Chantikul_Anstis-_part II_indentation_K1c

Chantikul_Anstis-_part II_indentation_K1c - A Critical...

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A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: 11, Strength Method P. CHANTIKUL, G. R. ANSTIS, B. R. LAWN,* and D. B. MARSHALL*’* Department of Applied Physics, School of Physics, University of New South Wales, New South Wales 2033, Australia An examination is made of the sharp-indentation technique of strength-test precracking for toughness evaluation. The ex- perimental 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 evi- dent 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 exten- sion. The main disadvantage is that only one result is obtained per specimen. Indentatiodstrength data from ceramics listed 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” K, appropriate to fracture properties at the flaw level is obtained. I. Introduction N THIS paper an alternative adaptation of the indentation ap- I proach to toughness measurement is investigated. The central idea is that the radial crack system be used as a dominant flaw in a strength test piece. Then, in conjunction with an appropriate fracture mechanics analysis for cracks in tensile loading, standard strength formulas may be used to determine K,. A form of this “controlled-flaw” approach has been used by others,’* on a range of brittle materials, in which detailed measurement of the flaw dimensions is an essential step in the analysis (for a review see Ref. 7). However, two major disadvantages are apparent in such studies: (i) The subsurface flaw geometry is not always clearly delineated on the section faces of the fractured test piece (in which case the method would appear to hold no real advantage over that described in Part 18); (ii) a systematic discrepancy exists between the toughness evaluated from the strength equation and that deter- mined from more conventional fracture specimens (e.g. double cantilever, double torsion), with the former consistently lower by some 30 to 40%.24 While it has been duly recognized that the residual contact field associated with the radial crack system is the chief source of the discrepancy in this case:.” no serious attempt has been made to incorporate a residual stress intensity factor term into the strengthltoughness formulation. Rather, special experi- mental stratagems aimed at nullifying the residual stresses (e.g. annealing, physical removal of central deformation zone) have been explored.’ Apart from greatly complicating the test
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Chantikul_Anstis-_part II_indentation_K1c - A Critical...

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