Tensile strength and fracture surface characterization of Hi-Nicalon™ SiC fibers

Dimensional, tensile strength and fracture surface characterizations were carried out for a particular batch of Hi-Nicalon™ SiC fiber. In general, filaments with larger cross-sectional areas (equivalent diameters) had lower strengths than filaments with smaller cross-sectional areas. A cyclic variation of fiber diameter along filament lengths was discovered with a repeat distance of about 16-cm and a maximum rate of change of about ±0.6 μm/cm. During tensile tests at room temperature, fracture originated at critical flaws that typically consisted of internal pores or carbonaceous inclusions. Well-demarcated mirror and hackle regions usually surrounded the critical flaws. With a few exceptions, the critical flaw size (ac) was linearly related to the mirror size (rm) by ac≈0.33rm. From fracture mechanics principles, values for the average mirror constant (Am) and effective fracture toughness for this batch of Hi-Nicalon™ fiber were estimated to be 2.99±0.33 and 1.1±0.2 MPa m1/2, respectively.