and it raises a whole set of experimental problems (texture, acquisition, filtering, etc.). Regarding an atypical application, balloons, the authors of [6] recall that the field of (very) large deformation is still wide open and depending on the use case, special specific experimental configurations may help. It is also a theme addressed by [7] where calibrated targets were used to evaluate measurement uncertainties in this large deformation regime. The possibility to bridge more intimately measurements and models is highlighted in [8] where experimental measurements are combined to a model to extract mechanical fields with a certain mechanical admissibility close to a shear crack at bi-material interface. A little further on in the coupling between models and measurements, [9] proposed an interesting methodology to quantitatively characterize mechanical (interlaminar) properties reputed to be difficult to identify using finite element model updating techniques. Among current topics, the coupling of DIC with other types of instrumentation techniques or more generally data fusion is discussed in Article [10]. A comparative analysis based on DIC and Accoustic Emission techniques is helpful to comprehend the characteristics of concrete fracture process zones. In addition to the classic 2D DIC, several variants are also illustrated in this special issue. For example, stereo-DIC is an ally of choice for the validation of models on complex or large poly-instrumented structures. The issue of calibrating several independent benches using valuable CAD information is discussed in [11]. Conversely, when non-planar tests are to be instrumented at small scales or in conditions of difficult access, stereo can be used with a single camera by adapting the mounting with, for example, prisms and mirrors [12]. Another variant of DIC, which is still in its infancy, relies on X-ray based digital volume imaging. Increasingly, the measurement of 3D fields in material bulk (DVC) is leading mechanical engineers to rethink the way they conduct tests, developing, in addition to new image correlation algorithms, special machines that allow in-situ testing. This trend is illustrated in article [13]. Volume measurement with X-ray tomography is not the only volume imaging method of interest in mechanics. For example, Optical Coherence Tomography (OCT) allows this kind of investigation to be carried out and is particularly interesting for biological materials. The results obtained, combined with identification techniques, make it possible to estimate some mechanical properties [14]. Last but not least, the last part concerns algorithmic issues. The choice of correlation metrics itself is still under investigation. For instance, in [15], the authors present different metrics based on the gradients of the image rather than on the grey level. The above-mentioned question of large deformations implies, in addition to experimental constraints, particular complexities from an algorithmic point of view. Initialization in the large deformations framework of the correlation algorithm is also a topical issue [16].  
This research received no external funding. This issue follows two quite successful special session related to Digital Image Correlation at the 18 th International Conference on Experimental Mechanics (ICEM18) in Brussels. We would like to thank the European Society for Experimental Mechanics (EuraSEM) for inviting us to edit this special issue in MDPI Applied Sciences journal. We would also like to record our sincere gratefulness to all authors and reviewers who contributed to this special issue. The authors declare no conflict of interest.