## 1 Introduction  
Researchers have attempted in recent years to develop non-destructive evaluation (NDE) techniques for subsurface defect detection in wind turbine blades (WTBs). Among these, shearography has shown its adaptability to the inspection of various composite materials. This is attributed to its feature of showing the stress concentration and the first derivatives of the displacement while loading on the material's surface. The novelty of using shearography as an NDE technique for inspecting WTBs is its integration with a robotic platform for on-board defect inspection. A few projects, such as DASHWIN [1] and WInspector [2], have used shearography for the inspection of WTBs using dynamic analysis methods. The reported results are qualitative fringe pattern evaluations, which are subtractions of loaded and unloaded speckle patterns, and no phase maps are produced during the process. The main focus of these projects has been dynamic manipulation and semi-automatic control with no manual work in the inspection, as fringe pattern analysis is more practical in real-time dynamic evaluation compared to the acquisition of a phase map. The development of shearography systems in recent years has also included the optimisation of phase results using different phase shift techniques for accuracy and quantitative analysis [3]. In the context of inspection, the retrieval phase is more sensitive to the defect than fringe pattern, as when the loading is small, the fringes may not form,while one can easily obtain the information from the phase map. Conventional temporal phase shift digital shearography (TPS-DS) [4; 5; 6] has been used largely for interferometric analysis. However, changing phase in the time-domain (i.e., at different points in time using phase shifting devices) does not meet the dynamic inspection requirement for WTB inspection. The reason is that in the recording process the shearogram will be easily affected by any fluctuations on the surface. Moreover, a piezoelectric stepper for shifting the phases is hard to control in an integrated robotic system, because the time for shifting the phase needs to be kept short to maintain the optimum fringes for phase map calculation. Furthermore, the relative motion between the sample surface and shearography system needs to be eliminated. If the vibration between the WTB and shearography system is intense to a critical level, the expected results, including phase information, will be lost. The possibility of deriving phase using temporal phase shift interferometry is low owing to the above-mentioned limitations. Thus, researchers have attempted to develop quasi-dynamic inspection processes using advanced algorithms to estimate phase results in a shorter time and with fewer phase-shifting steps. Among these, several algorithms have shown promising results for their fast and precise estimation. [PERSON] and [PERSON] [7] and [PERSON] et al. [8] have reported the use of temporal phase shift before loading and estimating the phase map using different algorithms to obtain a fringe pattern after loading. The estimating algorithms could be least-square-based, or clustering-based. The quasi-dynamic approaches with advanced algorithms have resulted in a reduction in the steps of temporal phase shift and calculation time. [PERSON] and [PERSON] [9] also used phase calculation by adopting two successive fringes without knowing their phase step value. However, their approach was limited in the on-board control of the temporal phase stepping. [PERSON] et al. [10] used principal component analysis on two successive fringes, where phase step was unknown, and achieved relatively more ideal results with less complex computations. They subsequently proposed further developments of their approach with the use of different algorithms, obtaining improved results [11; 12]. The application of these methods to real WTBs and their subsurface defects, however, has led to distorted and vague results in some areas that could not show full defect information.