Continuous Surfaces in DIC

Because of how we track and match images in order to obtain data, and because of the fundamentals of strain theory, we must treat the area of interest as a continuous surface.  Discontinuous surfaces can result in unreliable data and erroneous strains.

Tracking and Matching Data

We apply a random speckle pattern to track our Area of Interest (this is the region of the specimen that you wish to obtain data).  We do not track the individual speckles, rather we track groups of pixels that we call Subsets.  The Subset size is user defined, but in this case let's consider a subset size of 21.  If we have a subset size of 21 pixels, this means that we are tracking squares of 21x21 pixels throughout the surface of the specimen.  In order to track and match these subset points, the subsets themselves must remain continuous.  If the subsets break apart or have discontinuous behavior, then we cannot track them.  Typically subsets that break apart (for example, when the surface cracks) will be dropped from the data, however what data is included in the contour plots is partially determined by factors like subset size and thresholding (which you can change in the Run menu).  What is even more problematic is that sometimes it looks like the subset was dropped near a crack, but some of that false data inside the crack is included in the nearest subset.  We report data in the center of the subset, so if the subset is 21x21, the edge of the contour plot will be a subset that includes 10 pixels outside of where the plot is drawn.  So some of the data that looks like it is outside of the crack, could potentially be including the crack, which would result in unreliable data.  The data within that crack is unreliable because strain theory assumes a continuous surface.

Strain Theory

Strain is a way to quantify how a continuous body deforms.  It is, very simply, the percent change in elongation and it is a measure of how ductile or rigid a material is (however, there are different types of strain tensors available in the software).  If you have a discontinuous surface, like a hole or a crack, if we correlate over that void or crack, then we are essentially putting strain gauges over the crack.  So as a material separates, it can show a huge "strain," but the material has broken so that is not strain, rather than just simply displacements of crack openings. When cracks or discontinuities in the surface occur, our software is designed to typically drop those points.  Again, we can control how much data we choose include with subset size and thresholding.  But we must be careful when looking at strains where there are discontinuous surfaces, because those could be erroneous due to the discontinuous material behavior.

Special Cases: Composites and Textiles

Some materials with a micro-structure, such as composites and textiles, can behave as a continuous material on a macrolevel but then have fibers slipping past each other and discontinuities on a smaller scale.  This is something to note because we could potentially see a shear strain due to materials slipping past each other, when it's, in fact, just slippage and not strain.  On a more macro level, however, the material is deforming as a continuous surface.  It's important to have an idea of how the material is deforming on both a micro and macro scale when looking at fibrous materials.