Lessons Learned in Modeling Honeycomb Sandwich Fillet Fracture with the Extended Finite Element Method

Honeycomb-core sandwich composites provide efficient structural configurations for lightweight space structures. However, the damage tolerance of these composites poses a challenge. In particular, facesheet–core interface delamination caused by impact loads or internal core pressurization can lead to significant damage and eventual failure. The delamination strength of the facesheet–core interface is directly tied to the adhesive fillets that connect the core to the facesheet. The geometry of these fillets, as well as the defects that may arise within them, are functions of the amount of adhesive material used, the cure-process conditions, the adhesive resin properties, and the surface wettability of the core cell walls and facesheet. Because fillet geometry and defects exhibit significant variation in sandwich composites, characterization of the interface fracture strength is required for each design and manufacturing process. To reduce this empiricism, this paper investigates the use of extended finite element method (XFEM) modeling approaches to explicitly model adhesive-fillet failure in honeycomb-core sandwich composites and to understand the fracture and failure mechanisms at the local fillet scale that contribute to facesheet–core delamination crack growth. Significant difficulties were encountered in applying XFEM to model fillet fracture in sandwich composites. XFEM implementation in the ABAQUS software package was used to model fillet fracture for two demonstration problems: (1) longitudinal crack growth along a honeycomb core cell wall aligned with the L-direction to simulate Mode-I delamination growth, and (2) fillet fracture under a flatwise tension test. This paper documents the modeling approaches used, XFEM convergence behavior for 2D and 3D models, challenges encountered in predicting crack growth using XFEM, workarounds developed to address some of these challenges, and lessons learned in applying XFEM to sandwich-fillet fracture analysis.