If your initial crack length is 'a', can you try an element sizing of a/10 and a/20 to see if the SIF values are relatively close? Also, the SIF values for contours 3-6 should converge and will be more accurate than contours 1 and 2, which are typically the least accurate due to being closes to the stress singularity. Lastly, are your material properties linear elastic?
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