| Description |
The rise in popularity of sandwich composites has created a demand for methods to properly characterize their fracture behavior. This research builds upon previous development of the Separated End-Notched Flexure (S-ENF) test, a proposed test configuration for characterization of Mode II critical energy release rates for facesheet/core debonding in sandwich composites. The current work presents analytical methods for designing specimens that will produce stable crack growth behavior, and suggests a simplified approach for calculating energy release rates. The analytical model was utilized to study the effects of specimen design on the test behavior and the stress state within the core. Analyses included assessments of sandwich composites with foam and honeycomb core materials. Finite element analyses performed as part of the investigation supported the conclusions drawn from the analytical modeling. These analyses predicted that crack growth stability could be achieved by properly designing the sandwich specimen geometry. The pre-crack length was identified as a critical parameter, and a minimum required length was theorized for stable crack growth. inplane compressive stresses within the core were also of concern, and proper specimen design is required to prevent core failure in an undesired mode. The examination of empirical data suggests that crack growth behavior is sensitive to specimen compliance, and that the specimen compliance must remain below a threshold value to achieve stable crack growth. Results from mechanical testing showed that the closed-form method is iv suitable for calculating energy release rates and is preferred over finite element analysis. These results demonstrate the importance of proper specimen design and support the inclusion of the presented closed-form equations within a standardized test method. |
