13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- q-vector 110 mm 140 mm 125 mm δ (displacement) δ (displacement) Figure 3. Partitioned two-dimensional Wedge Splitting Test and mesh strategy adopted around crack-tip. For a sharp crack, the strain field becomes singular at the crack tip. Including the singularity at the crack tip for a small-strain analysis improves the accuracy of strain calculations, the J-integral and stress intensity factors. The partitioning of the geometry is defined by the circular lines centered on the crack tip (Figure 3); this partitioning strategy facilitates the generation of a focused mesh. The remaining portion of the model is free meshed using the “medial axis” meshing algorithm. The crack tip is meshed using a ring of collapsed 8-node bilinear, reduced integration (CPS8R) elements. To obtain a singularity term, the following conditions must be met: 1. The elements around the crack tip must be focused on the crack tip. One edge of each element must be collapsed to zero length so that the nodes of this zero length edge are located at the crack tip. 2. The “midside” nodes of the edges radiating out from the crack tip of each of the elements attached to the crack tip must be placed at one-quarter of the distance from the crack tip to the other node of the edge. 4. Digital Image Correlation Analysis (Q4-DIC) Digital image correlation is a technique that allows one to retrieve displacement fields separating two digital images of the same sample at different stages of loading. We summarize the main points here, for more information the reader shall refer for instance to [11, 15]. Generally, it is not possible to find the correspondence of a single pixel in an image in the deformed state to the image in the reference state. This is because there is not a unique correspondence. The subsets which are comprised of finite number of pixels are utilized to locate the same material point between deformed and reference states. To correlate the deformed image g to the undeformed reference image f, each image is divided into small subsets and the correlation algorithm is executed from subset to subset. A digital image of a body is simply a discrete intensity record of the light levels present at various positions of the body in the smallest unit of digitization, the pixel. In an eight-bit system, light intensity at each pixel ranges in value from 0 to 255 (represented as 0 < p(x,y) < 255). The point P(x,y) in
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