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Chevron-notched, three-point flexure specimens were used to study the quasistatic fracture behaviour of a variety of structural ceramics at temperatures to 1400°C. Types of materials tested included monolithic ceramics (SiC., Si3N4, MgAl2O4), self-reinforced monoliths (acicular-grained Si3N4, acicular-grained mullite), and ceramic matrix composites (SiC whisker/Al2O3 matrix, TiB2 particulate/SiC matrix, SiC fibre/CVI SiC matrix, Al2O3 fibre/CVI SiC matrix). Fracture resistance behavior of the materials was quantified as three distinct regimes of the fracture histories. At the initial part of the crack propagation, the apparent fracture toughness was evaluated as the critical stress intensity factor for the chevron notch, KIvM. During stable crack propagation, the crack growth resistance was characterized by the instantaneous strain energy release rate, GR, using a compliance method assuming linear-elastic unloading to calculate the effective crack lengths. At final fracture, the complete fracture process was quantified using the work-of-fracture, γWOF, which can be equated to the fracture surface energy for linear-elastic materials. Results indicate that the chevron-notched, three-point flexure specimen facilitates the study of fracture behaviour in a wide range of brittle and quasi-brittle materials at elevated temperatures. The unique features of the chevron geometry, which are automatic crack initiation and inherent stable crack growth, are crucial to the successful evaluation of the fracture tests.

Mechanical properties of a developmental high strength and high toughness SiC, Generation I SX, have been evaluated under a Department of Energy (DOE)/Oak Ridge National Laboratory (ORNL) subcontract. The mechanical properties determined included flexural strength, tensile strength, and fracture toughness at room and elevated temperatures. Stress rupture, dynamic fatigue and creep at elevated temperatures also have been evaluated. The strength limiting factors have been identified at room and elevated temperatures. The strength controlling mechanisms are discussed. The microstructure-mechanical property relationship has been established.

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