Fracture Toughness of Unidirectional Fiber Reinforced Ceramic Composites in Mode II Utilizing Laser Interferometry

A laser interferometry technique to measure crack opening displacement was developed and applied to end notched flexure specimens of a Corning Glass Works 1723 glass matrix with silicon carbide fibers. The laser interferometer displacement guage monitored the crack opening displacement COD at the location of a support while specimens were subjected to Mode II forward shear cracking at room temperature via three point bend tests on a standard Instron Compression machine. Vertical displacement was measured at the center of the test specimen using a linear variable differential transformer. Load verses displacement curves were generated for both the LVDT displacement and the COD. The COD curve showed a marked improvement in determining the onset of crack growth. From the enhanced critical load determination, the materials fracture toughness in Mode II, GI.Ic was calculated for varying crack lengths. The calculations provided estimates with a variance of only 10 from the mean thus illustrating the utility of this procedure.