Accession Number : ADA335068

Title :   Effect of Precrack "Halos" on KIc Determined by the Surface Crack in Flexure Method

Descriptive Note : Final rept. Aug 95-May 97

Corporate Author : ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD

Personal Author(s) : Swab, Jeffrey J. ; Quinn, George D.

PDF Url : ADA335068

Report Date : DEC 1997

Pagination or Media Count : 42

Abstract : The surface crack in flexure (SCF) method, which is used to determine the fracture toughness of dense ceramics necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this paper, fractographic techniques are used to characterize evidence of stable crack extension, a halo, around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al2O3, an AlN, a glass-ceramic, and a MgF2 revealed the presence of a "halo" around the periphery of each precrack. The halo in the AlN was merely an optical effect due to crack reorientation, while the halo in the MgF2 was due to indentation-induced residual stresses initiating crack growth. However, for the Al2O3 and the glass-ceramic, environmentally assisted slow crack growth (SCG) was the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size in order to determine the appropriate fracture toughness.

Descriptors :   *FRACTURE(MECHANICS), *CERAMIC MATERIALS, *CRACK PROPAGATION, CRACKING(FRACTURING), TOUGHNESS, STRENGTH(MECHANICS), FRACTOGRAPHY, ALUMINUM OXIDES, RESIDUAL STRESS, MAGNESIUM COMPOUNDS.

Subject Categories : Ceramics, Refractories and Glass
      Properties of Metals and Alloys

Distribution Statement : APPROVED FOR PUBLIC RELEASE