Non-Contact 3D Deformation and Strain Measurements using Digital
Image Correlation for Biomedical Applications
T. RUSIN1, C. WEISSMAN2
Key words: DIC, Strain, Deformation, Multicamera-DIC, Micro-DIC
In recent years, many material testing laboratories have employed non-contact optical imaging
techniques instead of standard strain gauges. In some applications, standard strain gauges can not be
used for various reasons such as fragile or thin materials or the need to obtain a full fieldmeasurement of the object without the need to mount numerous gauges. The most popular opticalmethod is 3D Digital Image Correlation (DIC) which uses stereoscopic imaging and analysis toobtain strain measurements results over the test sample. The 3D DIC technique uses two cameras
which are aligned to the object surface at appropriate angles for the field of view required. A
calibration target is then used to enable the calculation and reconstruction of the 3D space from eachof the 2D views of the two cameras. In order to make DIC measurements it is necessary to have astochastic pattern (random dots) on the sample so that a correlation algorithm can track thedisplacement or motion of the surface as it is deformed. Using this information allows for thecalculation the 3D deformation and resultant strain field from sequential images of the object.
The Digital Image Correlation technique was used for the first time in 1980s in a single camera
setup for measuring surface displacement providing in-plane 2D measurements of strain [1, 2]. The
method was successfully used for measurements of the mechanical properties of biological tissues[3]. Biomaterial research has several unique challenges for the measurement of deformation andstrain. In particular, specimens are typically small and delicate which makes them difficult orimpossible to measure using standard strain gauges. Furthermore, most organs and tissues are nonuniform in shape which makes data analysis of stress/strain relationship problematic. Thus, the DIC
technique is ideally suited for biomechanical measurements thanks to the ability of the method toobtain true full-field, non-contact and three-dimensional information of displacements and strains on
biomedical materials and components in both the macro and microscale. In addition, DIC can be usedfor other measurements including; tension, compression, torsion, bending and combined loading,peel, creep, relaxation and more on a wide range of biomaterials. The recent introduction of Stereo
microscopic DIC opens new avenues of research for micromechanical measurements of ultra-smalland sensitive materials [4]. The advantage of both high measurement sensitivity and accuracy [5]enables the measurement of deformation induced not only by direct mechanical loading but also by
thermal loading, changes of humidity, and physiological factors.
The DIC technique not only yields measurement results quickly, but it also provides theresearcher with these results as a comprehensive data set. Such data is necessary to fully understandcomplex materials and structures including those with anisotropic characteristics, as well as theability to compare the experimental results with computational (i.e. FEA) models. Biomaterial
deformation and strain measurements often face unique challenges such as; components withcomplex geometries, interactions of materials with dissimilar compositions (i.e. bone with implants,skin with sutures, etc.), and structures that have multivariable dependencies. Furthermore, modernDIC systems provide the user with numerous tools to assist with the analysis and presentation of theresults such as; report generation of measurements independent of the application and system
configuration, correlation of measurement results with numerical simulations via standardized dataexchange formats, validation of material behaviour with complex structures, and compatibility with3D printing. One of the latest and most useful developments for the DIC technique is the ability to1Tomasz Rusin, Dantec Dynamics GmbH, Ulm, Germany, e-mail: trn@dantecdynamics.com
2Cliff Weissman, Dantec Dynamics Inc. Holtsville, NY, e-mail: cwn@dantecdynamics.com

use more then 2 cameras in the measurement system. The multi-camera DIC solution provides theunique ability to acquire deformation and strain information all around a 3D object as well as verylarge objects and those with multifaceted and complex geometries in one single measurement step.
Multi-camera DIC configurations achieves a single coordinate system by arranging the individual
cameras in a strategic array such that their fields of view overlaps in pairs. The multi-camera setup
enables measurement of objects with high radii of curvature, around corners, cylindrical, cubic, andspherical geometries can be evaluated. The advantages of the multi-camera DIC solution overstandard 3D DIC systems includes; no need to stitch measurement results from non-simultaneously
acquired data sets, increased accuracy on large or obscured areas observed simultaneously by several
cameras, no coordinate transformations required, easy calibration of all cameras simultaneously,
flexible setup capable of accommodating various test object geometry.
Multi-camera systems provide significant advantages to biomechanical research applications
thanks to the ability to determine the real stress results during mechanical test of specimens with
arbitrary shapes, sizes, and compositions. Such objects like bones, spine and skull can be measured at
360 degrees using an 8 camera system. An example of a compression test of a cylinder is shown
below in Fig. 1. A flat specimen example is shown on the right which used a 4 camera setup. This
type of setup can be very useful for real stress measurements of flat specimens such as skin.

Fig. 1. Full-field evaluation of a cylindrical object with an 8 camera DIC System (left image). On the right tensile test of flat specimen with use of 4 camera system in bi-plane configuration. Strain distribution in X, Y andZ (thickness) direction (right 3 images).

Stereo Microscope 3D Digital Image Correlation opens new possibilities for very small anddelicate specimens. Standard DIC systems are not able to measure specimens smaller than about
10mm because both objectives in stereoscopic setup have limited depth of field. This limitation canbe overcome using stereo microscope. Micro DIC enables measurements of small objects such as;
dental implants, trabecular bone structure, and rings of a stent (Fig.2).

Fig. 2. Micro DIC results on 3 different small scale objects. First three images: dental implant, implant withstochastic (speckle) pattern, correlated result (best fit plane). Image 4 and 5 from the left show trabecular bonestructure (native structure image and correlated). It is possible to correlate image on single trabeculae. Last image
on the right presents sinusoidal ring of a stent (best fit plane).

References[1][2][3][4]

PETERS W. H., RANSON W. F., OPT. ENG. 1982, 21, 213.
CHU T. C., RANSON W. F., SUTTON M. A., PETERS W. H., Exp. Mech. 1985, 25, 232.
D. ZHANG, D. D. AROLA, J. Biomed. Opt. 2004, 9, 691.

RUSIN T. and KOPERNIK M., Characterization of Biocompatible Materials Using Stereo
Microscope 3D Digital Image Correlation, Advanced Engineering Materials 2016, 18, No. 9 © 2016
[5]
SIEBERT T., BECKER T., SPILTTHOF V, NEUMANN I. and KRUPKA R.. Error
Estimations in Digital Image Correlation Technique. Applied Mechanics and Materials Vols. 7-8(2007) pp 265-270.