Quantification of ductile damage in AL2024-T351 using x-ray tomography

Ductile damage mechanics models provide a tool for the prediction of ductile damage and tearing in ductile materials. Models require knowledge of key microstructural features that influence the ductile failure process in order to calibrate critical model parameters. These parameters include the initial and final void volume fraction. The approach widely used to define the initial void volume fraction is to relate this to the volume fraction of void-initiating precipitates quantified using 2D metallography. This paper illustrates how synchrotron X-ray tomography, and high resolution Focused Ion Beam tomography can provide a new and more reliable approach to defining such starting parameters and can provide additional insights into the ductile failure process. This enables more realistic parameters to be derived that predict the distribution of ductile damage below the final fracture surface and hence provide added confidence in the use of the model to predict global behaviour. Results of synchrotron and FIB tomography experiments undertaken with respect to an aluminium alloy AL2024-T351 are presented. The resulting data illustrates the initial microstructure, which includes shrinkage porosity,- coarse precipitates and fine dispersoids, and the final microstructure close to the fracture surface following failure. The data demonstrate that the void volume fraction initially increases by the growth of porosity and by precipitate fracturing. Final failure occurs by the rapid formation of nano-scale voids at dispersoids. Analyses of notched tensile specimens are undertaken using the GTN model with parameters derived from the tomography data. Conclusions are drawn regarding the use of microstructurally-derived parameters to predict ductile fracture behaviour. Copyright © 2009 by ASME.