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In this work, dislocation dynamics (DD) analysis is used to investigate the strength of nanoscale metallic multilayered composites. Several possible interactions between threading (glide) dislocations and intersecting interfacial dislocations are considered and found to lead to strength predictions in better agreement with experimental trends and significantly higher than the predictions of the simplified confined layer plasticity model based on Orowan bowing of single dislocation in a rigid channel. The strongest interaction occurs when threading and intersecting interfacial dislocations have collinear Burgers’ vector and involves an annihilation reaction at their crossing points followed by the resumption of threading with a new dislocation configuration. The other possible dislocation intersections involve the formation of junctions, which are found to be more complex than simple models suggest. When the layer interfaces are modeled as impenetrable walls, as in existing analytical and some dislocation dynamics (DD) models, the predicted strengthening effect is weaker than that predicted by DD with more physical boundary conditions at the interfaces.

The present work reports a novel approach to enhance the fracture resistance and notch sensitivity of carbon fiber-reinforced polymer composites utilizing additive manufacturing (3-D printing) fabrication. The 3-D printed composites utilize carbon fiber bundles to reinforce nylon/chopped fiber resin in a multilayered structure configuration. Single-edge (60°) notched samples were printed using Mark Two printer. Three reinforcement schemes were designed and used to manufacture the specimens. The focus was placed on selective reinforcement at the crack tip to arrest crack initiation. The mechanical properties, fracture toughness, and fracture behavior of the printed composites were evaluated. It was found that wrapping fiber around the notch effectively blunted the notch and redirected crack propagation away from the notch tip, thereby lengthening the crack path and leading to improved fracture resistance. It was also found that such improvement reaches a saturation level. Excessive notch reinforcement beyond optimal limit can reverse the gains in fracture resistance due to notch-targeted reinforcement. Examination of the fracture surface morphology of the printed composites reveals lack of fusion of the sizing of the individual continuous carbon fiber bundles and the lack of adhesion between the matrix layers (nylon/chopped fiber resin) and the adjacent carbon fiber bundle reinforcement. Damage to the fibers within the carbon bundle was also observed. Thus, a synergetic effect of the carbon fiber bundles reinforcement and the matrix requires more optimization to manufacture carbon-reinforced polymer composites using 3-D printing.