Effects of Subcritical Crack Growth on Fracture Toughness of Ceramics Assessed in Chevron-Notched Three-Point Bend Tests

Chevron-notched bend specimens in three-point loading are increasingly used for evaluation of fracture toughness of structural ceramics. Its advantages include stable precracking of the specimen during loading, a simple specimen geometry, and a compressive mode of loading that is convenient for testing ceramics at elevated temperatures. Subcritical crack growth caused by environmental interactions or mass transport phenomena at elevated temperatures, however, can affect the apparent fracture toughness measured. A numerical computational study was carried out to assess the effects of subcritical crack growth on crack stability in the chevron-notched three-point bend specimens. A power-law relationship between the sub-critical crack velocity (v) and the applied stress intensity (K_I) was used along with compliance and stress-intensity relationships for the chevron-notched bend specimen to calculate the load response under fixed deflection rate and a machine compliance. The results indicate that the maximum load during the test occurs at the same crack length for all the deflection rates; the maximum load, however, is dependent on the deflection rate for rates below a critical rate. The resulting dependence of the apparent fracture toughness on the deflection rate is compared to experimental results on soda-lime glass and polycrystalline alumina.