Issue 30

D. Nappini et alii, Frattura ed Integrità Strutturale, 30 (2014) 394-402; DOI: 10.3221/IGF-ESIS.30.47 400 Moreover, indentation sites at the same temperature performed on the spacer ring and on the 1 st and 2 nd stage of turbine wheels were considered equivalent points of measure. For each variable, the mean values among equivalent points were calculated and reported in function of the temperature. The analysis of all defined parameters had allowed to identify those which showed a monotonic and regular function. Among them, the most significant are the elastic unloading contact stiffness and the area included by loading and unloading curve (fig. 2). A good fit solution was performed using a linear functions of mean data. Fig. 9 shows the obtained trend. The quantities S and W extrapolated from the P– δ curves could be used for a first assessment of the mean temperature of the component in the site of analysis, so these parameters are also promising indicators to material damage estimation due to extensive service. Figure 9 : Total work done by the indenter a) and elastic unloading contact stiffness b) as function of mean temperature in indentation zone for serviced material (IN718). Following this analysis, various objective function f(temperature, stress) were tested in order to include the stress field effect on damage. The stress acting on components was not considered as important as the temperature; its effect was therefore introduced as an amplification factor of the temperature values. In particular the functions shown in fig. 10a. Evolutions of the various parameters vs. the modified temperature were carried out and the accuracy of the fits was determined by the correlation coefficient R 2 . All the factors performed an improvement of the correlation coefficient, but the better result were obtained using the amplification factor σ σmax as shown in fig. 10b for the S parameter. Figure 10 : a) Comparison between three amplification factors proposed to include stresses effect; b) The relation between the elastic unloading stiffness and the modified temperature in indentation zone for IN718 in serviced condition. C ONCLUSION n accurate analysis of several quantities evaluated from the load-indentation depth curve were performed on items sampling on turbine component in Inconel 718, with the aim to assess the capability of spherical indentation testing system to evaluate variations in the material properties due to damage resulting from service. R² = 0.9451 75 75.5 76 76.5 77 77.5 0 0.1 0.2 0.3 0.4 0.5 0.6 S (dP/dδ) modified temperature S, raw data A b) a) a) b)