Non-destructive sub-surface inspection of multi-layer wind turbine blade coatings by mid-infrared Optical Coherence Tomography

Lapre, Coraline; Petersen, Christian Rosenberg; Nielsen, Per; Wulf, Thomas; Bech, Jakob Ilsted; Fæster, Søren; Bang, Ole; Israelsen, Niels Møller

doi:10.5194/wes-2025-237

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https://doi.org/10.5194/wes-2025-237

Preprints

04 Dec 2025

| 04 Dec 2025

Status: this preprint is currently under review for the journal WES.

Non-destructive sub-surface inspection of multi-layer wind turbine blade coatings by mid-infrared Optical Coherence Tomography

Coraline Lapre, Christian Rosenberg Petersen, Per Nielsen, Thomas Wulf, Jakob Ilsted Bech, Søren Fæster, Ole Bang, and Niels Møller Israelsen

Abstract. Non-destructive inspection (NDI) is useful in the industrial sector to ensure that manufacturing follows defined specifications, reducing the quantity of waste and thereby the cost of production. Optical Coherence Tomography (OCT), a well-known diagnostic technique in medical and biological research, is increasingly being used for industrial NDI. In the mid-infrared (MIR) wavelength range, OCT can be used to characterise parts and defects not possible by other industry-ready scanners, and enables better penetration than conventional near-infrared OCT.

In this article, we demonstrate NDI of wind turbine blade (WTB) coatings using an MIR OCT scanner employing light around 4 µm from a supercontinuum laser source. We inspected the top two layers of the coating (topcoat and primer) in two different samples. The first is to determine the maximum penetration depth, and the second one is to imitate defect identification. The results of our study confirm that MIR OCT scanners are promising for coating inspection and quality control in the production of WTBs, with performance parameters not achievable by other technologies.

Received: 04 Nov 2025 – Discussion started: 04 Dec 2025

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Coraline Lapre, Christian Rosenberg Petersen, Per Nielsen, Thomas Wulf, Jakob Ilsted Bech, Søren Fæster, Ole Bang, and Niels Møller Israelsen

Status: final response (author comments only)

RC1:
'Comment on wes-2025-237', Anonymous Referee #1, 18 Dec 2025
The paper
“Non-destructive sub-surface inspection of multi-layer wind turbine blade coatings by mid-infrared Optical Coherence Tomography”
By
Lapre et al.,
presents an application of mid-infrared (MIR) optical coherence tomography for non-destructive inspection of wind-turbine-blade (WTB) coatings. The study is timely, addressing an industrially relevant need for high-resolution subsurface imaging that exceeds the penetration capabilities of near-infrared OCT while avoiding the logistical and safety constraints of X-ray computed tomography.
The paper is generally well structured, with an informative comparison against NIR-OCT and X-ray CT, and convincingly demonstrates that MIR OCT can resolve the topcoat and primer layers and detect various artificial defects.
Nonetheless, the manuscript requires revisions before it is suitable for publication, primarily to strengthen quantitative analysis, clarify methodology, and refine claims about industrial applicability.
More specifically:
Penetration depth, defect detectability, and layer-thickness extraction are treated mainly qualitatively. Quantitative rigor is insufficient in several key analyses. The study would benefit from statistical summaries over multiple locations and more formal error metrics.

The attenuation-based interface-detection algorithm seems underdeveloped. The conceptual description (Fig. 5) is helpful, but the lack of methodological detail, no validation against ground truth, and poor performance in defected areas weaken the Conclusions. Suggested claims about future integration of deep learning are reasonable, but they should not compensate for the current missing quantitative evaluation.

Industrial scalability claims seem overstated. The speculative calculation about faster scanning times lacks a realistic time-budget analysis and seems optimistic. These observations should be presented more cautiously, experimentally backed up, or omitted.

Key procedures, including refractive-index derivation, OCT post-processing, and sample preparation, need clearer and more rigorous descriptions to allow replicability.

Related to the above, the description of the sample preparation requires greater detail.

The state-of-the-art for damage assessment in GFRP is relatively limited. Many recent contributions can be found for this specific application, for example, in the field of vibration-based monitoring, https://doi.org/10.1016/j.compstruct.2020.112882 and https://doi.org/10.1002/stc.2805. These could be added to the discussion in the Introduction.

The text repeatedly highlights penetration of “~360 µm”. However, it seems that this value corresponds to a single dataset (sample no. 1) and thus is not a generally valid statement. It is one value of one measurement. The authors should provide statistics across several scans.

The figures could benefit from higher contrast and more explicit annotation

The English is overall good, but there are occasional grammatical errors, such as missing articles. A thorough review could be useful.
Citation: https://doi.org/10.5194/wes-2025-237-RC1
RC2:
'Comment on wes-2025-237', Anonymous Referee #2, 20 Dec 2025
As reviewer of the Manuscript wes-2025-237 entitled " Non-destructive sub-surface inspection of multi-layer wind turbine blade coatings by mid-infrared Optical Coherence Tomography", I have thoroughly reviewed the manuscript, and I would recommend addressing the below comments to make the study more wholistic nature to appreciate the NDI of multi-layer wind turbine blade coatings by MIR OCT Technique:
Under Abstract section, there should be executive summary of the quantitative conclusions of the proposed research. The abstract should be rephrased to make it more wholistic of the complete study on the manuscript.

Considering the inspection depth upto two layers, what structural advantages can be accomplished as initially is quoting that the reducing the quantity of waste and thereby cost of production. Please add justification for the proposed study?

What is the capability of the described NDI techniques application on field blades compared to conventional inspection techniques.

What is the threshold capability of inspection depth for the mentioned MIR OCT technique.

Apart from the introducing the application of MIR OCT technique to wind blades from other applications, what are the extended research capabilities addressed in the proposed manuscript?

I hope my critique helps the authors to improve their work and find useful in this review. Thank you!
Citation: https://doi.org/10.5194/wes-2025-237-RC2

Coraline Lapre, Christian Rosenberg Petersen, Per Nielsen, Thomas Wulf, Jakob Ilsted Bech, Søren Fæster, Ole Bang, and Niels Møller Israelsen

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Short summary

We are presenting an optical volumetric scanner used to characterise the coating on wind turbine blades. Two samples were characterised, the first sample was used to define the maximum depth measurement, and the second imitated the usual manufacturing defect inside the coating. We have also developed an algorithm to monitor the thickness of multiple coating layers, which is based on the scattering and absorption properties of the material.


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