Mechanical Properties of Fibers Using a Nano-Tensile Testing System

摘要

Due to the recent evolution of nano-structured polymeric, metallic, and biological multifunctional materials at rapid pace, there is an immediate need in determining mechanical properties using small volume of materials. Determination of modulus, stress state corresponding to yield and/or failure, and associated hardening or softening behavior is most reliably obtained using a direct tension test due to the simplicity in describing the state of stress and strain over a gage length. In the present research, a nano-tensile testing system is being developed for determining the tensile properties of fibers and related materials. The determination of fiber properties via tensile methods clearly requires specific consideration related to type of grips and specimen mounting procedures. Important questions regarding specimen gripping without compromising sample integrity and alignment as applied to fibers having a diameter in the range of 5 to 100 microns has been explored to date. The testing system that is being used has exceptional resolution related to the measurement of axial load (nano-Newtons) and displacement (nanometers) by using a nano-indentor head in an inverted configuration. The global displacement of these fibers is being measured at micron resolution, with axial force resolution at 50 nano-Newtons, exceeding the performance capabilities of any standard load cell. This study presents initial results of tensile testing with polypropylene, glass, copper, and carbon fibers. A review of currently existing international (ISO) and national (ASTM) standards for carbon fiber testing revealed a need for improved templates for mounting fibers. Details of a novel design of new attachment fixtures and template for easy mounting of fiber type specimens along with an experimental setup developed for specimen mounting and alignment aspects will be presented. The present testing system is currently being used to test new materials (for example: electro-spun polymeric fibers with single wall carbon nanotubes). In addition to obtaining stress-strain behavior, using the phase difference between applied sinusoidal load and corresponding displacement, storage and loss modulus as a function of global strain is being measured. This could provide new insight into damage evolution and phase transition during the deformation behavior of fibers or thin films subjected to a tensile state of stress at constant temperature.