A phenomenological theory of inelastic deformations including damage processes may be based on the fundamental assumption that the body can be considered as a classical continuum and that the principle of uniquely determined local thermodynamical state holds. Within this frame particular balance equations for energy and entropy can be stated describing the interaction between external energy supply and changes of the internal material structure. These balance equations are applied to elastic-plastic deformations including damage processes. The parameters describing the damage state can be considered either as thermodynamical state variables or merely as thermo-mechanical process variables. This distinction leads to different formulations.

Some thermodynamical considerations on inelastic deformations including damage processes

The article deals with a new phenomenological model of damage cumulation in metallic materials subjected to static tension. The main parameter of the running state of the material, which is put in correspondence with the degree of loosening, is the Poisson ratio at the stage of loss of strength (the descending part of the stress-strain curve). The suggested model describes satisfactorily the results of specially arranged experiments.

Model of damage cumulation in metallic materials under static tension

The article reviews the possibilities, technical peculiarities, and advantages of the new experimental method of determining quantitatively the level of damage to ductile structural materials at the onset of macrofailure The method is based on the analysis of the change of elastic properties of the ductile material in the zone of macrofailure at supercritical stages of loading (corresponding to the descending branch of the stress-strain diagram) On the example of the aluminum alloy AMtsS and steels St 3, 08Kh18N10T the article analyzes the kinetics of damage cumulation at the stages of deformation preceding the onset of macrofailure, and the effect of stress raisers on the limit damage to the material at the instant of the onset of macrofailure The article presents experimental data indicating that the limit damage of the tested materials is constant in the process of the self-similar growth (without change of micromechanisms) of the ductile macrocrack

Determination of the damage parameters of ductile materials at the stage of softening

In the paper we investigate the robust identification approach to identify the material parameters in the augmented Gurson model for the elasto-plastic porous media. We consider the robust loss function given by Huber [9], Beaton and Tuckey [38] and the loss function based on the l 1 -norm. The resulting minimization problem is solved by means of our own implementation of the Boender et al. global minimization method. Our aim is to compare the results with our earlier standard least squares estimates. In the paper, the effects of nucleation and growth of voids in the plastic porous media are investigated. Three different forms of the model are considered: the augmented Gurson model (total porosity model) with variable nucleation and growth material function, the same model with constant growth material function and the separated porosity model. The identification of the material functions parameters is based on Fischer's experimental data set for axisymmetric tension of steel specimens.

http://am.ippt.pan.pl/am/article/viewFile/v58p125/pdf

Robust identification of an augmented Gurson Model for elasto-plastic porous media

In the paper the effects of nucleation and growth of voids in the plastic porous media are investigated. Three different forms of the model are considered: the augmented Gurson model (total porosity model) with variable nucleation and growth material function, the same model with constant growth material function and the separated porosity model. The identification of the material functions parameters is based on Fischer’s experimental data set for axisymmetric tension of steel specimens and formulated as a typical nonlinear regression problem using the least squares approach. The resulting minimization problem is solved by means of our own implementation of the Boender at al. global minimization method. Calculations and statistical analysis (Akaike, FPE and Vuong tests) have led to a conclusion that the growth material function in the uniaxial tension for steel may be assumed to be constant although not necessarily equal to one.

/pdf/modelling-and-identification-of-voids-nucleation-and-growth-2b3ie8p6iu.pdf

P. Perzyna

Papers

Evolution equation for the void fraction parameter in necking region