Evaluation of Rib/Skin Fracture in Composite Isogrid

Abstract

Fracture mechanics was used to evaluate failure of the rib/skin connection of composite “isogrids.” The energy derivative method was used to determine the strain energy release rate (\iG) using a general purpose finite-element (FEM) program with an orthotropic material model. Flexure tests were conducted to observe when rib/skin interface fractures propagate, and the results were correlated with the FEM-based fracture mechanics model. A laser interferometry technique, electronic shearography was used to monitor crack propagation during the flexure tests. Environmental scanning electron microscopy (ESEM) was used to understand the micromechanical aspects of fracture. Variation of the fiber volume fraction, rib thickness, and the presence of weak shear zones were found to be critical to the failure process. The paper explores the concepts of linear elastic and nonlinear fracture as applicable to these graphite/epoxy composite structures. This study provides the designers and users of the isogrid with a methodology for quantifying structural degradation as a function of the rib/skin disbond.