As a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface, two-dimensional digital image correlation (2D DIC) is now widely accepted and commonly used in the field of experimental mechanics. It directly provides full-field displacements to sub-pixel accuracy and full-field strains by comparing the digital images of a test object surface acquired before and after deformation. In this review, methodologies of the 2D DIC technique for displacement field measurement and strain field estimation are systematically reviewed and discussed. Detailed analyses of the measurement accuracy considering the influences of both experimental conditions and algorithm details are provided. Measures for achieving high accuracy deformation measurement using the 2D DIC technique are also recommended. Since microscale and nanoscale deformation measurement can easily be realized by combining the 2D DIC technique with high-spatial-resolution microscopes, the 2D DIC technique should find more applications in broad areas.

https://iopscience.iop.org/article/10.1088/0957-0233/20/6/062001/pdf

Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review

Phase shifting algorithms for fringe projection profilometry: A review

Digital Image Correlation (DIC) has been established as a flexible and effective technique to measure the displacements on specimen surface by matching the reference subsets in the undeformed image with the target subsets in the deformed image. With the existing DIC techniques, the user must rely on experience and intuition to manually define the size of the reference subset, which is found to be critical to the accuracy of measured displacements. In this paper, the problem of subset size selection in the DIC technique is investigated. Based on the Sum of Squared Differences (SSD) correlation criterion as well as the assumption that the gray intensity gradients of image noise are much lower than that of speckle image, a theoretical model of the displacement measurement accuracy of DIC is derived. The theoretical model indicates that the displacement measurement accuracy of DIC can be accurately predicted based on the variance of image noise and Sum of Square of Subset Intensity Gradients (SSSIG). The model further leads to a simple criterion for choosing a proper subset size for the DIC analysis. Numerical experiments have been performed to validate the proposed concepts, and the calculated results show good agreements with the theoretical predictions.

Study on subset size selection in digital image correlation for speckle patterns.

Quality assessment of speckle patterns for digital image correlation

Digital image correlation for surface deformation measurement: historical developments, recent advances and future goals

Optical methods are becoming commonplace in investiga- tions of the physical and mechanical behavior of biological tissues. Digital image correlation (DIC) is a versatile optical method that shows tremendous promise for applications involving biological tis- sues and biomaterials. We present the fundamentals of DIC with an emphasis on the application to biological materials. An approach for surface preparation is described that facilitates its application to hy- drated substrates. Three examples are presented that highlight the use of DIC for biomedical research. The first example describes the use of DIC to study the mechanical behavior of arterial tissues up to 40% elongation. The second example describes an evaluation of the me- chanical properties of bovine hoof horn in the dehydrated and fully hydrated states. Uniaxial tension experiments are performed to deter- mine the elastic modulus (E) and Poisson's ratio (n) of both the arterial and dermal tissues. Spatial variations in the mechanical properties are evident from the full-field characterization of both tissues. Finally, an application of DIC to study the evolution of loosening in cemented total hip replacements is described. The noncontact analysis enables measurement of the relative displacement between the bone/bone ce- ment and bone cement/prosthesis interfaces. Based on the elementary optical arrangement, the simple surface preparation, and the ability to acquire displacement/strain measurements over a large range of de- formation, DIC should serve as a valuable tool for biomedical re- search. Further developments will enable the use of DIC for in vivo

/pdf/applications-of-digital-image-correlation-to-biological-3bb3a0mc7v.pdf

Applications of digital image correlation to biological tissues

Aging and the related changes in mechanical behavior of hard tissues of the human body are becoming increasingly important. In this study the influence of aging on the mechanical behavior of human enamel was evaluated using 3rd molars from young (18 ≤ age ≤ 30 years) and old (55 ≤ age) patients. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin–Enamel Junction (DEJ) and within three different regions of the crown (i.e. cervical, cuspal and inter-cuspal enamel). Results of the evaluation showed that the elastic modulus and hardness increased with distance from the DEJ in all three regions examined, regardless of patient age. The largest increases with distance from the DEJ occurred within the cervical region of the old enamel. Overall, the results showed that there were no age-dependent differences in properties of enamel near the DEJ. However, near the tooth’s surface, both the hardness (p < 0.025) and elastic modulus (p < 0.0001) were significantly greater in the old enamel. At the surface of the tooth the average elastic modulus of “old” enamel was nearly 20% greater than that of enamel from the young patients.

https://www.umbc.edu/lamp/pubwork/Park_et_al_J_Mater_Sci_2007.pdf

Mechanical properties of human enamel as a function of age and location in the tooth.

