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Title: Effect of precrack ``halos`` on fracture toughness determined by the surface crack in flexure method

Abstract

The surface crack in flexure 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 article, 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 Al{sub 2}O{sub 3}, and AlN, a glass-ceramic, and a MgF{sub 2} reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF{sub 2} is due to indentation-induced residual stresses initiating crack growth. However, for the Al{sub 2}O{sub 3} and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.

Authors:
 [1];  [2]
  1. Army Research Lab., Aberdeen Proving Ground, MD (United States)
  2. National Inst. of Standards and Technology, Gaithersburg, MD (United States). Ceramics Div.
Publication Date:
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
655383
DOE Contract Number:  
AC05-84OR21400
Resource Type:
Journal Article
Journal Name:
Journal of the American Ceramic Society
Additional Journal Information:
Journal Volume: 81; Journal Issue: 9; Other Information: PBD: Sep 1998
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; FRACTURE PROPERTIES; ALUMINIUM OXIDES; ALUMINIUM NITRIDES; GLASS; MAGNESIUM FLUORIDES; CRACKS; FRACTOGRAPHY; OPTICAL MICROSCOPY; RESIDUAL STRESSES; CRACK PROPAGATION

Citation Formats

Swab, J J, and Quinn, G D. Effect of precrack ``halos`` on fracture toughness determined by the surface crack in flexure method. United States: N. p., 1998. Web. doi:10.1111/j.1151-2916.1998.tb02620.x.
Swab, J J, & Quinn, G D. Effect of precrack ``halos`` on fracture toughness determined by the surface crack in flexure method. United States. https://doi.org/10.1111/j.1151-2916.1998.tb02620.x
Swab, J J, and Quinn, G D. 1998. "Effect of precrack ``halos`` on fracture toughness determined by the surface crack in flexure method". United States. https://doi.org/10.1111/j.1151-2916.1998.tb02620.x.
@article{osti_655383,
title = {Effect of precrack ``halos`` on fracture toughness determined by the surface crack in flexure method},
author = {Swab, J J and Quinn, G D},
abstractNote = {The surface crack in flexure 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 article, 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 Al{sub 2}O{sub 3}, and AlN, a glass-ceramic, and a MgF{sub 2} reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF{sub 2} is due to indentation-induced residual stresses initiating crack growth. However, for the Al{sub 2}O{sub 3} and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.},
doi = {10.1111/j.1151-2916.1998.tb02620.x},
url = {https://www.osti.gov/biblio/655383}, journal = {Journal of the American Ceramic Society},
number = 9,
volume = 81,
place = {United States},
year = {1998},
month = {9}
}