Damage parameter based fatigue assessment of cast steel components containing internal defectsTuesday (26.05.2020) 14:30 - 14:50
In steel constructions, components from cast steel are attractive alternatives to complexly formed weldments. However, depending on the geometry and the casting process internal defects such as pores or inclusions cannot be fully avoided. Under fatigue loading, such defects could act as -crack initiation sites, which lead to crack initiation and finally to failure of a component. For the fatigue assessment of the manufacturing-related internal defects in cast steel suitable engineering rules are still missing. This in turn leads to the extreme consequence that expensive components have to be discarded.
In the present work, the influence of manufacturing-related internal defects on the fatigue strength of components is investigated and engineering assessment procedures are validated and recommended.
Specimens from G20Mn5 and G22NiMoCr5-6 cast steel are manufactured using a special mold geometry to control the formation of internal defects. The specimens are examined by CT measurements to get a 3D representation of the defect shapes and distribution. The fatigue life of the specimen is determined by cyclic testing at various load levels. To employ fatigue assessment different modelling approaches are tested and evaluated with regard to their applicability, with a spotlight on the utilization of damage parameters to assess crack initiation life. The underlying material models are adjusted to experiments on nearly defect free material.
The experiments on specimen containing defects show a strong correlation between fatigue life and defect size and position. As expected, fully internal defects are less damaging than surface and subsurface defects -. For an efficient calculation of the damage parameters in a finite element model it proves to be suitable to consider the inner defects by using a porosity dependent material model. Based on the stress-strain field at the boundaries of the pores the damage parameter can be determined as measure for crack initiation, which conservatively describes the fatigue life. The damage parameter according to Fatemi and Socie was proven as a suitable parameter for the lifetime assessment of castings with internal defects.