On the Tensile Failure of Single Carbon Fibers Subjected to Transverse Loading

This study investigates the applicability of the transverse loading technique for inducing tensile failure in single IM7 carbon fibers of nominal diameter 5.2 µm over a range of microscale gagelengths. Transverse loading experiments and finite element simulations are performed using different indenter tip radii (R~500 µm, 100 µm, 50 µm, and 20 µm) and fiber starting angle configurations to characterize the resulting multiaxial stress state and identify loading configurations that promote tensile failure. Fiber failure under transverse loading is studied in terms of the experimentally determined global (far-field) axial failure stress and the locally induced axial tensile, axial compressive and transverse compressive stresses induced within the fiber–indenter contact region. SEM images of failed fiber ends retrieved after testing are also used to gain insights into the fiber failure under different loading conditions. The results show that bending-induced axial compressive stresses become more pronounced with decreasing indenter tip radii and increasing fiber starting angle and are expected to govern fiber failure. Tensile fiber failure is expected to occur only for R~100 µm indenter at all starting angles and for R~50 µm indenter at 0°. These loading configurations induce axial tensile stress concentrations of magnitude SCF~2.5–2.8 over a range of microscale lengths lσ,max~5−70 μm. However, fiber failure is not consistently expected to occur within the stress concentration region because the axial tensile stress field is comprised of both a localized stress-concentration region and a uniformly stressed region, allowing failure to initiate in either region in accordance with weakest-link statistics. Thus, the effective gagelength cannot be defined solely by the length of the induced stress concentrations and requires consideration of the entire axial stress distribution of the fiber along with weakest link statistics which is beyond the scope of the current study. The identified combinations of indenter geometry and fiber starting angle in this study provide a basis for determining fiber tensile strength distribution at microscale gagelengths.