Issue 30

J. Vázquez et alii, Frattura ed Integrità Strutturale, 30 (2014) 109-117; DOI: 10.3221/IGF-ESIS.30.15 115 Test Type N (N)/ Q (N) σ(MPa) N exp 1 1 5800 / 850 40 1070886 2 1 5800 / 850 40 1793368 3 2 5800 / 850 50 577540 4 2 5800 / 850 50 382064 5 2 5800 / 850 50 676704 6 3 5800 / 1100 70 303509 7 3 5800 / 1100 70 347072 8 4 5800 / 1100 55 252878 9 4 5800 / 1100 55 283110 10 5 5800 / 1350 70 167324 11 5 5800 / 1350 70 165421 12 6 4750 / 1100 110 52440 13 6 4750 / 1100 110 57032 14 7 3690 / 1350 110 32395 15 7 3690 / 1350 110 57479 Table 1 : Loads ( N and Q ), axial stress ( σ ) and experimental fatigue cycles ( N exp ) . E ν σ u σ y 70 GPa 0.33 572 MPa 503 MPa C n Δ K th Δ σ FL 8.83·10 -11 3.322 2.1 MPa√m 169 MPa σ f ´ b ε f ´ c 1231 -0.122 0.26 -0.806 K ´ n ´ 694 MPa 0.04 Table 2 : Mechanical and fatigue properties for the Al 7075-T651. The fretting fatigue predictions obtained for the 2D (plane stress or plane strain) and 3D models, are shown in Tab. 3. In addition, Tab. 3 indicates the experimental lives. To consider the full picture of the behaviour offered by each model (2Ds or 3D), Fig. 8 presents the predictions, N pred , versus the experimental lives, N exp , for all cases. This plot illustrates the solid predictions obtained using both 2D models; furthermore, it illustrates that both models provide similar predictions. Regarding the 3D model, this plot also indicates good behaviour; however, for test type 2, the results were worse than those obtained using the 2D models. To complete the prediction obtained using the models, Fig. 9 represents the percentage of N i and the crack initiation length as a function of the predicted life N pred . This plot shows that the crack initiation lengths are continuously longer than 20  m, justifying the low influence exerted on the fatigue predictions by the assumed initial crack aspect ratio. Similarly, this plot displays that the comparative relevance of both the crack initiation and the crack propagation phases can vary across test results. This fact shows that it is not possible to ignore either the crack initiation phase or the crack propagation phase when conducing these tests. Finally, Fig. 9 also indicates that the crack initiation length is always shorter than 70 µm; a model that considers the short crack growth phase is expedient.