Tensile specimen design and experimental procedures for characterizing polymeric composites using X-ray based micro-tomography

The recent rapid increase in the use of continuous and chopped fiber composites for automotive, aerospace, and naval applications demands an increased understanding of microstructure evolution with stress in order to understand potential failure locations. X-ray imaging with micro-focus source and optics with high resolution shows promise for exploring such technology to study the microstructure. Initial tomography and radiography results will be presented that clearly show individual fibers and their interface with the resin as a function of tensile stress. In this study, we focus on the design of miniature fiber-reinforced polymer specimens suitable for examination during tensile loading using a micro-tomography system. Issues related to potential stress concentrations and experimental boundary conditions are examined using finite element analysis. Two gripping designs and specimen geometries are examined analytically and experimentally. Specimens with cylindrical cross section with specimen ends bonded to a metallic threaded grip were considered for thermoset materials. Grips containing cavities with cylindrical and conical shapes were also examined. A dog-bone shaped, shoulder loaded, square cross section sample was considered for thermoplastic materials because of difficulties bonding them to a metallic threaded grip.