Objective Evaluation of Short‐Crack Toughness Curves Using Indentation Flaws: Case Study on Alumina‐Based Ceramics

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Abstract
An objective methodology is developed for evaluating toughness curves (T‐curves) of ceramics using indentation flaws. Two experimental routes are considered: (i) conventional measurement of inert strength as a function of indentation load; (ii) in situ measurement of crack size as a function of applied stress. Central to the procedure is a proper calibration of the indentation coefficients that determine the K‐field of indentation cracks in combined residual‐contact and applied‐stress loading, using data on an appropriate base material with single‐valued toughness. Tests on a fine‐grain alumina serve to demonstrate the approach. A key constraint in the coefficient evaluation is an observed satisfaction of the classical indentation strength–(load)−1/3 relation for such materials, implying an essential geometrical similarity in the crack configurations at failure. T‐curves for any alumina‐based ceramic without single‐valued toughness can then be generated objectively from inert‐strength or in situ crack‐size data. The methodology thereby circumvents the need for any preconceived model of toughening, or for any prescribed analytical representation of the T‐curve function. Data on coarse‐grained aluminas and alumina‐matrix material with aluminum titanate second‐phase particles are used in an illustrative case study.