Issue 37

Y. Hos et alii, Frattura ed Integrità Strutturale, 37 (2016) 234-240; DOI: 10.3221/IGF-ESIS.37.31 234 Focussed on Multiaxial Fatigue and Fracture Growth of long fatigue cracks under non-proportional loadings – experiment and simulation Y. Hos, M. Vormwald Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Straße 3 64287 Darmstadt/GERMANY hos@wm.tu-darmstadt.de, vormwald@wm.tu-darmstadt.de A BSTRACT . An experimental campaign was carried out on thin-walled tubes under tension and torsion. The results from experiments are measured and compared. It is observed that cracks follow a shear-dominated growth pattern with increasing crack length, instead of a tension-dominated one. The experiments are performed with high amplitudes applied to the specimens, resulting in large cyclic plastic deformations and crack growth rates up to 10 -3 mm/cycle. Stress intensity factors were calculated for the proportional loading case. K EYWORDS . Multiaxial fatigue; Non-proportional loadings; Crack growth; Fracture mechanics; Experimental mechanics; Numerical simulation I NTRODUCTION atigue crack growth under non-proportional mixed-mode loading depends on many factors. A recently issued review [1] identified seven such factors. The first one, mode-mixity, is arguably the most obvious one. The maximum tangential stress criterion has been supported by several experimental results [2-4], whereas Roberts and Kibler [5] have discovered cases where maximum shear stress criterion is more suitable. Natural mode I cracks were produced firstly, and then loaded with mode II loading, without observing a change in the direction of the crack. Only the maximum shear stress criterion can model this behavior. It could be concluded that increasing mode-mixity leads to a tendency for a shear-dominated crack growth. Secondly, the material itself is a dominant factor. For example, Qian and Fatemi [2] conclude that “mode II crack growth occurs more often in aluminum alloys than in steels”. Thirdly, the magnitude of cyclic plastic deformation is an influencing factor, as increasing cyclic plastic deformation creates a tendency towards shear-dominated crack growth. Results published by Tanaka [6], Brown and Miller [7], Otsuka and Tohgo [8], Socie [9] and Doquet and Bertlino [10] support this argument. The fourth factor to be specified is the crack closure, which is directly dependent on the cyclic plastic deformation. Roughness along the crack front leads sometimes to the interlocking of the crack fronts, and interlocking results in a shielding of the crack tip from mode II (especially in low stress intensity factor ranges). Moreover, the mean stress effect (the fifth factor) is very relevant to the crack closure and therefore needs to be taken into account. In addition, the geometry of a structure is obviously a very important factor in every type of crack growth phenomenon. Finally, the mode-mixity changes along the crack front in three-dimensional cases. With so many plastic effects playing a role in crack growth under non-proportional loading, a crack tip parameter, which takes plasticity into account, has to be used in further research. Several studies [11-15] have shown that  J is a very F