Effect of Griffith precracks on measurement of composite fracture toughness

The objective of this study was to produce true Griffith-type cracks in composite specimens for measuring fracture toughness. A compact test specimen geometry, conforming to the ASTM standard (E-399-84) was chosen for the evaluation. Five methods of precracking were evaluated: 1) placing no precrack (notch in specimen left blunt); 2) placing a sharp notch with a Bard Parker blade under hand pressure; 3) placing a sharp notch with a Bard Parker blade under hand pressure, followed by stressing the sample at 60% of the failure load for 25 cycles; 4) forming a sharp notch by placing a razor blade in the sample during polymerization; and 5) placing a Griffith-type crack. Results indicate that the method of precracking a sample does have a significant effect on the measured value of the fracture toughness.