Crack stability and its effect on fracture toughness of hot-pressed silicon nitride beam specimens

The effect of stable crack extension on fracture toughness test results was determined using single-edge precracked beam specimens, Crack growth stability was examined theoretically for bars loaded in three-point bending under displacement control, The calculations took into account the stiffness of both the specimen and the loading system, The results indicated that the stiffness of the testing system played a major role in crack growth stability, Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions, Hot-pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension, The fracture toughness values from the unstable tests were 10% higher than those from the stable tests, This difference, albeit small, is systematic and is not considered to be due to material or specimen-to-specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.