Fracture behavior of individual carbon fibers in tension using nano-fabricated notches

The fracture toughness of single Toray T700 polyacrylonitrile (PAN) carbon fibers is measured using focused ion beam (FIB) nano-fabrication techniques to induce controlled geometry of end notches with lengths 100 nm to 1 μm. These fibers were subjected to axial loading with a nano-tensile testing system for evaluating mode I fracture behavior. The test data indicates the mode-I fracture toughness (KIC) for these PAN based carbon fibers is 1.73 MPa m1/2 for notch sizes larger than 300 nm, which compares well with limited previous studies. However, measurement scatter in the current study is significantly reduced by using a FIB artificial notch opposed to the native flaw distribution for fracture initiation. In addition, improved fiber mounting and alignment procedures coupled with the precision of the nano-tensile testing system assist in unprecedented resolution in single fiber fracture toughness. For notch sizes less than 0.3 μm, the KIC fracture toughness increases with decreasing notch size, confirming PAN carbon fibers sheath/core microstructure hypothesis. The softer core and harder shell due to synthesis and processing steps suggest the measured data has important implications to single carbon fiber structure–mechanical property relationship.