The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

A Critical evaluation of indentation techniques for measuring fracture toughness

An examination is made of the sharp-indentation technique of strength-test precracking for toughness evaluation. The experimental approach follows that proposed by other workers but the theoretical analysis contains one vital new feature; the residual-stress term discussed in Part I of this study is now introduced explicitly into the strength formulation. This modification overcomes a major systematic discrepancy evident in the previous models and at the same time, by virtue of attendant changes in the nature of the crack stability prior to attaining a failure configuration, eliminates the need for frac-tographic measurements. Other advantages are also apparent, notably an insensitivity to postindentation radial crack extension. The main disadvantage is that only one result is obtained per specimen. Indentation/strength data from ceramics listed in Part I confirm the essential features of the theory and provide a suitable calibration factor. The method has special application to those materials which do not necessarily produce a well-defined radial crack pattern, in which case an “effective”Kc appropriate to fracture properties at the flaw level is obtained.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method

The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this flrst part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elasticlplastic indentation frac- ture mechanics. It is thereby asserted that the key to the radial crack response lies In the residual component of the contact fkld. This residual term has important implications concerning the crack evolution, including the possibility of postindentation slow growth under environment-sensitive conditions. Frac- tographic observations of cracks in selected "reference" mater- Ys are used to determine the magnitude of this effect and to investigate other potential complications associated with de- partures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely hpie in procedure and economic in its use of material.

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A review of the fracture energy and toughness data for dense ceramics at 22 °C shows maxima commonly occurring as a function of grain size. Such maxima are most pronounced for non-cubic materials, where they are often associated with microcracking and R-curve effects, especially in oxides, but often also occur at too fine a grain size for association with microcracking. The maxima are usually much more limited, but frequently definitive, for cubic materials. In a few cases only a decrease with increasing grain size at larger grain size, or no dependence on grain size is found, but the extent to which these reflect lack of sufficient data is uncertain. In porous ceramics fracture toughness and especially fracture energy commonly show less porosity dependence than strength and Young's modulus. In some cases little, or no, decrease, or possibly a temporary increase in fracture energy or toughness are seen with increasing porosity at low or intermediate levels of porosity in contrast to continuous decreases for strength and Young's modulus. It is suggested that such (widely neglected) variations reflect bridging in porous bodies. The above maxima as a function of grain size and reduced decreases with increased porosity are less pronounced for fracture toughness as opposed to fracture energy, since the former reflects effects of the latter and Young's modulus, which usually has no dependence on grain size, but substantial dependence on porosity. In general, tests with cracks closer to the natural flaw size give results more consistent with strength behaviour. Implications of these findings are discussed.

Grain size and porosity dependence of ceramic fracture energy and toughness at 22 °C

Controlled semi-elliptical surface flaws were produced in hot-pressed SiC by Knoop microhardness indentation. Flawed specimens were placed in 4-point bending in order to determine their critical stress intensity factor, KIC, at both room and high temperatures. Room-temperature fracture and KIC values after annealing were sensitive to the annealing environment; this behavior correlated with the active/passive nature of the oxidation process. Flaw healing was observed for annealing exposures in air. Room-temperature KIC values increased with increasing annealing temperature. High-temperature KICvalues decreased with increasing temperature as a result of a decrease in the fracture surface energy.

J. J. Petrovic

Papers

Controlled Surface Flaws in Hot‐Pressed SiC