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

摘要

The Separated End Notched Flexure (S-ENF) test is proposed for Mode IIfracture toughness characterization of facesheet/core disbonding in sandwichcomposites. While similar to the three-point End Notched Flexure test for monolithiccomposites and the Cracked Sandwich Beam test for sandwich composites, theproposed test utilizes a tensioned wire placed underneath the top facesheet to maintainseparation and prevent frictional forces during loading. The use of a tensioned wireeliminates the need to remove core material from the test specimen to achieve thedesired separation. Of particular interest in the development of the S-ENF test was theprevention of unstable disbond growth for a variety of sandwich compositeconfigurations. A combined numerical and experimental investigation was performedto identify factors contributing to stable Mode II disbond growth. Three-dimensionalfinite element analysis was used to investigate effects of the initial disbond length andcore thickness of the sandwich configuration as well as the span length of the flexuretest fixture. Numerical results suggest that ratio of the initial disbond length relative tothe span length must be selected properly for stable disbond growth. Mechanicaltesting confirmed that stable disbond growth can be produced using compositesandwich specimens with Nomex honeycomb and polyurethane foam cores. However,further investigation is needed to better understand the considerations leading tosuitable S-ENF specimen design.