Effect of defect size on fracture strength of dental low fusion porcelain.

Various grinding defects were produced on the surface of specimen dental low fusion porcelain in an attempt to establish the relationship between defect size and fracture strength. In addition, the applicability of the process zone size-fracture criterion in assessing the material properties of dental low fusion porcelain was examined. Super porcelain AAA E3 (Noritake Co., Japan) was used as dental low fusion porcelain. The bending strength and fracture toughness value were estimated by the three-point bending test. After glazing, grinding flaws were introduced by grinding the specimen with abrasive papers of various mesh sizes. In order to calculate the fracture toughness value of dental low fusion porcelain, we introduced a surface crack using a Vickers indenter. The results were discussed based on the process zone size-fracture criterion. The size of cracks caused by grinding was estimated with the process zone size-fracture criterion and Newman-Raju formula. As the defect size decreased, the fracture stress approached the strength for smooth specimen without defect. The K(c) value showed a tendency to approach the K(lc) value when the defect size increased. The relationship between the fracture stress, sigma(F), and the equivalent crack length, a(e), was in good agreement with the theoretical relations deduced from the criterion in dental low fusion porcelain.