| This work analyzed multi length-scale composites from an experimental and modeling perspective. Three main experiments were conducted: fatigue of foam and pin-reinforced foam cores, indentation prevention using load spreaders, and nanotube reinforcement for increased interlaminar strength.;A method for determining cycles to failure for a sandwich panel was established using an empirical model. The fatigue response is captured by conducting experiments at varying stress levels to determine empirical constants. Stress-softening is observed in 92% of tests under varying fatigue cases, even at low stress amplitudes. A stiffness criterion can be developed to estimate failure such that once a reduced level of stiffness is reached a structure is removed from service.;For load spreaders numerical models showed that a stress concentration develops under the corner of the load spreader. In light of this, a triangular collapse model was derived using a work balance. The model was found to predict the collapse load within 20 N or 12% of the failure load. For design purposes the model allows the minimum spreader dimension to be determined by inputting the expected load on a joint.;Carbon nanotubes were grown on the surface of carbon and glass fibers using CVD. The heat of the reaction and exposure to high temperature were limited to prevent fiber damage. Nanotubes longer than two times the fiber diameter negatively impact the fiber volume fraction which reduces interlaminar strength, but high densities of short nanotubes improve interlaminar strength. Gains of up to 29.4 % were seen in glass, 56.2 % were seen in Grafil and 36.2 % were seen in IM7. |
