Issue 33

F. Fremy et alii, Frattura ed Integrità Strutturale, 33 (2015) 397-403; DOI: 10.3221/IGF-ESIS.33.44 401 Second, in all mixed mode I+II experiments, below 1 mm of crack propagation, the crack path remained coplanar even for the “proportional” load path. The crack growth rate variations from one test to another can thus be attributed to a load path effect only. The number of cycles required to get a crack extent of 0.65 mm, for instance can typically varies by a factor 3, according to the load path applied in the experiment, though the extreme values and the mean values of the stress intensity factors are kept the same in each experiment. (a) (b) Figure 5 : (a) Evolutions of the crack lengths with the number of cycles applied for each load case in mixed mode I+II conditions. Two curves are plotted for the « windmill » load path, the curve 1, was plotted considering 1 cycle per load path, and the curve 2 considering 2 cycles per load path. (b) Evolutions of the crack lengths with the number of cycles applied for each load case in mixed mode I+II+III conditions. The results of the fatigue crack growth experiments conducted in mixed mode I+II+III conditions are plotted in Fig. 5b As in mixed mode I+II, a very significant load path effect is observed. However, the load path effect is smaller when the extreme values of the stress intensity factors are attained simultaneously for each mode (“cube” and “proportional” load paths). The largest difference is observed between the “cube” and the “star” load paths. As in mixed mode I+II, below 0.5 mm the crack path remains coplanar and the variations of the growth rate from one test to the other can solely be attributed to the load path effect. The number of cycles required to grow the crack by  a=0.5 mm in each mode I+II+III test are gathered in Tab. 1. According to the loading path the crack growth rate is found to vary by more than a factor 2.In addition, in mixed mode I+II+III, the crack path is also significantly varying with the loading case. The effects are more pronounced in mixed mode I+II+III than in mixed mode I+II. The overall inclination of the crack path was roughly characterized at the end of the test, by two angles, the tilt angle (  ) and the twist angle (  ).