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Carbon fiber reinforced composites often exhibit large amounts of property scatter. Attempts at understanding composite property scatter have led researchers to generate many 2D models which ignore the 3D phenomenon of entanglement. Previous studies of entanglement have suggested it is correlated to a length scale, but have not had large enough samples to determine its size. In this study, fiber paths of long, entangled, continuous fibers were extracted from CT data of an automotive grade, heavy tow composite. Descriptive metrics of these fiber paths were used to quantify the entanglement as a function of position along the fiber direction. Using this data, several minimum length scales for capturing the behavior of multiple descriptors were determined. These length scales revealed where statistical representation of 3D fiber models provides superior information to that of 2D models. 

Carbon Fiber Reinforced Plastics (CFRPs) are widely used due to their high stiffness to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of compacting a dry fibrous reinforcement prior to resin infusion. This process is meant to decrease the average inter-fiber distance, effectively increasing the fiber volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRP laminates, varying only the debulking step in the manufacturing process. High resolution serial sections of all three laminates were taken for analysis. Using these scans, the fiber positions were reconstructed. Statistical descriptors such as local fiber and void volume fractions, fiber orientation, and void distribution and morphology were then generated for each sample. Fiber clusters present within the material were identified and analyzed for each level of debulking applied. Using these descriptors, the effects of debulking on the morphology and organization of the composite microstructure was evaluated. 

Carbon fiber reinforced plastics (CFRPs) are widely used due to their high strength to weight ratios. A common process manufacturers use to increase the strength to weight ratio is debulking. Debulking is the process of transversely compacting a dry fibrous reinforcement prior to wet out with the matrix resin, in order to induce fiber nesting, effectively increasing the volume fraction of the sample. While this process is widely understood macroscopically its effects on fibrous microstructures have not yet been well characterized. The aim of this work is to compare the microstructures of three CFRPs, varying only the debulking step in the manufacturing process. The microstructural effects of debulking on three unidirectional CFRPs made from three different levels of debulking were studied. High resolution serial sections of all three samples were taken using the UES ROBO-MET at the NASA Glenn Research Center in Cleveland, Ohio. Using these scans, the fiber positions were measured and connected to make fiber paths. Statistical descriptors such as local fiber and void volume fractions, and void distribution and morphology were then generated for each sample and compared. Using these descriptors, the effects of debulking on the composite microstructure can be measured. 

Adhesively bonded composite joints can help reduce weight in structures and avoid material damage from fastener holes, but stress concentrations formed at the edges of the adhesive bond line are a main cause of failure. Stress concentrations within the adhesive can be reduced by lowering the stiffness at these edges and increasing the stiffness in the center of the joint. This may be achieved using a dual-cure adhesive system, where conventional curing is first used to bond a lap joint, after which high energy radiation is applied to the joint to induce additional crosslinking in specific regions. Anhydride-cured epoxy resins have been formulated to include a radiation sensitizer enabling the desired cure behavior. Tensile testing was performed on cured systems containing varying levels of radiation sensitizer in order to evaluate its effects on young’s modulus as a function of radiation dose. 

Adhesively bonded joints contain stress concentrations at geometric and material discontinuities within the joint, causing the joint to be inefficient. This study investigates a method to grade the material properties of an adhesive across the bondline to have a soft, flexible adhesive near the stress concentration and a stiff, strong adhesive elsewhere. Theoretical studies and a few experiemental studies have shown an that the load is distributed more evenly along the joint and strength is increased. Adhesive gradation is achieved through a secondary crosslinking system in the adhesive which is activated via radiation. After an adhesive is initially cured, the joint can be exposed to varying levels of radiation to grade the properties. Initial results demonstrate the ability to grade stiffness using radiation shielding, and final results will demonstrate the application in an adhesively bonded joint.