PSI - Issue 28

1182 Ping Zhang et al. / Procedia Structural Integrity 28 (2020) 1176–1183 Author name / Structural Integrity Procedia 00 (2019) 000–000 7 border between the two slip planes (point B in Fig. 4b) at the 20 th cycle. After that, the crack entered the green area controlled by the �1� 11� slip plane, switched its direction and propagated downwards (Fig. 4c). Finally, at the 23 rd cycle, the crack met the boundary and deflected again (point C).

Fig. 4. Evolution of slip plane activity with increasing loading cycles for [111] oriented sample at 24 °C.

It is also noticed that the slip plane area in front of the crack tip was pushed forward and its size diminished as the crack grew. For instance, an obvious difference can be observed in the blue area (see the white marked area in Fig. 4b and c) between the 20 th and the 23 rd cycle. Generally, the stress field produced by the crack alters the local activity of slip planes. As the crack grows, it tries to enlarge the domain to which it belongs to maintain its current extending direction. When this is not possible, the slip plane will be switched and a deflection appears. 4.3. Crack growth Besides the crack growth speed of numerical and experimental cracks for all samples were measured and grouped in Fig. 5. It should be clarified that the number of numerical loading cycles has been scaled up by an averaged scale factor to enable a direct comparison, which was calculated by matching the maximum number of cycles required for cracks to grow to the maximum length in experiments and simulations for each case. Also, the crack paths were projected to the normal direction of the loading axis since the propagation paths were not horizontal in most cases.

Fig. 5. Comparison of experimental results (Zhang et al., 2019) and numerical prediction for the crack propagation rates of all specimens.

It is noted that most curves exhibit a step change of shape with increasing loading cycles, which is attributed to the multiply deflected crack paths discussed earlier. In fact, both [111] orientation samples exhibit this response (Fig. 3)