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Textile composites manufactured using Resin Transfer Modeling (RTM) can offer advantages in some automotive applications including reduction in weight, while being relatively simpler to fabricate than standard laminated composites used for aerospace applications. However, one of the challenges that arise with these textile composite materials is that the mechanical properties are inherently dependent on the local and final (in-situ) architecture of the textile itself as a result of the molding and curing processes. While this provides additional latitude in the composite design process it also necessitates the development of analytical models that can estimate the mechanical properties of a textile composite based on the textile architecture and the properties of the manufactured component. In this paper, an analytical model is developed and its estimations are compared against experimental in-plane engineering properties for composites with various textile architectures. Results from the model are also compared against finite element (FE) based computational results. The microstructures of the 2D triaxially braided composite (2DTBC) studied were extensively characterized. The microstructure properties thus measured were used in the analytical model to estimate the mechanical properties. Uniaxial tension and V-notched rail shear tests were conducted on 2DTBC with different textile architectures. Good agreement between the analytical, computational, and experimental results were observed and are reported here. Furthermore, computational estimations of matrix mechanical properties are limited to the linear elastic range of a representative material volume (unit cell) and coupon data. Full mechanical response of larger 2DTBC structures, albeit of prime interest, is beyond the scope of this work and could be the focus of follow up studies.

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 nondestructive 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 throughtransmission 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.

Foam core sandwich composites are finding a wider use in aerospace, automotive, and construction applications. These structures present unique challenges in terms of material failure and interaction and are sensitive to damage and imperfections introduced during manufacturing. An emerging class of 3D fiber reinforced foam core aims to replace monolithic foams used in sandwich structure cores particularly in demanding high-performance aerospace applications. This research is focused on investigating the development of testing methods capable of measuring the effective interface fracture properties between the facesheet and the core in 3D fiber reinforced foam cores. Double cantilever beam and end-notched flexure specimens are developed to evaluate the mode I and mode II fracture properties of a 3D fiber reinforced foam core. The design, development, and initial failure of a mode I interface fracture test for 3D fiber reinforced foam cores are presented. The digital image correlation results on the failed tests allowed for a different approach to be utilized in designing a new bonded double cantilever beam specimen for testing the mode I fracture of a 3D fiber reinforced foam core sandwich structure that resulted in a successful interface fracture test. The bonded DCB specimens exhibited relatively smooth crack propagation and produced GIc values similar to honeycomb sandwich structures and significantly higher than comparable foam structures.

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