Issue 49

B. El-Hadi et alii, Frattura ed Integrità Strutturale, 49 (2019) 547-556; DOI: 10.3221/IGF-ESIS.49.51 554 obtained for the angle Ө =60° and located at approximately 60% (fig. 9.b) of the thickness from the patched side to the free side and this also makes it compatible with the shape of the crack in fig10.a. Fig. 10.a also shows the maximum length of crack whose position exactly corresponds to the position for the maximum value of stress intensity factor. Fig.10.b compares the normalized crack-front shapes between repaired plates with 2.29 mm and 6.35 mm thickness. As can be seen in the figure, the difference between the maximum length crack positions confirms the current results. Thus, it is useful to give a precise prediction of the crack behavior and to identify the adequate value of the stress intensity factor for an accurate estimate of the lifetime of the patch-repaired structure and this for the two thicknesses studied in this work. In this respect, Fig.11 shows a comparison between average of the stress intensity factors versus the crack front inclination for thin and thick panels. Incontestably, the obtained results indicate horizontal monotony of the average stress intensity factors for a thin panel. Accordingly, one can suggest that the ideal choice for the calculation of the crack propagation is the average of the SIF's values along the crack front. This will be confirmed, hereinafter through the analyzed fatigue life results. Conversely, the curve of the thick panel (t=6.35 mm) shows a change of the average stress intensity factor when the skew changed. Here, the averaged SIF icreases with the increase of the skew angle. Figure 11 : Average stress intensity factors versus angle inclination in the crack plane for different thickness Figs. 12.a and 12.b show the comparison of fatigue life between experimental results and FEM results obtained from using different averaged stress intensity factor, namelly maximum value in the thickness direction (K.max), average all values in the thickness direction (avg-all), mid-point value in the thickness direction (K.mid) , root mean square value of all values (avg-K.rms), average all values from patched side to mid-point (avg-H.P.S), average all values from unpatched side to mid-point (avg -H.un- P.S), average all values except the values of both side element in the thickness direction (avg- 9.mid). Figure 12 : Comparison of fatigue life between experimental results and predicted values using averaged FEM results: (a) t=2.29mm (b) t=6.35 mm.