Abstract
Imperfections due to the manufacturing process can significantly affect the local fatigue strength of the bulk material in cast aluminium alloys. Most components possess several sections of varying microstructure, whereat each of them may inherit a different highly-stressed volume (HSV). Even in cases of homogeneous local casting conditions, the statistical distribution parameters of failure causing defect sizes change significantly, since for a larger highly-stressed volume the probability for enlarged critical defects gets elevated. This impact of differing highly-stressed volume is commonly referred as statistical size effect. In this paper, the study of the statistical size effect on cast material considering partial highly-stressed volumes is based on the comparison of a reference volume V 0 and an arbitrary enlarged, but disconnected volume V α utilizing another specimen geometry. Thus, the behaviour of disconnected highly-stressed volumes within one component in terms of fatigue strength and resulting defect distributions can be assessed. The experimental results show that doubling of the highly-stressed volume leads to a decrease in fatigue strength of 5% and shifts the defect distribution towards larger defect sizes. The highly-stressed volume is numerically determined whereat the applicable element size is gained by a parametric study. Finally, the validation with a prior developed fatigue strength assessment model by R. Aigner et al. leads to a conservative fatigue design with a deviation of only about 0.3% for cast aluminium alloy.
Highlights
Complex cast aluminium parts possess a severely heterogeneous microstructure and it is essential to consider its interaction with the highly stressed volume (HSV)
The aim of this work is the validation of the statistical size effect with consideration of the microstructural properties, as introduced as probabilistic design method for aluminium castings in References [3,4,5]
Previous investigations [3] indicated that the transition knee point is close to about two million load cycles for such unnotched samples made of aluminium alloy
Summary
Complex cast aluminium parts possess a severely heterogeneous microstructure and it is essential to consider its interaction with the highly stressed volume (HSV). The aim of this work is the validation of the statistical size effect with consideration of the microstructural properties, as introduced as probabilistic design method for aluminium castings in References [3,4,5]. The local fatigue strength correlates well with the dedicated microstructure because of the statistical distribution of the defects, apparent in preliminary studies [6,7,8,9,10,11]. Fatigue initiating defects in cast parts can be described well with extreme value statistics, like the generalized extreme value distribution (GEV) or the Gumbel distribution [12,13,14]. Further methodologies to assess the statistical size effect with regard to volumetric
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