Use of digital image correlation to solve solid mechanics problems

Digital Image Correlation (DIC) is an optical, non contact, computer based whole-field surface displacement measurement technique which uses speckles as an information carrier. In principle it is applicable to any type of material under any kind of loading. The whole-field information can be obtained by comparing two images of the specimen at different states of deformation using pixel and its signature (subset) in the undeformed image. The technique gives reasonable accuracy for the materials which undergo large deformation. The measured displacement and strain results are influenced by the quality of illumination, speckle characteristics, technique used for making the speckle pattern and subset size.

To understand the algorithms used in the DIC technique and the mathematical development of the technique in a simple manner, literature survey is carried out and efforts are taken to consolidate various algorithms suggested by various authors in a unified manner with standard notations, symbols and nomenclature.

Illumination is one of the basic factors affecting accuracy of the DIC measurement. One needs to illuminate the specimen uniformly, which can be ensured by having unimodal histogram. Efforts are taken to get the histogram, which is a plot of gray scale level along x-axis and number of pixels along y-axis. The preliminary experiments are carried out on various simple problems to understand its effect on the DIC results.

Speckle pattern on the test specimen is a key aspect in DIC to achieve reasonable accuracy. Formation of fine, uniform size, random, non-repetitive and highly contrast speckle pattern requires skill and experience. Efforts are taken to improve the quality of speckle pattern over the period of time.

To understand the methodology of 2D-DIC and the effects of the methods used for making speckle pattern on the accuracy of the DIC results, benchmark problems undergoing in-plane displacements are analysed using DIC technique and the results are validated using analytical solutions and FE analysis. Analysis of shear effect for a cantilever beam under bending is carried out to evaluate whether DIC is sensitive enough to recognize the effect of shear, which is often neglected in the mathematical modeling. To understand the effect of the methods used for making speckle pattern, various possible methods like spraying and hand sketch are used to make speckle patterns and experiments are carried out on standard problems in the increasing order of complexities such as finite plate with a hole, ring under diametral compression followed by practical problems of a spring transducer element and then flange coupling.

To understand the methodology of 3D-DIC and the effect of the methods used for making speckle patterns on the accuracy of DIC measurement, specimens are speckled using three different methods namely, spraying, screen printing and hand sketch, and an experiment is carried out on a cantilever beam subjected to out-of-plane displacement.

After understanding the 2D and 3D-DIC methodologies along with the various factors affecting accuracy of the DIC measurement like illumination and technique used for making speckle pattern, a real life problem i.e., nut tightening by a spanner is studied, to ensure the applicability of the technique to problems of graded complexities.

The factors other than illumination and speckle pattern making technique, which can affect accuracy of the DIC measurement are a) subset size and b) characteristics of the speckle pattern. Combined effect of the subset size and characteristics of speckle pattern is studied on the spanner specimen.