Issue 10

G. Bolzon et alii, Frattura ed Integrità Strutturale, 10 (2009) 56-63; DOI: 10.3221/IGF-ESIS.10.07 61 Figure 5 : Contour map of the crack opening displacements of the half-penny shape fracture surface produced by indentation (simulation results) According to this popular formulation, the normal cohesive stress σ transferred during progressive mode I fracture processes can be described by the relationship: ( ) c c c w w w w e we     (5) where: e is the Neper number; c  is the maximum cohesive normal traction; w represents the opening displacement and c w is a characteristic value of it, associated to the fracture energy f G of the material by the formula: 0 ( ) f c c G w dw e w       (6) In mixed model fracture formulation, the opening displacement w is replaced by a scalar measure of the displacement jump vector across the interface and a further parameter is introduced to describe the shear strength of the interface [32, 33]. Preliminary studies have shown that the indentation curve, the geometry of the residual imprint and the crack length reflect to different extent fracture properties like c  and f G , entering relation (5) and (6). The biggest sensitivity to a change in these parameters is shown by the fracture length but, in turn, the actual position of the crack tip is rather difficult to establish with the required precision. For this reason, information about the geometry of the residual imprint and in-plane displacement components on the specimen surface after fracture propagation can be acquired by several kinds of microscopes nowadays available on the market, possibly endowed with suitable image correlation tools, see e.g. [35, 36]. These data can be exploited to enhance the identification of fracture properties through indentation test [12, 14, 16], including them in the discrepancy function (1), compared to the corresponding computed quantities. The application, now in progress, of reliable and robust identification procedures should also allow to verify the range of applicability of approaches based on modified mixture laws, proposed for fracture properties of metal-ceramic composites b y Jin et al. [37], in analogy wit h [23]. C LOSING REMARKS he envisaged increasing use of metal-ceramic composites for structural applications in the near future require adequate mechanical characterization, which can be better based on instrumented indentation and inverse analysis rather than on standard laboratory tests, due to difficulties and costs associated with material production and specimen extraction processes. Recently developed parameter calibration methodologies, gathering experimental information from the measurement of the residual deformed configuration of the specimen after the test, besides from the indentation curves, are generally bound to enhance the identification of both bulk and fracture parameters. T