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Experimental Methods to Characterize the Heterogeneous Strain F ield 123

proposed a method for full-field deformation measurement through pixel-by-pixel
mapping using a B-spline functional form. Schreier et al. (2000) emphasized the impor-
tance of image reconstruction in improving the accuracy of the matching process. 2D-
DIC was also combined with the scanning electron microscope (SEM) for large deforma-
tions. Inverse methods emerged recently for characterizing elastic properties, properties
in heterogeneous materials, hyperelastic properties, micromechanics, and composites.
Other applications in material characterization include 2D-DIC measurements in films,
polymers, metals, heterogeneous composites, wood, bio-materials, alloys, asphalt, ce-
ramics, concrete, and geomaterials such as clays and sands.

4.4.3 Use of DIC in 3D Surface Analysis
The development of more complicated 3D-DIC surface analysis was intended to over-
come the inability of 2D-DIC to handle the out-of-plane motion that changes the mag-
nification and introduces errors in the measured in-plane displacement. With a 3D
analysis, the error can be removed by incorporating the displacement in the third direc-
tion. Luo et al. (1994) combined stereovision principles with 2D-DIC concepts in single
camera imaging and developed a two-camera stereo vision system for the measure-
ment of 3D crack tip deformations. Synnergren and Sjödahl (1999) successfully em-
ployed stereovision to make deformation measurements, where the investigators ob-
tained flash X-ray images of a specimen from two directions before and during high-
rate loading. The investigators extracted 3D estimates for specimen deformations by
comparing features in both sets of X-rays. Andresen (1999) analyzed images of a grating
undergoing large deformation using a stereo system. Using SEM imaging, Lockwood
and Reynolds (1999) obtained surface stereo images of an in-situ specimen from two
orientations.
Entering the new century, 3D digital image correlation has seen a wide range of ap-
plications on both large and small structures. Basic studies of typical high-speed imag-
ing systems were performed by Tiwari, et al. (2007). Stereovision system applications
have included measurements on flexible wings undergoing aerodynamic loading (Al-
bertani et al., 2007), as well as measurements of shape and deformation on cylindrical
surfaces (Luo et al., 1993). Using 3D-DIC, fracture studies were performed under mixed
mode I/II (Luo and Huang, 2000) and mixed mode I/III loading conditions (Sutton et
al., 2007). Material characterization has also been an active area including microscale
studies in engineered materials (Florando et al., 2007) and biomaterials (Nicolella et al.,
2001), foam, ceramics, composites, polymers, and high-rate events.

4.4.4 Use of DIC in 3D Volume Analysis
Different from the 3D-DIC stereo surface analysis, 3D volumetric DIC also characterizes
the internal changes in deformation and strain of an object, and has gained attention as
an active area of research recently, resulting in matured imaging capabilities. 3D volu-
metric DIC requires technologies for internal characterization. Micro- and macro-com-
puter-aided X-ray tomography (X-ray CT) and magnetic resonance imaging (MRI) are
examples of such technologies that are available for general image analysis.
Complementary to development in imaging technology, a wide range of software is
now available for image analysis to extract useful information. Veress et al. (2003) de-
veloped a method for comparing images to extract full-field estimates for deformations.
Bay et al. (1999) extended 2D-DIC concepts to match small sub-volumes before and af-
ter undergoing loading to obtain a full volumetric field of 3D motions. Germaneau et al.