Y.W. Zhang

Extracting the mechanical properties of a viscoelastic polymeric film on a hard elastic substrate

A semi-analytical approach was developed to study the creep response of a viscoelastic polymeric thin film on a hard elastic substrate under flat-ended punch indentation. This approach allows one to separate mechanical properties of the film from its substrate. To verify the approach, flat-ended punch creep indentation tests were performed to obtain the long-time viscoelastic behaviors of a polymethyl-methacrylide film overlying on an aluminum substrate. Three viscoelastic models describing the constitutive equations of the polymer film are compared in this paper. “Film-only” viscoelastic parameters were extracted by fitting with the experimental creep curves of flat-ended punch indentation. It is shown that the present approach provides an effective way to characterize the mechanical properties of a polymeric film on an elastic substrate.

Structure-Mechanical Property of Individual Cobalt Oxide Nanowires

We present a comprehensive approach to address the correlation between mechanical properties of nanowires (NWs) with their characteristic size, microstructure, and chemical composition. Using this technique, the Youngs modulus of Co3O4 NWs with different sizes was evaluated. Thermal annealing in inert atmosphere was found to induce chemical reduction of as-grown Co3O4 NWs into CoO NWs without modifying their geometrical shape. Both Co3O4 and CoO NWs exhibited a size-dependent variation in Youngs modulus.

Annealing effects on the elastic modulus of tungsten oxide nanowires

Three-point bend test coupled with transmission electron microscopy (TEM) analysis was carried out on individual tungsten oxide nanowires (NWs) before and after annealing. Three-point bend test monitors the change in the Young’s modulus of the NW after annealing, while TEM provides nanostructural detail changes on the same NW. In this way, insight into the correlation between the mechanical properties of a NW and its nanostructure details can be obtained. Annealing increased the diameter of the NWs by forming a uniform amorphous/polycrystalline outer coating. The coating results in a decrease in Young’s moduli for thicker NWs. On the other hand, annealing led to increased Young’s moduli of thinner NWs, which is attributed to the improved crystallinity in these NWs after annealing. This study points to a more refined strategy for more in-depth understanding of the relationship between the nanostructures and elastic mechanical properties of NWs.

Studying Visco-Plasticity of Amorphous Polymers by Indentation Tests

A constitutive model for thermoplastic polymeric materials and its finite element implementation are presented. A five-step indentation scheme was formulated to extract a complete list of the parameters in the constitutive model. Indentation tests on polymethylmethacrylate (PMMA), following the five-step and other schemes, were performed. The experimental data using the five-step scheme were used to extract all the parameters in the model. The extracted parameters were then used to predict the other experimental results. Good agreements between the experimental results and model prediction indicate that the five-step indentation scheme is a practical approach to determine the mechanical properties of thermoplastic polymers.