Fracture Resistance of a Transparent Magnesium Aluminate Spinel

The fracture resistance of a fully dense, transparent, polycrystalline magnesium aluminate spinel was measured from room temperature to 1400°C using the chevron-notched beam and the straight-notched beam macroflaw techniques, as well as the indentation-induced, controlled-microflaw test method, all in three-point bending. Flexural strengths were also measured for the same range of temperatures to compare with the fracture toughness measurements. From the load vs load-line displacement (P-u) curves of the chevronnotched test specimens, the crack growth resistance curves (R-curves) and the total work-of-fracture were determined. It was observed that polycrystalline MgAl2O4 exhibits rising R-curve behavior which increases with increasing test temperature. The R-curve increases are attributed to the geometric constraints due to grain bridging and grain wedging phenomena as well as secondary grain boundary microcracking processes, all of which occurred in the wake region behind the advancing crack. The work-of-fracture and the R-curves increased rapidly above 800°C coincident with the onset of increased secondary grain boundary microcracking.

[1]  R. C. Bradt,et al.  Fracture of single crystal MgAl2O4 , 1980 .

[2]  R. Cook,et al.  Microstructure and the Strength of Ceramics , 1986 .

[3]  R. C. Bradt,et al.  Room‐Temperature KIc Values for Single‐Crystal and Polycrystalline MgAl2O4 , 1981 .

[4]  A. Kobayashi,et al.  The toughness of polycrystalline MgAl2O4 , 1988 .

[5]  Asish Ghosh,et al.  Elevated‐Temperature Fracture Resistance of a Sintered α‐Silicon Carbide , 1989 .

[6]  R. C. Bradt,et al.  Fracture of Polycrystalline MgAl2O4 , 1980 .

[7]  L. Pook.  An approach to a quality control KIc testpiece , 1972 .

[8]  D. Munz,et al.  Fracture toughness calculation from maximum load in four point bend tests of chevron notch specimens , 1980 .

[9]  K. Kromp,et al.  Crack resistance curves of alumina at high temperatures , 1987 .

[10]  A. Heuer,et al.  Precipitation Strengthening in Non-Stoichiometric Mg-Al Spinel , 1974 .

[11]  R. Bradt,et al.  The Crack Growth Resistance Curve of Non-Phase-Transforming Ceramics , 1988 .

[12]  A. Heuer,et al.  Precipitation in non-stoichiometric magnesium aluminate spinel , 1974 .

[13]  M. Swain.  R-curve behaviour in a polycrystalline alumina material , 1986 .

[14]  W. Jillek,et al.  Sub-critical crack extension and crack resistance in polycrystalline alumina , 1977 .

[15]  Dong M. Chay,et al.  Microstructure and Room‐Temperature Mechanical Properties of Hot‐Pressed Magnesium Aluminate as Described by Quadratic Multivariable Analysis , 1968 .

[16]  R. Bradt,et al.  Graphical Methods for Determining the Nonlinear Fracture Parameters of Silica and Graphite Refractory Composites , 1986 .

[17]  A. Kobayashi,et al.  A 3-D finite element analysis of a chevron-notched, three-point bend fracture specimen for ceramic materials , 1987 .

[18]  N. H. Macmillan,et al.  The hardness, stiffness and toughness of diphasic abrasive materials prepared by sol-gel techniques , 1985 .

[19]  H. Tattersall,et al.  The work of fracture and its measurement in metals, ceramics and other materials , 1966 .

[20]  R. Steinbrech,et al.  Increase of crack resistance during slow crack growth in Al2O3 bend specimens , 1983 .

[21]  Y. Mai,et al.  Crack‐Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina , 1987 .

[22]  M. Fine,et al.  Precipitation Strengthening of MgO by MgFe2O4 , 1972 .

[23]  Junn Nakayama.  A Bending Method for Direct Measurement of Fracture Energy of Brittle Material , 1964 .