1 Modeling Failure of 3D Fiber Reinforced Foam Core Sandwich Structures with Defects

Foam core sandwich composites are widely used in primary structural components of launch vehicles and spacecraft. These structures exhibit complex failure modes that are sensitive to butt-joints, core mismatches, impact damage, voids and facesheet delaminations. 3D Fiber Reinforced Foam Core (3DFRFC) represents a new class of core material designed to replace standard foam core in future aerospace structures. An analysis, test, and non-destructive evaluation program was developed to estimate the strength reduction for debonds between the facesheet and the 3DFRFC. A compression test method was selected to evaluate fully bonded samples and samples with debonds of varying diameters: 0.5 inch, 1.0 inch, 1.5 inch, and 2.0 inch. The manufactured debonds were verified using through-transmission ultrasonic inspection. Nonlinear finite element analysis with a progressive failure methodology was used to understand the failure processes and predict strength reduction. The typical failure mode for 0.5 inch, 1.0 inch, and 1.5 inch debonds was facesheet compression failure, while analysis showed that for larger debonds failure consisted of a rapid sequence of buckling, delamination, and fiber failure. Testing and analysis demonstrated significant strength reductions for the 2.0 inch debond. I