Issue 41

S. Beretta et alii, Frattura ed Integrità Strutturale, 41 (2017) 269-276; DOI: 10.3221/IGF-ESIS.41.36 275 successively were correlated using a commercial software (VIC 2D) and the displacement field for each load increment was successively used to calculate the SIF. The SIF evolution with the load P is then plotted and compared with the SIF calculated analytically. It is important to observe that, since the SIF is calculated based on the displacement fields calculated from the reference image captured at the minimum load, the measured SIF is interpreted as a variation ΔK form the reference condition. Since the crack propagation is also influenced by crack closure, we also provide an analysis of the measured effective ΔK eff . The plot in Fig. 5b shows that the measured SIF remains approximately zero before crack opening which occurs at point 3” where it can be defined the opening SIF as the K open . The ratio between the opening SIF and the maximum SIF calculated analytically K open /K theo is used to quantify the crack closure effect. This approach has been already successfully adopted by Carroll and co-authors [7]. Figure 6 : DIC measurements and regression of the displacement field: a) contour plot of the DIC displacement field; b) analysis of the displacement fields by the 3-terms regression; c) ΔK calculation and comparison with the analytical solution for the 2-terms regression and for the d) 3-terms regression. In order to precisely measure the SIFs, the area adopted for the two regressions are different according the analysis performed in the first part of the paper, for more details refer to the text. Fig. 6 shows the main results of the experiment conducted on the SENT specimen. Contour plot in Fig. 6a shows the vertical displacement field as calculated from the DIC algorithm for a defined load step. Fig. 6b indicates the vertical displacement field (blue lines) and the displacement field calculated from the fitting of the William expansion adopting a correlation basted on the first 3 terms (red lines). Accordingly, Fig. 6c and 6d include the analyses of the variation of the SIF during loading for the same crack length but different number of terms in the regression. In particular, Fig. 6c shows the SIF variation according the 2-terms regression calculated using a DIC regression area of 0.03 mm 2 which corresponds to a field-of-view dimension of d=0.17mm . The crack length at this test interruption was a/W=0.4 . According to these parameters, Fig. 4 shows that the SIF calculated should provide accurate results. As a comparison, Fig. 6d shows the 3- terms regression. In this case, the numerical analysis (Fig. 4) suggests that, for a crack length of a/W=0.4 , the regression performed with 3 terms provides accurate SIF values when the field-of-view dimension is between d=0.2mm and d=0.6mm . The DIC regression area was then selected to be 0.18 mm 2 which corresponds to a field-of-view dimension d=0.42mm . Comparing the results in terms of SIF values it is observed a good agreement between the two types of regressions. This results also in the correct estimation of the closure levels of approx. K open /K theo =0.4 for both the types of regressions.