Effect of Pop-in Precrack Length on the Fracture Toughness of Porcelain-Fused-to-Metal

Abstract

Using the single edge precracked beam (SEPB) method, the fracture toughness (K_IC) of porcelain-fused-to-metal (PFM) was measured to evaluate the effect of pop-precrack length. The K_IC was increased with increases in the pop-in precrack length, as observed in ceramics exhibiting R-curve behavior caused by the crack interface interaction force such as crack bridging due to interlocking grains. To investigate the effect of such an interaction force, a double notch was introduced. However, no decrease on K_IC was found despite exclusion of most of the precrack. Other experiments suggested that the growth of the pop-in precrack did not occur during the measurement of K_IC. From these results, it was concluded that the increase in K_IC with increases in the pop-in precrack length was due to the presence of the interaction force which had already reached the maximal value a very short distance from crack tip. After removing the resistance force, K_IC; was estimated as 0.85±0.06 MPam^1/2. Considering that such an interaction force should be also included as a part of K_IC, the length of the pop-in precrack should be as short as possible, when measuring the K_IC of ceramics such as PFM using the SEPB method.