Impact of microstructural properties on the crack threshold of aluminium castings
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This work focuses on the technological size effect of the cast aluminium alloy EN-AC 46200. Herewith, specimens inheriting specific local cooling rates and therefore varying local dendrite arm spacings are taken from crankcases with hot isostatic pressing (HIP) post-treatment. In order to evaluate the statistically distributed microstructural properties, the dendrite arm spacing λ2 is probabilistically assessed. Thus, the λ2 values follow the lognormal distribution, detached of local solidification times. Crack propagation tests are conducted at varying load ratios, such that the impact of the microstructure by means of λ2 is evaluated. The statistically assessed investigations propose the intrinsic crack threshold to be independent of microstructural properties, in opposite to the long crack threshold which is significantly affected by varying λ2 values due to enhanced crack closure effects. The evaluated cyclic R-curve results can be generalized into a microstructural independent manner. Moreover, the load ratio effect and its convolution with microstructural properties is evaluated based on the Newman approach. The linear extrapolation of the newly raised three-dimensional Newman approach, considering arbitrary λ2 values as technological effect, is validated by literature data and the proposed stress intensity factor threshold course is revealed to be slightly conservative by about twelve percent on average.