Ultra-thin Composites Materials
A recently discovered property of composites is that when the plies are manufactured thin enough, they can withstand very large surface strains under bending (ranging from 2% to 8% strain or more), and recover their initial shape when unloaded without damage. This remarkable resilience, combined with their high specific stiffness, makes thin-ply shells comprising of 2 to 7 layers an ideal choice for designing the next generation of lightweight booms, longerons, antennas, radiators, and other space structures that can be folded or coiled for efficient packaging. In addition, due to their ability to undergoe large geometric deformations, ultra-thin composite materials are also finding broader structural applications in areas such as robotics, morphing wings, energy harvesters, and biomedical devices.
Large Curvature Column Bending Test
The testing and characterization of ultra-thin composites, also called high strain composites (HSC), in the large curvature region is currently poorly understood. Due to the differences in microstructure and manufacturing approaches, the material response of HSC differs significantly from that of standard composite materials. Despite their remarkable properties stemming from thinness and compliance, this poses challenges for conducting conventional experiments for constitutive modeling. The recently introduced column bending test (CBT) fixtures offer a solution, allowing the characterization of not only bending stiffness and strength but also fracture properties such as toughness. This capabilities are critical for future design of crack-resistant materials and deployable structures.
Fracture Mechanics of Nonlinear Plates and Shells
Spacecrafts made of HSC materials undergo various processes which can induce material failure due to large curvature loads. These include localized buckle formation due to coiling or packaging, viscoplastic rupture due to long stowage times, propagation of folds due to dynamic deployment, and extreme environmental conditions while in-space. The high localization of the fracture process zone in the material is indicative of a quasi-brittle failure under bending in the HSC material. The formation of cracks can adversely effect the overall stiffness of the shell structure or lead to further crack growth processes, which can induce catastrophic failure of the overall spacecraft.The lack of approaches to characterize the toughness of HSC materials under large curvatures leads to the adoption of conservative design margins that have limited the achievable coiling radius and led to lower packaging efficiencies.
We consider the use of the CBT, in tandem with a numerical analysis of the energy release rate using the virtual crack extension method, to determine the critical bending mode fracture toughness of thin-ply composite laminates. Notched samples are tested in the geometrically nonlinear regime until failure, from which the fracture toughness of the material is calibrated with finite element simulations. The development of the testing approach will enable for future design of tougher laminates that suppress crack growth through new combinations of fiber and matrix constituents (thermoplastic vs thermoset), ply orientations, ply type (unidirectional, glass fiber plane weave), etc. In addition, due to the stringent demands by the space industry and the increasing adaptation of HSC as the material of choice for spacecraft designs, we are also interested in characterizing the effects of in-space environmental conditions on toughness. These include material degradation due to radiation, temperature fluctuation, and long-term viscoplastic rupture.
