Tensile Fracture Toughness of Ceramic Materials: Effects of Dynamic Loading and Elevated Temperatures

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Abstract
Experimental methods are presented for the determination of fracture initiation toughness of ceramics and ceramic composites in pure tension under quasi‐static and dynamic loading conditions over a range of temperature spanning 20° to 1300°C. Circumferentially notched and cyclic fatigue precracked rods of a variety of ceramic materials were subjected to quasi‐static tensile fracture (rate of stress intensity factor loading, K1∼ 0.1 MPa ∼ m1/2∼ s−1) in an electroservohydraulic test machine and to dynamic tensile fracture (K1∼ 106 MPa ∼ m1/2· s−1) using a modified tensile Kolsky (split‐Hopkinson) bar. For the quasi‐static and dynamic fracture tests at elevated temperatures, the ceramic specimen was inserted into an air furnace where either friction grips or stress wave loading outside the furnace subjected the specimen to fracture. Dynamic finite‐element analyses of the circumferentially notched cylindrical rod have been conducted to develop the optimum specimen dimensions and test procedures for the measurement of dynamic fracture toughness at ambient and elevated temperature. Experiments conducted on Al2O3, Si3N4, and SiC, and an Al2O3‐25 vol% SiC whisker composite at room temperature indicate that the dynamic to quasistatic fracture initiation toughness ratio KId/KIc is in the range of 1.1 to 1.4. Elevated‐temperature fracture tests for the polycrystalline Al2O3 of 3‐μm average grain size reveal that KId is only mildly sensitive to temperature over a range of 20° to 1100°C, whereas it suffers a precipitous drop above 1100°C. Over the temperature range 20° to 1300°C, the ratio KId/KIc is found to be in the range 1.2 to 1.5. Scanning electron microscopy observations show that failure above 1100°C usually evolves by the nucleation, growth, and coalescence of cavities. The mechanisms of elevated‐temperature quasi‐static and dynamic fracture in polycrystalline Al2O3 are examined and possible causes for the apparently higher dynamic fracture initiation resistance are discussed. The significance and limitations of the proposed experimental techniques are highlighted.