Issue 33

M. Vormwald, Frattura ed Integrità Strutturale, 33 (2015) 253-261; DOI: 10.3221/IGF-ESIS.33.31 253 Focussed on multiaxial fatigue Multi-challenge aspects in fatigue due to the combined occurrence of multiaxiality, variable amplitude loading, and size effects M. Vormwald Technische Universität Darmstadt, Materials Mechanics Group, Franziska-Braun-Str. 3, D-64287 Darmstadt, Germany vormwald@wm.tu-darmstadt.de A BSTRACT . The three major issues which contribute to uncertainties in fatigue life estimations are the limited transferability of material data obtained in a laboratory for describing the fatigue behavior of components, the lack of damage accumulation rules to realistically take into account the effects of various load sequences, and the uncertainty in the assessment of multiaxial stress states. The three issues contribute to life prediction errors in a balanced way. Some modeling effort was spent in joining solution proposals in a unifying short crack model. The actual state is presented in the paper together with some ideas for further improvement as well as simplification for promoting the model’s acceptance in practical applications. K EYWORDS . Multiaxial fatigue; Short crack; Critical plane. I NTRODUCTION or the stage of technical fatigue crack initiation conventional approaches like the local strain approach are applied which do not explicitly refer to the physical process of defect growth. This stage is treated as a black box where the failure process is disguised behind strain or stress life curves, damage accumulation rules, and multiaxial failure hypotheses. In such simulations, three major issues contribute to uncertainties in fatigue life estimations, the limited transferability of material data obtained in a laboratory for describing the fatigue behavior of components, the lack of damage accumulation rules to realistically take into account the effects of various load sequences, and the uncertainty in the assessment of multiaxial stress states. In a multi-scale modeling of the fatigue process, the next smaller scale of observation is the short crack growth regime. A large amount of research was dedicated to the investigation and accompanying modeling of short crack growth behavior with the aim to reduce the inaccuracies of conventional fatigue life estimation approaches. The accuracy of damage accumulation rules under variable amplitude loading was significantly enhanced by modeling the sequence-dependent opening and closure of short cracks. Origins of limited transferability of material data may be identified in the size effect, the roughness effect, or the presence of residual stresses. Some of these influences can be well explained using a short crack approach. For example, the non-homogeneous stress field in notches affects the crack driving force. Considerable effort has been dedicated to model the statistical size effect, too. From the viewpoint of a short crack approach, the probability of exposure of a strength-reducing defect increases with the surface of the highly stressed material. Finally, the short crack approach was introduced for the assessment of multiaxial fatigue. The three mentioned issues contribute to life prediction errors in a balanced way. Some modeling effort was spent by Hertel and Vormwald [1,2] during the last years in joining solution proposals in a unifying short crack model. The actual state is presented together with some ideas for further improvement and simplification for promoting the model’s acceptance in practical applications. F