Issue 10

G. Bolzon et alii, Frattura ed Integrità Strutturale, 10 (2009) 56-63; DOI: 10.3221/IGF-ESIS.10.07 57 components. The combination of experimental information with the test simulation and inverse analysi s [9] represents the envisaged approach to material characterization as the properties to be determined becomes numerous and less amenable to direct separate measurement, as in the present context. This approach can return the bulk and fracture properties of metal-ceramic composites in a robust and reliable manner, by exploiting experimental data relevant to both the traditional indentation curves and the mapping of the residual displacement field, as shown e.g. in [10-16] and illustrated in the next Sections with the aid of some meaningful application example. I NDENTATION T OOL ndentation test represents a practical methodology for the characterization of traditional and innovative materials. With respect to more traditional experimental investigation, indentation can be performed on small material portion, does not require to extract laboratory specimens, can be performed even in situ, directly on the structural component, to continuous monitoring and diagnosis purposes. The outcome of instrumented indentation, namely the curves that represent the relationship between the penetration depth and the force exerted on the equipment tip, see Fig. 1, reflect constitutive and fracture properties of the sampled material, though in an indirect way. Quantitative calibration of parameters entering the selected constitutive model can then be returned by inverse analysis procedures, which combine experimental data and the simulation of the laboratory test as shown, e.g., in [9, 17, 18]. The inverse analysis problem can be formulated as the minimisation, with respect to the unknown parameters, of a norm that quantifies the overall discrepancy between the measured quantities and the corresponding values computed through a mathematical or numerical model. Indentation test can be analysed in the large deformation regime by finite element (FE) approaches, as already done in previous analyses [10-16] or by alternative numerical techniques, like in [19, 20]. Numerical simulation represents an useful tool also to the purpose of designing the experimental test, and selecting the most appropriate measurements, e.g. by sensitivity analysis [21]. Figure 1 : Comparison between the experimental mean indentation curves obtained from conical (Rockwell) tests on an Al/TiB 2 specimen with 50% TiB 2 weight content and the corresponding curves recalculated by a FE model of the test, supplemented by the identified material parameter set. Let the indentation forces i F ( i=1, …, N ) and the corresponding penetration depth Mi d represent the experimental information collected from the mean indentation curves, visualised in Fig. 1. For any given value i F , numerical analysis returns the corresponding penetration depth ( ) Ci d z as a function of the constitutive parameters contained in the selected material or fracture model (elastic moduli, yield limits, fracture energy, …), here collected by vector z . The discrepancy between quantities gathered from the experimental apparatus and from the simulation of the test can then be defined by the norm: I