I am looking for a method to calculate the stress intensity factor (SIF) by using XFEM at different crack lengths. As far as I know, XFEM only allows the calculation of SIF for stationary cracks. However, I came across a research paper where XFEM was used to obtain the SIF at various crack length positions, and a graph was plotted accordingly. But i didn't understand how the author calculate it and i need help. The graph and its explanation are provided in the attached file.