Digital Image Correlation

Accurate and reliable deformation measurements are a cornerstone of materials testing.  Traditional methods, including strain gauges and extensometers, are both accurate and reliable; however, they provide information only at the site of application.  A more advanced technology, Digital Image Correlation (DIC), is capable of producing non-contact full-field strain measurements of deforming bodies.  Veryst analyzes global deformation measures, such as specimen elongation, and a global view of strain localizations with this technology.  Veryst also uses this methodology to measure local deformation, such as the true strain in a necked tensile specimen.  We can also perform biaxial measurements, permitting the calculation of Poisson’s ratio or Lankford coefficients (r-values).   

The use of non-contact measurements also allows for the testing of fragile or soft materials which may be negatively influenced by an extensometer that is physically attached to the specimen.  When coupled with our high-speed camera, Veryst can determine surface strains of objects deforming at high rates of strain.

DIC_Fig.1
Example of a strain field on an ASTM D638 Type I thermoplastic tensile specimen moments before failure occurs


While DIC is useful in measuring deformation for standard tensile and compressive tests, it also proves helpful during custom or non-standard tests.  It is possible to track the speckle pattern for a wide range of geometries and test types, including shear testing [lap shear and Iosipescu (V-Notch)].  We can also measure properties of adhesives during peel tests, cleavage tests, and cantilever beam tests.  We additionally use this technology to determine other important variables such as shear strains and rotational measurements.

DIC Fig.2
Mode I adhesive test of a foam adhesive in a double cantilever beam experiment using DIC to track the strain of the adhesive under loading. Data collected in this strain field can be interpreted in different ways, depending on the information needed

 

Video showing strain field during double cantilever beam adhesive test from above image


 

DIC  Fig.3
V-Notch (Iosipescu) shear test on a polyurethane material. This test is primarily used for in-plane shear testing. Typically a strain gauge would be mounted on the specimen in the center notched region, however using DIC we clearly see the bands of shear strain (εxy) throughout the specimen

 

Video of V-Notch (Iosipescu) shear test from above image

 

This video below shows a thin film notched tension test of low-density polypropylene.  The sample has been speckled with paint to measure the strains using DIC up until failure.  We use this data to validate material models and determine the failure behavior of the material.   

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