Issue34

S. Henkel et alii, Frattura ed Integrità Strutturale, 34 (2015) 466-475; DOI: 10.3221/IGF-ESIS.34.52 474 Figure 11 : Comparison of configurations with two and four (branched) crack tips with linear elastic FEA. a) Time dependent change in stress intensity factors K I and K II for four crack tips. b) Stress intensity factors for two crack tips, c) von Mises stress distribution for a branched crack configuration. S UMMARY AND CONCLUSIONS he influence of an additional tensile load parallel to the crack propagation direction on the fatigue crack growth was investigated. In the examined stress intensity factor range, only a small influence of a crack parallel load F X = 40 kN was detected for the fatigue crack growth at a constant load amplitude in the aluminum alloy 6061 T651. At R = 0.1 a slightly lower crack growth rate was found with the tensile load F X . The known retardation in the overload case was found. The tensile load in crack direction F X caused a significant longer retardation by a factor of about 1.5 to 1.8 for 1.5 times overloads at R = 0.1. For the loading with and without overload, these effects are not pronounced at R = 0.5. It seems that this effect is primarily caused by a gain of crack closure effects caused by positive T-stresses. However, the influence of crack length and previous overload, was not yet considered. Less pronounced overload retardation can be assumed with tensile force F X for R = 0.8 and 1.3 times overloads. In principle, numerous mechanisms lead to delay after overloads [21], which are all more or less influenced by a crack parallel load (crack blunting, crack deflection, crack branching, formation of secondary cracks, strain hardening and residual stresses ahead of the crack tip, roughness and plasticity-induced crack closure). To evaluate the influence of individual factors further analysis and investigations are necessary. Crack branching was observed for a crack aligned under 45° to the loading axes and 90° phase shifted loading between the load axes. The branched cracks propagated nearly perpendicular to the loading axes which can be understood as a crack direction with high mode I fraction. A CKNOWLEDGMENT he authors thank the AMAG rolling GmbH, Austria for supporting the aluminum material. T T a) b) c)