The Separated End Notched Flexure Test for Mode II Fracture Toughness Characterization of Sandwich Composites

Abstract

The Separated End Notched Flexure (S-ENF) test is proposed for Mode II fracture toughness characterization of facesheet/core disbonding in sandwich composites. While similar to the three-point End Notched Flexure test for monolithic composites and the Cracked Sandwich Beam test for sandwich composites, the proposed test utilizes a tensioned wire placed underneath the top facesheet to maintain separation and prevent frictional forces during loading. The use of a tensioned wire eliminates the need to remove core material from the test specimen to achieve the desired separation. Of particular interest in the development of the S-ENF test was the prevention of unstable disbond growth for a variety of sandwich composite configurations. A combined numerical and experimental investigation was performed to identify factors contributing to stable Mode II disbond growth. Three-dimensional finite element analysis was used to investigate effects of the initial disbond length and core thickness of the sandwich configuration as well as the span length of the flexure test fixture. Numerical results suggest that ratio of the initial disbond length relative to the span length must be selected properly for stable disbond growth. Mechanical testing confirmed that stable disbond growth can be produced using composite sandwich specimens with Nomex honeycomb and polyurethane foam cores. However, further investigation is needed to better understand the considerations leading to suitable S-ENF specimen design.