Journal Published Online: 01 September 1993
Volume 21, Issue 5

Some Microstructural Conditions for Evaluating Fracture Toughness and -Curve Behavior in Platelet-Reinforced Composites

CODEN: JTEVAB

Abstract

The validity of Kk determinations by various techniques usually employed and compared with each other is discussed in detail for two ceramic materials. They are a simple Si9N4 material fracturing with transgranular crack propagation and a Si9N4-25vol% SiC-platelet measured by four different methods: single-edge V-notched beam (SEVNB), single-edge precracked beam (SEPS), indentation fracture (IF), and chevron-notched beam (CNB). It is generally recognized that, in platelet composites, Kk values can be reliability obtained only by procedures adopting notched specimens in which the wake-zone of precracking is mechanically removed (SEVNB or renotched SEPB). In the present investigation the CNB method, in which the wake contribution to fracture resistance is generally small but not negligible, was also found to give stress-intensity factors at maximum load whose value was almost coincident with the KIc determined by SEVNB method. Using stereological concepts, fractographic observations, and acoustic emission (AE) experiments, the microstructural conditions for negligible wake-contribution during the subcritical stable crack extension in the CNB experiment were worked out and applied to rationalize the behavior of the present composite. Apparent toughness values markedly higher than the “true” Kk of the composite were measured both by the standard SEPB and IF method. Such a discrepancy is considered due, in the former case, mainly to traction forces between the precracked surfaces while, in the latter, to an insufficient account of the residual stress field associated with the indentation.

Author Information

Pezzotti, G
The Institute of Scientific and Industrial Research, ISIR, Osaka University, Osaka, Japan
Niihara, K
The Institute of Scientific and Industrial Research, ISIR, Osaka University, Osaka, Japan
Nishida, T
Kyoto Institute of Technology, Faculty of Polytechnique Science, Kyoto, Japan
Pages: 8
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Stock #: JTE11779J
ISSN: 0090-3973
DOI: 10.1520/JTE11779J