Indentation fracture mechanics approach

Lac Each, R., Gyurova, L. A.,Grellmann, W. Application of indentation fracture mechanics approach for determination of fracture toughness of brittle polymer systems. Polym. Test. 26 (2007) 51-59. 08Pin Pinter, G. Slow crack growth in PE-HD under static and cyclic loads. Habilitation thesis, Montanuniversitat Leoben, 2008. 

The indentation fracture mechanics approach likens abrasive-workpiece interactions for grinding of ceramics to small-scale indentation events. The deformation and fracture patterns observed for normal contact with a Vickers pyramidal indentor under an applied load P are illustrated in Figure 3.1. A zone of plastic deformation is foimd directly imder the indentor. Two principal crack systems emanate from the plastic zone med-ian/radial and lateral cracks. Median/radial cracks are usually associated with strength degradation and lateral cracks with material removal. 

Most past research on grinding mechanisms for ceramics has followed either the indentation fracture mechanics" approach or machining" approach. The indentation fracture mechanics approach would seem to offer the possibility of describing both the material removal process and its influence on strength degradation in terms of the force or depth of cut at an... 

Indentation-fracture mechanics approach, which models abrasive-workpiece interactions by the idealized flaw system and deformation produced by an indenter. 

The development of indentation fracture mechanics has also allowed fracture toughness to be determined using indentation cracks. Indeed, the ease with which these cracks can be introduced and the simple specimen preparation involved has popularized this approach. Moreover, it has allowed crack behavior to be studied for cracks in a size range that is close to that found in practice. There are two main approaches for determining fracture toughness from indentation cracks. In the first approach, the size of the radial cracks that emanate from the hardness impression are measured. It is recognized that the parameter x depends on the elastic properties of the indented material. It has been proposed that x=P E H), where )8 is a constant that depends only on the indenter geometry and H is hardness. Equation (8.64) can, therefore, be written as... 

Ceramics can often be damaged under low contact loads and, thus, it is important to understand these damage mechanisms. These processes not only give rise to strength degradation but are linked to wear and erosion. Indentation fracture mechanics has been found to be a very useful approach in understanding these contact-damage processes (see Section 8.8). 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

Micro-mechanical properties were determined for the QllOO coating and the uncoated steel substrate using a Fischerscope HIOOC microhardness indenter. Measurements were taken on mounted polished cross-section coupons. Several regions within the coating were surveyed. A fracture toughness value was determined for the coating using a hardness indentation approach [12, 13]. The indentation test conditions and mechanical properties measured are shown in Table 25.2. 

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