Fabrication and dynamic mechanical behavior of nanocomposites
Polyester/TiO2 nanocomposites have been fabricated using an in-situ polymerization technique coupled with ultrasonics, and an investigation has been conducted to characterize their mechanical and fracture behavior under quasi-static and dynamic loading conditions. The presence of the particles had the greatest effect on fracture toughness; negligible particle influence was observed in the remaining mechanical properties obtained using quasi-static loading. Scanning electron microscopy analysis of fracture surfaces was carried out to identify toughening mechanisms. Dynamic fracture toughness testing was carried out, and an increase in dynamic fracture toughness relative to quasi-static fracture toughness was observed. High strain rate testing conducted using a split-Hopkinson pressure bar apparatus revealed a moderate stiffening effect with increasing particle volume fraction. ^ Dynamic photoelasticity coupled with high-speed photography was used to obtain the dynamic fracture constitutive behavior of polyester and nanocomposites. Birefringent coatings were used to conduct the photoelastic study due to the opaqueness of the nanocomposites. Two different specimen geometries were employed to obtain a broad range of crack velocities. Crack run-arrest, propagation, and branching events in polyester and nanocomposites were investigated and compared. Crack arrest toughness in nanocomposites was found to be 60% greater than that in polyester. Crack propagation velocities in nanocomposites were found to be 50% greater than those in polyester. Incipient branching values were 2.4 and 2.6 times the corresponding values of KIC in polyester and nanocomposites, respectively. ^ A one-point strain measurement technique using a modified Hopkinson pressure bar apparatus was employed in a parametric study to evaluate the limiting conditions of validity of employment of quasi-static relations in the determination of dynamic fracture initiation toughness in brittle and moderately brittle polymers. The technique was combined with photoelastic analysis and high-speed photography to validate results obtained in polyester and PMMA (polymethylmethacrylate) three-point bend specimens. Specimen size, crack length, incident pulse length and amplitude, and loading rate were variables used in the parametric study. A hypothesis was proposed and tested in an effort to explain how inertia effects interfere with the measurement of dynamic fracture initiation toughness using the one-point strain measurement technique. ^
Engineering, Mechanical|Engineering, Materials Science
Victor Manuel Fortes Evora,
"Fabrication and dynamic mechanical behavior of nanocomposites"
Dissertations and Master's Theses (Campus Access).