An evaluation of the crack growth resistance of human coronal dentin was performed on tissue obtained from patients between ages 18 and 83. Stable crack extension was achieved over clinically relevant lengths (0 ≤ a ≤1 mm) under Mode I quasi-static loading and perpendicular to the nominal tubule direction. Results distinguished that human dentin exhibits an increase in crack growth resistance with extension (i.e. rising R-curve) and that there is a significant reduction in both the initiation (Ko) and plateau (Kp) components of toughness with patient age. In the young dentin (18≤age≤35) there was a 25 % increase in the crack growth resistance from the onset of extension (Ko =1.34 MPa·m0.5) to the maximum or “plateau” toughness (Kp = 1.65 MPa·m0.5). In comparison, the crack growth resistance of the old dentin (55≤age) increased with extension by less than 10 % from Ko = 1.08 MPa·m0.5 to Kp = 1.17 MPa·m0.5. In young dentin toughening was achieved by a combination of inelastic deformation of the mineralized collagen matrix and microcracking of the peritubular cuffs. These mechanisms facilitated further toughening via the development of unbroken ligaments of tissue and posterior crack-bridging. Microstructural changes with aging decreased the capacity for near-tip inelastic deformation and microcracking of the tubules, which in turn suppressed the formation of unbroken ligaments and the degree of extrinsic toughening.

https://www.umbc.edu/lamp/pubwork/Nazari_et_al_2009.pdf

Aging and the reduction in fracture toughness of human dentin

We conduct displacement/strain measurements on the micro- scale using light microscopy and digital image correlation DIC. Errors in the measurements attributed to the optical arrangement and aberration induced at high magnification are identified using a warping function. Coefficients of the warping function are determined using a simple tech- nique that employs a precisely made orthogonal cross-grating plate. By acquiring images of the grating and identifying the nodes using subpixel techniques, a relationship between the object and the image planes is established. Thus, the displacement/strain derived by means of DIC is corrected by converting the displacement components in the image plane to the coordinate system existing on the object's surface. The approach is validated through a determination of the elastic properties of common metals; errors in estimation of the elastic modulus were within 4%. Although surface preparation generally plays a critical role in suc- cessful application of DIC, it is found to be of minimal importance under high magnification. Instead, the natural surface texture can be used with adjustment of the light incident angle. Results of the study show that DIC is a powerful tool in performing displacement/strain measurements on the microscale using a light microscope provided that an adequate cor- rection is employed for image distortion. © 2006 Society of Photo-Optical Instru-

https://www.umbc.edu/lamp/pubwork/Zhang_et_al-_Optical_Engg_2206.pdf

Displacement/strain measurements using an optical microscope and digital image correlation

Tooth fracture is a major concern in the field of restorative dentistry. However, knowledge of the causes for tooth fracture has developed from contributions that are largely based within the field of mechanics. The present manuscript presents a technical review of advances in understanding the fracture of teeth and the fatigue and fracture behavior of their hard tissues (i.e., dentin and enamel). The importance of evaluating the fracture resistance of these materials, and the role of applied mechanics in developing this knowledge will be reviewed. In addition, the complex microstructures of tooth tissues, their roles in resisting tooth fracture, and the importance of hydration and aging on the fracture resistance of tooth tissues will be discussed. Studies in this area are essential for increasing the success of current treatments in dentistry, as well as in facilitating the development of novel bio-inspired restorative materials for the future.

/pdf/on-the-mechanics-of-fatigue-and-fracture-in-teeth-51cxs5qog6.pdf

Dongsheng Zhang

Papers

Applications of digital image correlation to biological tissues

Mechanical properties of human enamel as a function of age and location in the tooth.

Aging and the reduction in fracture toughness of human dentin

Displacement/strain measurements using an optical microscope and digital image correlation

On the Mechanics of Fatigue and Fracture in Teeth