# Dynamic Wind Turbine Blade Inspection Using Micro-Polarisation Spatial Phase Shift Digital Shearography

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## Abstract

**:**

## 1. Introduction

## 2. Methodology

#### 2.1. System Principle and Setup

#### 2.2. Carrier Mask Modulation and Window Selection Phase Map Retrieval

#### 2.3. Filtering Algorithms and Phase Sequence Retrieval

#### 2.4. Steps of the Proposed Method

- S1
- Set up the proposed SPS-DS system described in Section 2.1 and use a heating gun to heat up the area of the WTB surface where there is known defect at the subsurface.
- S2
- Record a sequence of the speckle patterns at the heated area. Decompose the recorded video into frame-by-frame images.
- S3
- From the initial frame at time ${t}_{0}$ to time ${t}_{x}$, generate carrier masks at four phase values in different pixels and form a large mask to modulate the whole image, as expressed in Equation (2).
- S4
- Transform the modulated speckle patterns from time ${t}_{0}$ to time ${t}_{x}$ into 2D in the frequency domain and select the central speckle rings in each speckle pattern as described above with $\mathcal{R}$ denoting the lower frequency in Equation (4). Transform the derived frequency domain patterns into the original domain using inverse 2D Fourier transform to form the complex terms from ${\mathcal{R}}_{0}$ to ${\mathcal{R}}_{x}$.
- S5
- Multiply the derived ${\mathcal{R}}_{x}$ at time ${t}_{x}$ with conjugates of that at time ${t}_{0}$, as described in Equation (7), and calculate the phase map $\Delta {\varphi}_{{t}_{x}}$ at time ${t}_{x}$, as in Equation (8).
- S6
- In each of the initial phase maps derived at ${t}_{x}$, adopt the WFF algorithm in their complex domain and obtain a new filtered phase $\dot{\Delta {\varphi}_{{t}_{x}}}$.
- S7
- Form the sequence from time ${t}_{0}$ to time ${t}_{x}$, as described in Equation (9), produce the dynamic phase map series, and develop the defect variation along with the temperature change due to the dynamic thermal loading as a video sequence.

## 3. Experiments and Discussion

#### 3.1. Pre-Test Using a Composite Sample

#### 3.2. Test on the WTB with Known Defects

#### 3.3. Phase Maps

## 4. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Illustration of the polarisation directions for the micro-polarisation image sensor applied and the corresponding phase shift.

**Figure 4.**(

**a**) Sample composite surface; (

**b**) A continuous force used as the loading method exerted on the composite sample.

**Figure 5.**(

**a**) Setup of conventional TPS-DS system with three-step phase shift; (

**b**) Wrapped phase map retrieved by TPS-DS system.

**Figure 6.**Frequency domain of a modulated shearogram and the window for selecting the lower frequency.

**Figure 7.**(

**a**–

**c**) Development of fringe patterns at time points 10th, 40th, and 70th frame in SPS-DS system; (

**d**–

**f**) Retrieved wrapped phase map at time points 10th ($\dot{\Delta {\varphi}_{{t}_{10}}}$), 40th ($\dot{\Delta {\varphi}_{{t}_{40}}}$), and 70th ($\dot{\Delta {\varphi}_{{t}_{70}}}$) frame in SPS-DS system.

**Figure 8.**(

**a**) WTB sample for inspection and the delamination defects at subsurface circled in red; (

**b**) Heat gun for loading the blade surface.

**Figure 10.**Results derived at three time instants for defect 1 on the WTB; (

**a**–

**c**) Fringe patterns at time instants of 65th frame, 130th frame, and 200th frame; (

**d**–

**f**) Phase maps derived with the proposed method at time instants of 65th frame ($\dot{\Delta {\varphi}_{{t}_{65}}}$), 130th frame ($\dot{\Delta {\varphi}_{{t}_{130}}}$), and 200th frame ($\dot{\Delta {\varphi}_{{t}_{200}}}$); (

**g**–

**i**) Phase maps without passing through WFF filter at time instants of 65th frame ($\Delta {\varphi}_{{t}_{65}}$), 130th frame ($\Delta {\varphi}_{{t}_{130}}$), and 200th frame ($\Delta {\varphi}_{{t}_{200}}$).

**Figure 11.**Results derived at three time instants for defect 2 on WTB; (

**a**–

**c**) Fringe patterns corresponding to the time instants of 65th frame, 130th frame, and 200th frame; (

**d**–

**f**) Phase maps derived with the proposed method at the instants of 65th frame ($\dot{\Delta {\varphi}_{{t}_{65}}}$), 130th frame ($\dot{\Delta {\varphi}_{{t}_{130}}}$), and 200th frame ($\dot{\Delta {\varphi}_{{t}_{200}}}$); (

**g**–

**i**) Phase maps derived without going through WFF filter at time instants of 65th frame ($\Delta {\varphi}_{{t}_{65}}$), 130th frame ($\Delta {\varphi}_{{t}_{130}}$), and 200th frame ($\Delta {\varphi}_{{t}_{200}}$).

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**MDPI and ACS Style**

Li, Z.; Tokhi, M.O.; Marks, R.; Zheng, H.; Zhao, Z.
Dynamic Wind Turbine Blade Inspection Using Micro-Polarisation Spatial Phase Shift Digital Shearography. *Appl. Sci.* **2021**, *11*, 10700.
https://doi.org/10.3390/app112210700

**AMA Style**

Li Z, Tokhi MO, Marks R, Zheng H, Zhao Z.
Dynamic Wind Turbine Blade Inspection Using Micro-Polarisation Spatial Phase Shift Digital Shearography. *Applied Sciences*. 2021; 11(22):10700.
https://doi.org/10.3390/app112210700

**Chicago/Turabian Style**

Li, Zhiyao, Mohammad Osman Tokhi, Ryan Marks, Haitao Zheng, and Zhanfang Zhao.
2021. "Dynamic Wind Turbine Blade Inspection Using Micro-Polarisation Spatial Phase Shift Digital Shearography" *Applied Sciences* 11, no. 22: 10700.
https://doi.org/10.3390/app112210700