CRACK DECORATING TECHNIQUE FOR FRACTURE-TOUGHNESS MEASUREMENT IN ALUMINA

A technique for fracture toughness measurement in alumina is presented . Alumina with bimodal grain-size distribution and containing about 10 % intergranular glassy phase was used as test material. Controlled def ects were introduced by indentation using loads from 1.5 N to 39.2 N. Some specimens were annealed at 1000 degrees C in order to remove the indentation residual stress field. Crack shape and dimension were anal yzed by a decorating technique. Red colorant was poured on indented sp ecimens and decorated fracture surfaces were then observed by optical microscopy on later drying. This technique allowed the complete analys is of indentation crack evolution upon bending. Both indented and anne aled specimens were considered. Shape-factor evolution during crack gr owth was investigated and constant fracture toughness values (3-3.5 MP a root m) independent from crack growth and indentation load were calc ulated. (C) 1997 Elsevier Science Limited.