Time Dependent Failure of Flexible Composite Structures
Researchers
Kanthasamy (Uba) Ubamanyu
Daniele Ghedalia
Armanj Hasanyan
Sergio Pellegrino
Description
Ultra-thin composite laminates are used to manufacture coilable thin shells that achieve high packaging efficiency for deployable spacecrafts such as satellites, solar panels, and antennas. In the stowed configuration, the laminates undergo large curvature changes over timescales ranging from weeks to years. Under these curvatures, the composite experiences stress relaxation, dominated by the polymer matrix constituent, which can lead to rupture or localized cracking.
The microstructural changes that lead to rupture due to curvature loads not well understood. In this study, a new experimental approach, Flattening to Rupture (FTR) test is developed, which enforces a constant curvature on the composite. In this experimental setup, the initially curved sample is flattened between transparent glass plates to impose a curvature change in the laminate. To accelerate the relaxation process, the flattened composites are placed at an elevated temperature (time-temperature superposition principle).
The first advantage of the FTR is that in the flattened configuration, the surfaces of the composite are exposed for visual inspection of damage throughout the process. This allows one to record the exact time at which the surface cracks form.
Novel Flattening to Rupture (FTR) experimental setup in its undeformed configuration.
Flattened configurations for inducing rupture due to a constant curvature, applied for extended timescales.
The second advantage of the test is that it overcomes the geometric constrains of the X-ray scanner and allows one to conduct high-resolution micro scale X-ray computed tomography (micro-CT). Because the sample is in the loaded configuration throughout the process, the micro-CT imaging is conducted in an in-situ manner. To view the microstructural changes, micro-CT is conducted at various times throughout the relaxation process of the composite, until rupture. The dominant micro-scale failure mechanisms are microbuckling (kinking), fiber-matrix debonding, inner-ply delamination, etc., which lead to the formation of macroscopic surface cracks.
Volumetric view (4 mm x 4 mm x 4 mm) of a 7-ply composite in its initially deformed and ruptured states.
Snapshots of the damaged region in the 7-ply composite at various loading times.
ZEISS Xradia 510 Versa 3D X-ray microscope (XRM) was used for generating the volume imaging of the flexible composites. Visualization and image processing of the X-ray scans was done using Avizo software.
Publications:
Ubamanyu, K., Hasanyan, A. D., and Pellegrino, S. (2020). Experimental study of time-dependent failure of high strain composites. SciTech 2020, Orlando (FL), AIAA-2020-0207.
