Issue 41

D.-Q. Wang et alii, Frattura ed Integrità Strutturale, 41 (2017) 143-148; DOI: 10.3221/IGF-ESIS.41.20 146 Figure 3 : Crack growth morphologies during the overload process ( (a) , (b) , and (c) are the crack tip before, at and after the overload, respectively; while (d) , (e) and (f) are the corresponding strain distribution based on DIC measurement). Figure 4 : The strain evolution at the crack tip at OL-1, OL and OL+1 cycles (data points are extracted from the strain distribution with a distance of 50  m in front of the crack tip) Crack opening displacement evolution Crack opening displacement (COD) values at crack wake was acquired by subtracting the vertical displacements of the top flank and bottom flank of the crack mouth, which varies as a function of distance behind the crack-tip [14]. In this study, the measurement point was 50  m behind the crack-tip. The COD loops for the three full cycles, as shown in Fig. 5, have similar trend with that of crack tip strain in Fig. 3. It seems the loading process at the crack wake follows the mechanical response at the crack tip under the stress level in this work. As observed from Figs. 4 and 5, the crack closure varies little before and after the overload, probably due to the OLR is not very high, and the material investigated here has a low hardening coefficient. Crack growth rate and plastic zone size Apart from crack closure, other parameters, i.e., fatigue crack growth rate, plastic zone size and strain increasing rate, are calculated based on the experimental measurement, and the results are illustrated in Fig. 6. Strain increasing rate was defined as the slope of fitting curve of  max /  0 –N relationship, while plastic zone size was defined as areas where normal strain  yy is larger than 2%. Here,  max and  0 are the strain values at peak load and minimum load. It is observed that crack growth rate is decreased from 3.3  10 -4 to 1.5  10 -4 mm/cycle just after the overload, and then gradually restored to pre-overload 0.00 0.02 0.04 0.06 0.08 0.10 0 20 40 60 80 100 120 140 N OL -1 N OL N OL +1 P/P max (%) Crack tip strain