Issue 30

D. Nappini et alii, Frattura ed Integrità Strutturale, 30 (2014) 394-402; DOI: 10.3221/IGF-ESIS.30.47 399 R ESULT AND DISCUSSION Data analysis he comparison between the load-indentation depth curve obtained for ASTM A471 and IN718 was shown in fig. 6. The sensibility of instrumented indentation testing system to characterise different materials is highlighted. The analysis of the P–δ raw data is a good way to find promising indicators which might be useful to describe the behaviour of superalloy as well as alloy steel. Figure 6 : Comparison between the load-indentation depth curve for ASTM A471 and IN718 alloys. An accurate analysis of all the previous detailed quantities obtainable from the load-indentation depth curves was performed for each items, to evaluate the possibility to correlate some parameters to damage. Conventional hardness measurements were also carried out in order to find a relationship between these tests and indentations parameters. In fact, the hardness characteristic is correlate, to a certain extent, with the characteristics of many mechanical properties of a material, but the measured values was exhibit low sensitivity to structural transformations related to ageing in service, as observed also by the work of Lebedev [21]. The damage resulting in a serviced component is a function of time, stress and temperature. Turbine wheels and the spacer ring had seen the same extensive serviced time, about 10 5 hours, so it could not be a variable. Also the material was the same, so the variables of interest were the stress and the temperature fields. As examples of the obtained results, in fig. 7 was reported for the rotor of the first stage the evolution of δ max , δ i and δ f as function of stress, while the fig. 8 shows the variation with temperature of HB 1 , HB 2 and HB 3 . Figure 7 : Evolution of penetration values δ in function of the stress field for the 1 st stage wheel. Figure 8 : Evolution of hardness values in function of the temperature field for the spacer ring. Since an indentation test is not very appropriate for detecting a mechanical damage, such voids or micro-cracks, firstly the stress was not considered in the analysis and the results were modelling as a function of the temperature. 0 10 20 30 40 50 60 70 80 90 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 δ [μm] Dimensionless stress δmax δf δi 200 300 400 500 600 700 800 0.80 0.84 0.88 0.92 0.96 1.00 Hardness [N/mm2] Dimensionless temperature H1 H2 H3 T