In the virtual simulation of toughness using a standard steel specimen (A516-Gr.70), the maximum equivalent (Von-Mises) stresses are uniform along the entire length of the crack. However, when the KIC was calculated, the maximum values of the stress intensity factor were only concentrated in the center of the specimen crack, as shown in graph in red, and I would expect it to be uniform throughout the crack, as in the previous case. I would like to know why the KIC found is not uniform along the crack.
This is from memory, many moons ago, but you can verify it. The lack of material on the side of the piece allow for strain laterally. In the center these oppose each other from either side of the unit being considered. Since KI is calculated relative normal of the force applied and you have lateral strain, this does not enter into KIC. Basically for thin ( in terms of Z axis ) sample, the plain stresses dominate whereas for a thick sample the plane strain dominates. For the purpose of estimating KIC, you care about the central portion of the sample rather than the sides. As you increase the Z-thickness, you will note the value approaches its real value. So try modeling a piece 4x wider or 4x thinner and you will see his this changes the KIC for the interior. Not sure if I have the explanation right, but its a starting point.
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