_Applied_  
_sciences_  
_Article_  
**Boundary Deformation Measurement by Mesh-Based Digital Image Correlation Method**  
**[PERSON] 1, [PERSON] 1,*, [PERSON] 1, [PERSON] 2 and [PERSON] 3,*  
**Abstract: Digital image correlation (DIC) is a popular photomechanics method for deformation measurement. The conventional subset-based DIC method obtains the displacement vectors at the subset centers, but cannot calculate the deformation on the specimen edges, which may contain very useful information because specimens usually have greater deformation on edges, especially on curved edges due to stress concentration. In this study, the capability of a mesh-based DIC method using 8-node quadrilateral elements (Q8-mesh-DIC) in boundary deformation measurement was investigated and highlighted for specimens with non-uniform deformation. The results were compared with those obtained by some conventional subset-based DIC methods, and the accuracy of the boundary deformation measurement was verified through simulated and real experiments. The Q8-mesh-DIC appears to be more suitable for the boundary deformation measurement of non-uniform deformation fields. **  
**Keywords:digital image correlation; Q8-DIC; non-uniform deformation field; boundary deformation measurement**  
**Clation: [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Boundary Deformation Measurement by Mesh-Based Digital Image Correlation Method.**_Appl. Sci._**2021**, _11_, 53. [[https://dx.doi.org/10.3390/app11010053](https://dx.doi.org/10.3390/app11010053)]([https://dx.doi.org/10.3390/app11010053](https://dx.doi.org/10.3390/app11010053))  
Received: 3 December 2020  
Accepted: 21 December 2020  
Published: 23 December 2020  
**Published: 2  
From this perspective, the accurate measurement of deformations near or on the curved edges of specimens or structures plays a very important role in practical applications. However, based on conventional subset-based DIC, more information is missed near curved edges than straight ones. To obtain this useful information near the edges, some methods, including extrapolation or changing the subset shape, have been employed in references [6; 7; 8; 9]. In the extrapolation method [6], the deformation near the specimen edges that cannot be measured directly is extrapolated from the inner measured displacement field. In principle, the extrapolation method is easy to implement, but the deformation pattern near the edges may substantially differ from that of the inner region owing to factors like high-stress concentration; as a result, the extrapolated result may fail to reflect the true deformation. The half-subset method [8; 9] is another method for calculating the boundary deformation based on the conventional subset-based DIC model. In this method, valid and invalid pixels are labeled in the subsets across the edge, and only the valid points are used in the correlation analysis. That is, only parts of the subsets or irregular subsets are used to calculate the displacements near or on the edges, which generally results in insufficient matching information and low measurement accuracy. From the above discussion, it can be concluded that it is difficult to calculate the displacements near the specimen edges accurately using the subset-based DIC methods. As alternatives to overcome the drawbacks of the subset-based DIC methods in boundary deformation measurement, mesh-based DIC methods [10; 11; 12] have their potential prospects. Following the concept of the finite element method (FEM), mesh-based DIC methods discretise the area of interest (AOI) of a speckle image into several elements to ensure continuity of the deformation field within the entire AOI. By placing the element nodes on the edges, boundary deformation can be achieved using such methods. [PERSON] et al. [10] and [PERSON] et al. [11] established the 4-node-quadrangular mesh-based DIC (Q4-DIC) method for deformation measurement.