Issue34

G. M. Domínguez Almaraz et alii, Frattura ed Integrità Strutturale, 34 (2015) 498-506; DOI: 10.3221/IGF-ESIS.34.55 503 Figure 8 : Bubble or protuberance at the especimen neck section, under ultrasonic fatigue testing (before crack propagation). Very often, fatigue ctack initiation and propagation were associated with the bubble or protuberance generated during the high frequency loading. Figs. 7 a) and b) show the principal crack patterns of this material under ultrasonic fatigue testing: for specimens immersed in water a noticeable plastic deformation [19, 20] is observed at the specimen corner where stress concentration is higher; then, crack propagates following the protuberance contour. For specimens immersed in oil, crack initiation is localized at the top of the bubble and propagates along the principal elliptical axis of the protuberance, perpendicular to applied loading, Fig. 7 b). A study is in cours in order to formulate a comprehensive understanding of these behaviors, taking into account the involved physical-chemical and mechanical parameters [21, 22]. Crack propagation along the specimen lateral side is shown on Figs. 9 a) and b), for specimens immersed in water and in oil, respectively. In both cases, low plastic deformation is observed and low thermal effect can be assumed. a) b) Figure 9 : Crack propagation along the specimen lateral side immersed in water a) and in oil b) . Numerical evaluation of J integral and stress intensity factor K, with crack propagation on polymer ABS A numerical analysis was carried out to evaluate the two parameters controlling the energy relaxation and stress state of material during crack propagation. A bi-dimensional numerical model using PLANE183 element (Ansys software), was developed to obtain J and K under plane strain condition, Fig. 10. Crack length was imposed from 0.5 mm to 4 mm with increments of 0.5 mm; the symmetrical condition of specimen allows developing a half of specimen surface, as shown in Fig. 10. Three applying loads were imposed corresponding to: 1.67, 2.5 and 3.34 MPa, and these loads were maintained constant during crack propagation. The numerical model uses 6 close trajectories in order to evaluate the J integral. The numerical results are plotted on Fig. 11 a) and b) for the J integral and the stress intensity factor K, respectively. Values for these two parameters present an exponential evolution with crack length, as shown in Fig. 11; these results are of same order in regard previous results obtained on the polymeric blend PC/ABS [23].