Issue 50

J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 49 (2019) 658-666; DOI: 10.3221/IGF-ESIS.50.56 662 Figure 4 : Magnified view of the crack tip region illustrating how CTOD measurements are based on a particular pair of points behind the crack tip. The coordinate axes have been modified with their origin at the crack tip. Figure 5 : Graphs showing the effect of the location behind the crack tip of the selected points used for the CTOD measurement. (a) As a function of the distance along the crack plane L x ; (b) As a function of the distance in the perpendicular direction L y to the crack plane. EXPERIMENTAL RESULTS nce the values of L x and L y were established, the CTOD through a loading cycle for any particular crack length could be obtained from analysis of the DIC data. A typical plot showing CTOD at increments of 25 N throughout a full loading cycle for a crack length of 9.40 mm in the case of the specimen tested at R = 0.1 and a crack length of 9.20 mm in the case of the specimen tested at R = 0.6 is shown in Fig. 6. The elastic and plastic CTOD values can be obtained from the data given in Fig. 6, using an offset procedure similar to that reported by Skorupa et al. [10] for compliance measurements. In this process, a least squares straight line was fitted to experimental data over the part of the load cycle that corresponds with elastic deformation (see lines drawn in Fig. 6). The elastic portions of the loading cycle were identified using a 3 point sliding average, founding a range between 15% and 55% of the loading range from the minimum applied load for the specimen tested at R = 0.1 and between the minimum applied load and a 40% of the loading range from the minimum load for the specimen tested at R = 0.6. To evaluate how the components of CTOD can change depending on the loading range selected, a sensitivity analysis was performed to evaluate the errors made when variations of ±10% for the lower and upper limits are analysed. According to this sensitivity analysis, the variations of the lower limit involved errors between 3–4% for the plastic CTOD and 1.0–1.5% for the elastic component. On the other hand, the variations on the upper limit involved errors between 6.5–10% for the plastic CTOD and 3–4.5% for the elastic CTOD. From these results it is evident that the selection of the upper limit is more restrictive than the lower limit. This is 0.000 0.005 0.010 0.015 0.020 0.025 0 20 40 60 80 100 120 140 160 180 200 220 CTOD (mm) distance at the perpendicular direction to the crack, L y (μm) Lx=-1 pix (-13.7 microns) Lx=-2 pix (-27.4 microns) Lx=-3 pix (-41.2 microns) Lx=-4 pix (-54.8 microns) Lx=-5 pix (-68.4 microns) Lx=-6 pix (-82.1 microns) Lx=-7 pix (-95.8 microns) Lx=-8 pix (-109.5 microns) Lx=-9 pix (-123.2 microns) Lx=-10 pix (-136.9 microns) Lx=-11 pix (-150.6 microns) Lx=-12 pix (-164.2 microns) Lx=-13 pix (-177.9 microns) Lx=-14 pix (-191.6 microns) Lx=-15 pix (-205.3 microns) O (a) (b)