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 defects 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 analyzed by a decorating technique. Red colorant was poured on indented specimens and decorated fracture surfaces were then observed by optical microscopy on later drying. This technique allowed the complete analysis of indentation crack evolution upon bending. Both indented and annealed specimens were considered. Shape-factor evolution during crack growth was investigated and constant fracture toughness values (3-3.5 MPa root m) independent from crack growth and indentation load were calculated. (C) 1997 Elsevier Science Limited.