Mechanics of Growth and Coalescence of Pre-existing Voids in a Ductile Matrix

This research project experimentally and numerically investigates the growth and coalescence of pre-existing voids for a wide range of stress states in a ductile matrix with a special focus on shear dominated loading conditions and presents quantitative kinetics data for these processes. The results obtained contribute to further development of current fracture models by generating reliable experimental data for void growth and coalescence, which can be used to evaluate the existing fracture models. A lead-containing copper alloy has been selected as model material, in which lead inclusions act as pre-existing voids – as demonstrated by unit cell numerical simulations for a wide range of stress states. A second copper alloy of similar composition, but without lead allows us to obtain mechanical properties of the matrix. By comparing the behavior of the two alloys it is possible to evaluate the effect of the voids on the macroscopic mechanical behavior. The model materials are deformed and fractured in combined torsion / tension tests and notched tensile tests to subject them to the desired wide range of stress triaxialities. Quantitative fractography and metallography provide the desired kinetics data on void shape, size and orientation, that can be correlated with numerical simulations, as well as existing void growth and coalescence models. The influence of stress-state and micro-structural characteristics on void growth and void coalescence – particularly for zero nominal stress triaxiality conditions – is illustrated at the hand of the results of the investigation. Contrary to the predictions of current micromechanical models, significant growth of small voids resulting in moderate increases in the total void volume is observed in torsion specimens. This growth may be triggered by the developing texture of the matrix material and particularly by twinning. By means of FEM computations with 3D single cell simulations comprising initially ellipsoidal voids, the effect of the initial texture and texture evolution is emphasized. The numerical results showed that the experimentally observed texture of the material accelerates void growth under relatively high stress triaxialities, and possibly allows growth of small voids under low and zero stress triaxialities. The initial shape of the lead inclusions and possibly their growth, was found to be dependent on their initial size. These results emphasize the need for a complete and rather detailed modeling of microstructural and deformation features in order to model damage process correctly. The presented work contributes to a better understanding of void growth and coalescence fracture processes, particularly under shear dominated loading conditions.

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