Some microstructural conditions for evaluating fracture toughness and R-curve behavior in platelet-reinforced composites

Journal Article · Wed Sep 01 00:00:00 EDT 1993 · Journal of Testing and Evaluation; (United States)

OSTI ID:6147951

Pezzotti, Giuseppe; Niihara, Koichi ^[1]; Nishida, Toshihiko ^[2]

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1. Osaka Univ. (Japan). Inst. of Scientific and Industrial Research
2. Kyote Inst. of Tech., Kyoto (Japan). Dept. of Materials Engineering

The validity of K[sub Ic] fracture toughness determinations by various techniques usually employed and compared with each other is discussed in detail for two ceramic materials. They are a simple Si[sub 3]N[sub 4] material fracturing with transgranular crack propagation and a Si[sub 3]N[sub 4]-25vol% SiC-platelet measured by four different methods: single-edge V-notched beam (SEVNB), single-edge precracked beam (SEPB), indentation fracture (IF), and chevron-notched beam (CNB). It is generally recognized that, in platelet composites, K[sub Ic] values can be reliably 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 K[sub Ic] 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'' K[sub Ic] 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.

• OSTI ID: 6147951
• Journal Information: Journal of Testing and Evaluation; (United States), Vol. 21:5; ISSN 0090-3973
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
COMPOSITE MATERIALS
FRACTURE PROPERTIES
SILICON CARBIDES
SILICON NITRIDES
CERAMICS
CRACK PROPAGATION
EXPERIMENTAL DATA
MICROSTRUCTURE
RESIDUAL STRESSES
CARBIDES
CARBON COMPOUNDS
DATA
INFORMATION
MATERIALS
MECHANICAL PROPERTIES
NITRIDES
NITROGEN COMPOUNDS
NUMERICAL DATA
PNICTIDES
SILICON COMPOUNDS
STRESSES
360203* - Ceramics
Cermets
& Refractories- Mechanical Properties
360603 - Materials- Properties