Three-Dimensional Digitial Image Correlation Systems
The Dynamic Mechanics of Materials Laboratory uses VIC-3D, a commercial three-dimensional digital image correlation (3D DIC) software package that is maintained by Correlated Solutions. Static and dynamic camera systems are available for use. The static system consists of two Point Grey GRAS-20S4M-C cameras. These cameras have 1624 pixel by 1224 pixel resolution and can capture images at a maximum frame rate of 19 fps which is fast enough for most experiments conducted on a servohydraulic load frame. The dynamic system employs two Photron Fastcam SA1.1 high speed cameras. These cameras can achieve 125,000 fps at 256 pixel by 128 pixel resolution. A table that describes the frame rate and resolution capabilities of these cameras is found here. The dynamic system is used for split-Hopkinson bar and intermediate strain rate experiments.
The Dynamic Mechanics of Materials Laboratory uses this technique in experiments regularly. Some examples are highlighted below.
Uniaxial tension experiment of an axisymmetric 2024-T351 aluminum specimen:
In this experiment, a round tension specimen is tested in uniaxial tension. One might think that this would be a relatively simple test and yield rather uninteresting results. The 3D DIC data, however shows an interesting deformation phenomenon that takes place in the specimen (click on the image to play the movie). As the specimen is loaded, the deformation initiates in a region near the left end of the specimen and propagates to the right. This repeats itself several times until the necking localization forms and the specimen fails shortly afterward.
Cyclic loading of a resin mandible specimen:
In this experiment, a cyclic compressive load (that mimics typical chewing forces) is applied to two non-splinted dental prostheses that are retained with screws to dental implants embedded in a resin mandible specimen. 3D DIC is used to monitor the maximum principal strain distribution in the mandible specimen (click the image to play the movie). The data clearly shows a strain concentration underneath the anterior (left) prosthesis. These data help dental researchers evaluate their implants, prostheses and installation techniques.
Comparison of 3D DIC data to numerical simulations:
3D DIC data of a thick, notched tension specimen are compared to data from an LS-DYNA simulation below. Maximum and minimum principal strains measured with DIC are shown in (a) and (c), respectively. Simulated maximum and minimum principal strains are respectively presented in (b) and (d) below. In this case, the experimental strains are similar to simulated strains. This example illustrates that 3D DIC can be used to make detailed critiques of the constitutive models used in numerical simulations.
3D DIC is an optical measurement technique that is rapidly enhancing the experimental mechanics discipline. The technique can determine the three dimensional contour of an object's surface and track the surface displacement field of the object in a series of images. 3D DIC uses digital images from two cameras and the principles of optics to stereo-triangulate the surface contour of the object. An algorithm defines a field of "subsets" on the object's surface using the digital images. These subsets are N by N pixel boxes that contain an array of pixel gray-scale values. An advanced tracking algorithm can determine the translation, rotation and deformation of these subsets in loaded images with respect to a reference frame. The result is a time history of the specimen's surface displacement field. The displacement field is used with continuum mechanics definitions of strain to calculate the strain field on the specimen's surface.
3D DIC is an extremely useful tool for experimental mechanics. It allows the experimentalist to examine, in detail, complex behavior that exist even in relatively simple mechanical tests, such as the necking localization in tension and shear band formation in torsion of a thin-walled tube. The full-field displacements and strains measured using the technique give the user access to significantly more detailed information than previously available with strain gage measurements. These additional data can be used to make detailed comparisons and critiques of numerical simulations.
More detail on 3D DIC can be found in the following reference:
Sutton, M.A., Orteu, J.-J., Schreier, H.W., Image Correlation for Shape, Motion and Deformation Measurmeants, Springer, New York, NY, 2009.