Experimental study and modeling of damage of Al alloys using tensor theory |
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Authors: | Maxim A Zapara Nikolay D Tutyshkin Wolfgang H Müller Ralf Wille |
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Institution: | 1. Department of Technological Mechanics, Tula State University, Tula, 300600, Russia 2. Lehrstuhl für Kontinuumsmechanik und Materialtheorie, Technische Universit?t Berlin, Sekretariat MS 02, Einsteinufer 5, 10587, Berlin, Germany
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Abstract: | This article presents and evaluates experiments for the characterization and modeling of damage of structural aluminum and
aluminum–magnesium alloys. Tensile tests were performed for specimens with artificial defects (voids) represented by different
arrangements of pre-drilled micro-holes. The corresponding stress–strain curves were experimentally obtained. Plastic dilatation
and deviatoric strain were determined both for the local zones with artificial defects and directly for meso-elements (i.e.,
material cells with artificial defects). A symmetric second-rank order tensor of damage was applied for a quantitative estimate
of the material damage connected with the volume fraction and shape of micro-defects. The definition of this tensor is physically
motivated, since its hydrostatic and deviatoric parts describe the evolution of damage connected with a change in volume fraction
and shape of micro-defects, respectively. Such a representation of damage kinetics allows us to use two integral measures
for the calculation of damage in deformed materials. The first measure determines damage related to an increase in void volume
fraction (i.e., plastic dilatation). A critical amount of plastic dilatation corresponds to the moment of macro-fracture of
the deformed metal. By means of experimental analysis, we can determine the function of plastic dilatation which depends on
the strain accumulated by material particles under various stress and temperature-rate conditions of forming. The second measure
accounts for the deviatoric strain of meso-elements, and is related to the change in their shapes. The critical deformation
of ellipsoidal voids corresponds to the onset of their coalescence and to the formation of large cavernous defects. The second
measure is considered as a criterion of micro-destruction due to formation of cavities in the deformed material. Based on
the experimental data, some numerical modeling is realized for the investigated Al alloys to taken a change in stress triaxiality
into account. It shows that a change in triaxiality toward smaller values results in an appreciable decrease of damage induced
by strain. Both damage measures are important for the prediction of the meso-structure quality of metalware produced by metal
forming techniques. |
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