Issue34

Y. Hos et alii, Frattura ed Integrità Strutturale, 34 (2015) 133-141; DOI: 10.3221/IGF-ESIS.34.14 133 Focussed on Crack Paths Measurement and simulation of crack growth rate and direction under non-proportional loadings Y. Hos Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Str. 3, D-64287 Darmstadt, Germany hos@wm.tu-darmstadt.de M. Vormwald Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Str. 3, D-64287 Darmstadt, Germany vormwald@wm.tu-darmstadt.de A BSTRACT . A series of fatigue experiments on thin-walled tubes under tension and torsion, the experimental results – crack path and crack growth life – are measured and compared. It is observed that the cracks follow a curvature from a tensile to a shear dominated growth with increasing crack length. The results are enforced by the high amplitudes applied to the specimens causing large cyclic plastic deformations and crack growth rates in the order of 10 -3 mm/cycle. The non-linear nature of the cyclic deformation has been taken into account by applying a cyclic plasticity model, and plasticity-induced crack closure is captured by a contact formulation. Already for the uniaxial reference case the current limitations in modelling plasticity induced crack closure – a prerequisite for achieving realistic simulation results – have become obvious. Measurements have shown that friction and roughness induced closure processes come up, especially for non-planar crack surfaces, challenge to be met in the future. K EYWORDS : Multiaxial fatigue; Mixed mode; Fatigue crack growth. I NTRODUCTION he fatigue crack growth under non-proportional mixed-mode loading depends on many influence factors besides the mode-mixity. Increasing mode-mixity creates a tendency for a shear dominated crack growth instead of a tensile stress dominated one, the latter being the usual case. With increasing cyclic plastic deformation the shear dominated fatigue crack growth becomes more important. The cyclic plastic deformation is also the origin of the plasticity induced crack closure. Especially in non-proportional cases, the roughness and friction induced crack closure occurs and interacts with the plasticity induced crack closure. These influence factors are closely interrelated, therefore a study of individual factors is hard to achieve. Originating from information gathered from a literature overview [1], a research project was launched, seeking further knowledge on the mechanisms. Results achieved so far are the subject of this paper. In order to connect the results to the state of the art the experimental methods are first applied to cases with proportional and mode I dominated fatigue crack growth. T