Modeling 3D Fiber Reinforced Foam Core Sandwich Structures Using a Multi-Scale Finite Element Approach.

Abstract

An engineering challenge of composite sandwich structures is quantifying their ability to tolerate damage, particularly in launch vehicles and spacecraft, where mission assurance is critical. Recently, there has been a development of new core materials that may alter their damage tolerance through the use of a three-dimensional, truss-like network of reinforcing fibers inside a lightweight foam core. This research focuses on the testing and developing a multi-scale approach to model 3D Fiber Reinforced Foam Core (3DFRFC) sandwich composites with defects across typical operating temperatures. Details of the 3DFRFC measured directly from the microstructure were utilized to develop a parametric code for generating detailed embedded element models. These models were used for direct detailed modeling of fracture, edgewise compression, flatwise tension, flatwise compression, and three point bending test specimens. The embedded element models were also used as the cornerstone of a new method of developing effective homogenized properties for 3DFRFCs based on the details of the microstructure. Improved homogenization techniques developed by including the local interaction between the facesheet and the core are also included. The development of a new bonded double cantilever beam (BDCB) specimen for testing the Mode I fracture of a 3DFRFC sandwich structures is presented. The BDCB specimens exhibited relatively smooth crack propagation and produced GIc values similar to honeycomb sandwich structures and significantly higher than comparable foam structures. A full fabrication, testing, and evaluation of 3DFRFC specimens with differing sizes of facesheet-to-core interface debonds is also presented. The analysis methods were able to predict the failure load and modes within 5%. The 3DFRFC proved to be tolerant to facesheet-to-core debonds with only the largest debond demonstrating a statistically significant reduction of 22%. Finally, a detailed investigation of the through thickness behavior of a 3DFRFC composite under ambient and cold conditions is included demonstrating better through thickness ambient performance than unreinforced cores and relatively small reductions in strength at cold temperatures. The investigation into the performance of 3DFRFC composite structures highlights the robust behavior of the structure to cold environments while underscoring the importance of loading direction on the structural response of these highly orthotropic composites.