of 72 inches, is placed along the length wind blade. The length of the second axis is 24 inches so that it can cover typical shear web configurations.

GLIDER scanner

Test #3: Thin spar cap volume inspection

This test was performed on a sample featuring a 12.5 mm flat-bottom hole (FBH), simulating lamination in the spar cap. In this case, the spar cap is relatively thin (7.7 mm). For that reason, the Aquelene holder (AQ25) was chosen because of its ability to detect defects closer to the surface. The probe is a 1 MHz I5.

Aquelene holder

In the image below, we have a clear view of the simulated defect located at 3.6 mm under the surface.

A clear view of the simulated defect located at 3.6 mm under the surface.

Conclusion

Olympus has developed a phased array solution dedicated to the inspection of spar cap and shear web bonding. Although the acoustic attenuation and shape and structure of wind blades makes them a challenge to inspect, the carefully conceived design of this solution resolves those issues while providing high-resolution data and imaging. Inspecting the structural integrity of wind blades can now benefit from all the advantages of phased array ultrasound, enabling higher PODs and less operator dependent inspections.

Olympus IMS

Products used for this application


GLIDER Scanner

The GLIDER scanner is a 2-axis (X-Y) encoding scanner for the manual inspection of slightly curved or flat composite surfaces. The scanner, held by suction cups is well suited for raster scanning. Technologies: ECA, EC, UT, PA.

OmniScan MX2

The OmniScan MX2 now features a new phased array module (PA2) with a UT channel, and a new two-channel conventional ultrasound module (UT2) that can be used for TOFD (Time-of-Flight Diffraction), as well as new software programs that expand the capabilities of the successful OmniScan MX2 platform.

OmniScan SX

The single group, lightweight OmniScan SX features an easy-to-read 8.4 inch (21.3 cm) touch screen and provides cost-effective solutions. The OmniScan SX comes in two models: the SX PA and SX UT. The SX PA is a 16:64PR unit, which, like the UT-only SX UT, is equipped with a conventional UT channel for P/E, P-C or TOFD inspections.

Phased Array Probes

Phased array application-specific probes have a range from 0.5 MHz to 18 MHz and may come with 16, 32, 64, or 128 elements. Special probes may have up to hundreds of elements.
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