Y. Hos et alii, Frattura ed Integrità Strutturale, 34 (2015) 133-141; DOI: 10.3221/IGF-ESIS.34.14 139 step were applied. Nevertheless, before dealing with the more complicated combined loading cases producing non- proportional mixed mode a realistic cyclic plasticity model has to be implemented. Such models are available [13]; however, their application requires determination of material’s ratcheting behaviour first and identification of the corresponding material parameters second. Such work is ongoing. The numerical as well as the experimental determination of crack closure is accompanied by some uncertainties which are transferred to the effective ranges of the crack driving force parameter. Fig. 10 is shown here with the intension to unravel the uncertainties. First, an expected crack growth rate as function of the effective cyclic is plotted as solid line. A power law is assumed to correlate both variables. eff d d m a C J n    (5) The parameters C and m are determined following an older recommendation [14] that the exponent might be set to m= 1.5 and the coefficient can be estimated from a fatigue crack growth rate of 10 -5 mm/cycle at  J eff = E /(5 . 10 5 mm -1 ). Second, the experimentally determined crack growth rates of specimen R-001 are plotted as line- connected symbols over  J eff where this effective crack driving force is estimated based on both numerically and experimentally determined effective ranges. It can be concluded that the measured effective ranges provide more realistic estimates of the crack driving force. Figure 9 :  J eff as function of the applied nominal stress range  , specimen R-001, pure tension-compression with max 45kN F  and 1 F R   , steel S235. C ONCLUSIONS he path of fatigue cracks in thin walled tubes under combined tension/compression and torsion has been experimentally determined for proportional and non-proportional loading. As a general trend, it is observed that the cracks follow a curvature from a tensile to a shear dominated growth with increasing crack length. This behaviour had been observed also in previous investigations, however, an influence of the specimen geometry explaining the effect had bee postulated because the former specimens were shouldered. Here, it must be stated that the change of the growth mode is a property of the material itself. This change was observed to occur either in a continuous way or abruptly producing a kink. Both phenomena could appear in the same specimen at the both crack tips. The results summarized above are probably 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. Any fracture mechanics based model for T