While it is true that honeycomb core can trap moisture, it is a myth that widespread moisture ingression is an inevitable outcome when honeycomb sandwich structures are exposed to real world environments. It is also a myth that when moisture ingression occurs, progressive weight gain and strength loss are inevitable outcomes. Using a rebuttal of a 2004 honeycomb-critical paper as the focal point, this paper summarizes multiple sources which indicate both that moisture ingression is preventable and, even if it does occur, that proper material choices can prevent severe consequences. These claims are further supported by results from in-plane compression testing of typical (0.064 g/cm 3 aramid-paper/phenolic core with 0.116 cm carbon/epoxy faces) composite honeycomb sandwich specimens after impacting, water submersion, and extensive thermal cycling.

The design, analysis and testing of composite-to-metal joints for the ARPA Man-Rated Demonstration Article are discussed. The first is a composite-to-titanium sphere joint designed to react sphere membrane stress with block compression loading. A series of uniaxial compression tests to simulate this joint assembly was conducted. Test results successfully demonstrated the ability of the joint to withstand block compression loading even in the presence of assembly fitup gaps. A polymer shimming material (Hysol EA9394) was also evaluated as a means to fill potential fitup gaps and was found to effectively reduce stress concentrations resulting from the gaps. The second joint discussed is one between 120-degree segments that make up a stiffener ring for a ringstiffened composite cylinder. Joint design includes Hysol EA9394, boltedon titanium splice plates, and bondedon thermoset flange straps. 2D joint tests, single stiffener tests, and sub-scale ring-stiffened cylinder hydrostatic tests to validate the joint design are described.

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Using established principles of laminate buckling theory and postbuckling load redistribution, a theory is derived for crippling analysis of specially orthotropic or very nearly specially orthotropic laminated composite stiffeners. Results from testing of SCS-6/Ti-15-3 titanium matrix composite (TMC) stiffener segments are also presented and show good correlation with theoretical predictions. The testing program includes three different layups, three different geometries, and a repetition of two tests for each possible geometry/layup combination for a total of 18 tests. The results of the theory are presented in the form of easy-to-use design curves.