Stress intensity factors in thin-walled cracked structures are computed using a mixed-dimensional finite element (FE) shell/extended finite element (XFE) 3D formulation. The present approach belongs to the family of ‘substructuring’ methods. The domain is decomposed into cracked and safe subdomains which are solved by the XFE-code and the FE-software, respectively. The XFE-3D domain may contain cracks that do not necessarily join the top and bottom faces of the plate. The interface problem is solved using the finite element tearing and interconnecting method. Several validations are provided. Copyright © 2007 John Wiley & Sons, Ltd.