A Novel Procedure for the Inspection of Wind Turbine Bolts

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Bolts are used as the fasteners on various sections of the Offshore Wind Turbine (OWT), and they require regular Non Destructive Testing (NDT). The current procedure requires permanent strain gauges, single element ultrasonic inspection, or a mixture of both in order to ensure that the specific preload is maintained throughout an OWT’s operative cycle. Both of these methods have drawbacks: Strain gauges are mainly used on a smaller, experimental scale; and the ultrasonic method, traditionally carried out using single element transducers, has low accuracy and, additionally, the existence of the volumetric defects in the bolt (or a corrosion layer) can affect the stress measurement. This paper suggests a novel method to test bolts in an OWT quickly and efficiently, using a Phased Array Ultrasonic Transducer (PAUT) probe as opposed to a single element transducer. A PAUT probe has the ability to produce both a swept scan and a linear A – scan for each pair of transmitting and receiving elements or, if using the Full Matrix Capture (FMC) technique, a number of acoustic paths equal to the number of elements squared (i.e. 400 paths if using a 20 element probe).
A sector scan was initially performed to identify and roughly size defects, and if any were detected that were within the acceptance criteria a second, targeted scan was performed. Following this, the probe was moved to an optimum position where the acoustic paths will have minimal interaction with the defects and an FMC scan to measure Time of Flight (ToF) was performed, which allowed for stress calculation.
To justify the necessity of considering small acceptable defects in the follow-on ultrasonic stress measurement, two experiments were performed to study the effect of filling engraved text (0.5 mm depth) on the bolt head with gel used to represent the dust and debris from general use. We used a Detachable Active Array Head (DAAH) probe (2.25 MHz, 20 elements, Sonatest, UK) and a PEAK Micropulse 6 (PEAK Ltd, UK) as the controller. Using the PAUT system revealed that the gel caused a 10 ns difference in the ToF. This is equivalent to 40 MPa of stress, or an error of up to 20% of the yield strength (200 MPa). This shows the importance of pre-investigation of defects (PAUT defect detection) and choosing optimum acoustic paths for the follow-on stress measurement to avoid said defects and reduce measurement error.
Original languageEnglish
Publication statusPublished - 16 Jun 2023
EventWind Energy Science Conference 2023 - University of Strathclyde, Glasgow, United Kingdom
Duration: 23 May 202326 May 2023


ConferenceWind Energy Science Conference 2023
Abbreviated titleWESC 2023
Country/TerritoryUnited Kingdom
Internet address


  • offshore renewable energy
  • non-destructive evaluation (NDE)‎
  • ultrasonic transducer
  • phased array (PA) ultrasonic probe
  • stress monitoring


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