Issue 37

Y. Hos et alii, Frattura ed Integrità Strutturale, 37 (2016) 234-240; DOI: 10.3221/IGF-ESIS.37.31 235 successful parameter to model fatigue crack growth under proportional and non-proportional loadings, therefore an opportunity might arise to extend this solution to long cracks. The  J-integral should be interpreted according to the definitions of Dowling and Begley [16], Wüthrich [17] and Hertel et al. [18]. However, short cracks usually grow in a single plane without significant kinks or curvature, so the modeling could be simplified to an iterative search of this plane. More about the plastic simulations can be found in [19, 20]. A research project was launched on the abovementioned basis, seeking further knowledge on the mechanisms. This paper covers the achievements of this project. The investigation embarked on high amplitude loading, accompanied by large cyclic plastic deformation, high crack growth rates and short fatigue lives, and the cracks being all naturally produced. D ESCRIPTION OF THE PROBLEM onstant amplitude fatigue tests have been performed on thin-walled tubes which were put under either uniaxial tension-compression load, or torsional load, or a combination of both (proportional and non-proportional). The specimen geometry is depicted in Fig. 1. Longitudinally welded tubes were sawed and the starter notch was machined onto the specimen. The centers of holes at the ends of the starter notch are 10 mm apart from each other (length of an arc at the outer surface). The notch was positioned opposite to the weld. Figure 1 : Specimen geometry Inducing cyclic plastic deformation was a primary goal in the experiments. For this purpose, the tubes were ordered to be manufactured from the low strength constructional steel S235. The experimental results are shown in Fig. 2. The blue points in Fig. 2 correspond to strain-controlled tests and they show the upper end of the hysteresis at each strain level, measured at half-life each. A more detailed discussion of material properties could be found in previous works of the authors. [19, 20, 25, 26]. Figure 2 : Cyclic stress-strain curves of the used material. C