Fracture Toughness of Calcium Hydroxyapatite and Spinel at Different Humidities and Loading Rates

The fracture toughness of spinel (magnesium aluminate) and calcium hydroxyapatite is investigated at different humidities and loading rates at room temperature using a 4 point SENB set-up. It is revealed that the fracture toughness of spinel shows a sharp decrease below 10% humidity and remains constant for humidities above 10%, like alumina's, PZT and zirconia. On the contrary, the fracture toughness of calcium hydroxyapatite decreases more gradually, like MnZn ferrites. A model combining kinetical and adsorption effects, which is developed for MnZn ferrites, is proven to be valid for calcium hydroxyapatite too.