Aluminium castings often possess complex geometries with only selected surfaces machined. Thus, the overall fatigue assessment needs to cover the heterogeneously textured as-cast surfaces considering local casting process conditions as well.
Recent research activities of Pomberger and Oberreiter introduced an alternative sub-area based fatigue assessment methodology for as-cast and pore surface-layer afflicted specimens. The conducted research work focusses on flat sample geometries taken out of aluminium crankcases covering different manufacturing process conditions.
The new testing concept features the experimental fatigue assessment of as-cast surfaces on specimens taken out of the water-cooling-jacket volume crossways to the axial piston movement. The transverse arrangement of the specimen enables a testing load step which is most representative to the actual loading conditions of combustion engines. In addition, the transversal sampling position implies that the local casting process condition is nearly constant throughout the tested volume. A pronounced variation in microstructure and spatial roughness parameters was observed in previous studies for axial orientated samplings. Finally, the new transversal specimen arrangement features the study of local process conditions, as the highly-stressed volume is maintained within a specific height of the cylinder covering statistically coherent roughness parameters.
The tangential ring segments possess a circumferential as-cast surface, which is bend by applying an exterior compressive load at the sector ends. Thus, featuring a cyclic tension load with high stress ratio at the rough surface.
The experimental work covers two cast aluminium alloys, focussing on the effect of local casting conditions. Therefore, several specimens are additionally analysed with computed tomography to evaluate the distribution of casting inhomogeneities within the surface layer. The sub-area based fatigue assessment is statistically applied for the previously non-destructive scanned as-cast surfaces. The conducted experimental results show a sound correlation to the numerical fatigue assessment which enhances the current validation range of the method significantly.