## 3 Experiments and Discussion  
### Pre-Test Using a Composite Sample  
The method was initially tested on a simple artificial composite sample surface to establish its feasibility. The composite sample used is shown in Figure 4a. A continuous force, as shown in Figure 4b, exerted at the centre of the sample, was used as the loading method. A conventional TPS-DS system with three-step phase shift using piezoelectric stepper, as shown in Figure 5a, was applied for comparison of the results with the proposed SPS-DS system and the affiliated algorithm. The resulting wrapped phase map calculated with this TPS-DS system is shown in Figure 5b. The temporal phase shift shearography setup with triple captures of speckle frames enables three intensity speckle patterns to be obtained, and this can be derived, using coarse phase distribution, as:  
\[\Delta\phi=arctan\frac{\sqrt{3}(I_{13}-I_{12})}{2I_{11}-I_{12}-I_{13}}-arctan \frac{\sqrt{3}(I_{3}-I_{2})}{2I_{1}-I_{2}-I_{3}} \tag{11}\]  
where \(I_{11}\), \(I_{12}\), and \(I_{13}\) are the loaded speckle patterns and \(I_{1}\), \(I_{2}\), and \(I_{3}\) are the unloaded speckle patterns. The coarse phase map was filtered by the same filtering methods as the proposed algorithm to control the major influencing aspect for comparison purposes.  
Figure 4: (**a**) Sample composite surface; (**b**) A continuous force used as the loading method exerted on the composite sample.  
Figure 3: Flow chart of the proposed method.  
The proposed system's feasibility verification used the same composite sample, while the force exerted on the surface was instantaneous. The adoption of force loading in this step also aimed to validate that the proposed method could be generalised to other loading methods, apart from heat flow, used in the subsequent defect detection hereinafter. The speckle pattern captured by the shearography system is the recording of the sample's recovery process from a loaded state to the original state. Therefore, the results derived from this loading process can be deemed as dynamic. An example of the transformation to frequency domain from the carrier mask-modulated shearogram is shown in Figure 6, where the yellow marked rectangular window set for selecting the lower frequencies converged in the central area of the whole spectrum. The window size could be adjusted according to the filtering effect. The fringe patterns at three points of the time span (10 th, 40 th, and 70 th of a total of 70 frames) can be viewed in Figure 7a-c. Accordingly, the wrapped phase maps retrieved using the proposed method at these time points can be viewed in Figure 7d-f. The development of the phase can be viewed in Video S1. As noted, the accuracy of the result was close to that obtained with static measurement method referred to in Figure 4b (TPS-DS inspection in static condition). This implies that it could be fully used as a novel NDE method for detecting true subsurface flaws on WTBs and even for in situ inspection as it is as dynamically stable as the fringe analysis shearography system. Hereinafter, two true subsurface defects on an actual WTB section were detected by the proposed method to confirm its usability and capability as a dynamic NDE method.  
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.