Issue 33

F. Fremy et alii, Frattura ed Integrità Strutturale, 33 (2015) 397-403; DOI: 10.3221/IGF-ESIS.33.44 402 Load path Number of cycles ΔN i to get Δa=0.5 mm ΔN i /ΔN C 1 m eq N K C a           Cube  N A =4678 0.43 30.82 Proportional  N B =6216 0.57 28.08 Star  N C =10920 1.00 23.34 Table 1 : Number of cycles required to grow the crack by fatigue by  a=0.5 mm in each experiment, equivalent stress intensity factor   eq K MPa m   required to get a growth rate   i a / N m / cycle   assuming Eq. 1, with m=3 and C=3.08 10 -12 . Load path “Prop.” “ Cube ” “ Star ” Tilt angle  -10° none 40° Twist angle  50° 15° 10° Table 2 : Twist angle (  ) through the thickness of the specimen (5 mm) and tilt angle (  ) after the crack has propagated in mixed mode by  a=2mm. Effect of the load path on the crack path, after the crack has propagated in mixed mode I+II+III from 2a=34mm up to 2a=38mm. . The “proportional” loading path has promoted the most severe change in the crack path, since the crack plane has twisted by an angle of about 50°. On the contrary, the fatigue crack growth remains more or less coplanar under the “cube” load path. Most surprisingly, the “star” load path is producing a significant tilt and a small twist, while no bifurcation was observed in mode I+II under the “cross” load path. Adding mode III loading steps (“star”) to a mixed mode I+II loading cycle (“cross”) did not promote the twisting of the crack path, but induced a tilt. C ONCLUSIONS xperiments were conducted in mode I+II and in mode I+II+III non-proportional loading conditions in order to characterize the load path effect in fatigue crack propagation in a 316L stainless steel and the contribution to fatigue crack growth of mode III loadings. Since the same maximum, minimum and mean values of the stress intensity factors were applied in each experiments, the load paths are all considered as “equivalent” with respect to most of the fatigue crack growth criteria, in particular with those based on   1 n n n n eq I II III K K K K          . The main result of this set of experiments is that very different crack growth rates are observed even though the extreme values and the mean values of the stress intensity factors are the same in each test. A variation by up to a factor three of the crack growth rate according to the loading path was observed in these experiments, even when the crack path remained coplanar. In addition, it was shown that the crack path is also significantly dependent of the load path. For instance, the crack path remains coplanar for the “square” load path while a tilt is observed for the “proportional” load path in mixed mode I+II. In these two cases, the extreme values of the mode I and mode II stress intensity factors are attained simultaneously. R EFERENCES [1] Qian, J., Fatemi, A.,Mixed mode fatigue crack growth: A literature survey. Engineering Fracture Mechanics, 55(6) (1996) 969 ‐ 990. E