3D imaging of damage in biaxially loaded composites at cryogenic temperatures using a novel micro-CT experiment

This project provided unprecedented insight into damage initiation and evolution in composite materials subjected to extreme aerospace-type loading environments. Specifically, this research aimed to create a comprehensive understanding of through-thickness crack formation in cryobiaxially-loaded composite cryotanks. Cryobiaxial loading is caused by internal pressurization of liquid fuel coupled with low-temperature thermal strain. Considering this goal, two novel apparatuses were developed that leveraged X-ray computed tomography (CT) to 3D image biaxially-stressed composite specimens exposed to room and cryogenic temperatures. The first accomplishment was the development of a biaxial test methodology for characterization of ply-level damage in composites subjected to biaxial loading. A novel biaxial cruciform specimen was designed to promote damage formation in the gage region and enable ex-situ imaging using X-ray CT. This methodology provided, for the first time, a comprehensive view of damage initiation and progression during biaxial loading. The second major milestone was the evaluation of damage formation in three stacking sequences at both room and liquid nitrogen (LN2) temperatures while subjected to biaxial loading. As a result, it was discovered that stitch cracking was the dominant failure mode in all test cases. Four mechanisms contributing to this type of damage were identified. First, strains induced by in-plane mechanical loading caused damage initiation and evolution. Second, thermal-induced strains “pre-loaded” each ply in the laminate and led to increased crack area at a given loading level when compared to results from room temperature (RT) tests. Third, the relative fiber orientation between adjacent plies (Δθ) dictated the density, length, and through-thickness propagation of cracks within a laminate. Large Δθ’s were most effective at resisting through-thickness crack propagation, but resulted in the formation of more crack networks across the XY-plane of the specimens. Conversely, small Δθ’s allowed cracks to readily propagate through-thickness but minimized the total number of crack networks that formed in a specimen. Lastly, cryogenic temperature appears to have a significant influence on material properties and could ultimately accelerate or delay crack formation. Based on the findings in this study, a set of guidelines were proposed for the design of through-thickness damage resistant tape-laminates for future composite cryotanks. The final product of this research was the creation of a novel micro-CT experiment that enabled near-real-time, high-resolution, 3D imaging of damage formation in biaxially loaded composite specimens. Image data from these experiments has resulted in a new understanding of ply- and constituent-level composite materials failure under realistic loading conditions. This research provided a novel approach for ex situ and in situ visualization of structural failure that will lead to unprecedented advances in composite design and validation of laminate failure theories. This data will be invaluable for development and validation of current and future progressive damage models and will enhance our ability to predict failure and structural life of a wide range of composite structures.