field inspection of wind turbine blades using a microwave

Copyright © 2016 Wetzel Engineering Inc., All Rights Reserved.

BLADE INSPECTION & REPAIR – FIELD INSPECTION OF WIND TURBINE BLADES USING A MICROWAVE INTERFEROMETRIC METHOD

KYLE K. WETZEL, PH.D. [email protected]

WETZEL ENGINEERING, INC. 3914 GATTIS SCHOOL RD., SUITE 103 ROUND ROCK, TEXAS 78664 TEL: (512) 989 9194

CANWEA O&M SUMMIT FEBRUARY 24, 2016

Copyright © 2016 Wetzel Engineering, Inc. All Rights Reserved.

Wetzel Engineering Inc • Established 2001

• New Product Engineering Wind Turbine System Optimization

Rotor Blade Aero and Structure

Wind Turbine Controls

Composites Manufacturing & Repair

• Blade Forensics Engineering Root Cause Analysis

Design

Manufacturing

Operational Conditions

Engineered repair solutions

Assessing residual life

Copyright © 2016 Wetzel Engineering, Inc. All Rights Reserved.

Acknowledgements • Alex Tran, WEI Engineering Lead for

Structural Engineering • Responsible for honing the MW

technique and shop validation • Rob Woodward, Evisive Technologies

• Sr. Technician on the Project • Key Expert on the MW technique


CASE STUDY: BLADE MANUFACTURING DEFECT Situation –

6 year old blades failing due to OEM-acknowledged manufacturing defect No other engineering support from the OEM Owner trying to claw back warranty support to the extent of blade replacement Owner needs to understand long-term risk

Visual inspections could not fully characterize extent of damage internally and no assessment externally

Microwave NDT campaign on a sampling of blades combined with engineering analysis of the impact of the defect on life estimation provided a quantitative assessment of the number of blades at risk for premature failure – defined a monitoring protocol


BLADE ANATOMY & INSPECTION SCOPE

Blade Sandwich Shell Construction –

Embedded spar caps & sandwich balsa

core material

Fiberglass face sheets

Standard layout & construction – shell

panel buckling resistance

Thick wall, twall > ~ up to 30mm or 1.5 in.

Blade Inspection Area –

Internal & external blade surfaces

Manufacturing defect – wrinkles/marcelling

of fiberglass layers

Impact of marcelling, cracking & disbonding

evaluated

Correlation of defects/wrinkles in sandwich

structure

Near root region to area of max chord


APPLIED MICROWAVE THEORY

B A Transmitter

¼ λ

Transmitter

A B

Receivers Object being examined

Defect

Microwave NDT & Setup –

Near field technique – interferometric response

Non time-based signal, one-sided technique

Sum of transmitted and received signal – phase &

amplitude

Change in voltage – presence of indications

24 GHz probe setup:

Penetration depth adequate to penetrate

fiberglass layers, not completely through

balsa core

Freq. provide sufficient resolution in C-Scan

images to distinguish many features to

interpret & evaluate


LAB INSPECTION VALIDATION

Marcelling Crack

Marcelling

Multiple wrinkles in close proximity

Single, large wrinkle

Crack and surrounding delamination

Scores in Balsa Wood

Test samples fabricated in-house

Validate defects can be identified

Establish criteria for distinguishing – wrinkles,

cracks and disbanding for tracking damage

progression

Scores in balsa core identified in scan image

Verified depth of penetration through fiberglass achieved

Sufficient lab samples for validation

Viable solution for identifying volumetric indications on blade in the field


FIELD IMPLEMENTATION OF MICROWAVE NDT

Challenges in hardware –

Scan imaging inspection hardware

Speed & accuracy

Lightweight & robust

Adaptable to internal & external blade contours

Modular for assembly & disassembly

Tight clearance to area of interest –

Internal blade access

External blade platform


Evident in images – Balsa scores (running horizontal & vertical)

Fiberglass fabric overlay

Spar cap is also evident

External Scan Results – Linear indications run axially, appear jagged in shape

Indications could not be attributed to any type of embedded geometry

Indication of marcelling or cracking

Indications/defects in the bonds within fiberglass to spar were easier to interpret

FINDINGS & EVALUATION OF SCAN RESULTS

Interior Scan Results– Surface irregularities were present on interior

scanning surface

Scan images were similar to the interpretation of radiograph

Images circled are result of either cracks or marcelling

Combined visual identification of some findings - surface is not painted and natural light shine through to the inside

Allows for distinguishing difference between subsurface geometry and flaws


CONCLUSIONS First article application for field inspection

Microwave NDT technique identified subsurface fiberglass wrinkling &

marcelling

Scanning/inspection hardware setup:

Effective but can be improved

Need for a practical setup to maintain probe normality to surface

Improvements in setup will aid in improving accuracy of data

interpretation and reduced data processing time.

Value of one sided technique to provide similar C-scan image results

as ultrasonic or radiograph – couplant is not required

Lab validation & field implementation of technique was proved to be

successful and a cost effective solution for blade inspections

field inspection of wind turbine blades using a microwave

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