Development and evaluation of the sandwich open-hole flexure test

Abstract

An open-hole flexure test method has been developed to assess the notch sensitivity of sandwich composites under flexural loading. This test method, recently standardized as ASTM D8453, utilizes the current long beam four-point flexure fixture with the addition of a centrally-located open through-hole in the standard specimen. Previous research has identified an open-hole diameter that minimized finite-width effects while producing a statistically significant reduction in strength. Finite element analyses focused on quantifying interaction between the open-hole stress concentration and the load span length, hole location tolerance, and misalignment of the specimen and fixture. The effect of non-linear geometry due to large deflection was investigated and limits prescribed for linear geometry assumptions to remain valid. The sandwich configurations investigated consisted of a Nomex honeycomb core and carbon/epoxy facesheet laminates with a range of material orthotropy ratios. A series of mechanical tests were performed to evaluate the proposed specimen design, and the existing test fixture and procedure. The first set of experiments were performed to assess the notched strength sensitivity to the load span length. A second set of experiments assessed the effect of different core densities and thicknesses.