Nanomechanical characterization of soft materials such as polymers and polymer composites represents a unique and challenging research topic of current interest. In this presentation, two characterization methods are compared directly using a set of model PDMS polymer systems. Experiments were conducted using both nanoindentation and atomic force acoustic microscopy (AFAM) to determine the dynamic mechanical properties. The experiments were conducted to determine reduced elastic modulus E* via AFAM measurement and storage modulus E via nanoindentation. The AFAM analysis was done using both a Hertzian contact model and a flat punch model. While the two methods showed marginal agreement, the resulting AFAM moduli were dependent on the model used in the analysis. Further investigation is needed to improve the accuracy of these measurements. In addition, further modeling is needed to improve the understanding of the tip-sample interaction in such systems. The results are anticipated to lead to the development of improved characterization methods for compliant materials. [Work supported by ARL.]
Nanoindentation and nanoscratch tests made on the surface of Kevlar KM2 fibers impart contact geometries similar to a typical contact between a particle and a fiber in a particle-infused fabric. In this study, the forces required for indentation and scratching are used as a measure of a single particle gouge of the surface of a Kevlar fiber. The gouging forces and a geometric model are used to calculate the apparent friction and energy associated with particle gouging during impact. The friction associated with particle gouging can be increased up to ∼240% compared to Kevlar yarn-yarn friction levels. The energy of gouging a distribution of particles is on the order of 10% of the energy required for axial tensile failure and 25% of the energy required for transverse compression of the fiber (for a limiting transverse strain of ∼0.3). The relative contributions of the friction and energy associated with particle gouging to the overall energy dissipated by a textile composite are discussed.