Giuseppe Mirone

Effect of stress triaxiality corrected plastic flow on ductile damage evolution in the framework of continuum damage mechanics

Abstract Most of the damage models derived from the theory of Lemaitre have been developed using the hypothesis based on “proportional loading” for the determination of some material parameters. This hypothesis is adopted in such a way that a material coefficient (a scale factor for damage values) can be evaluated from tensile tests on unnotched specimens, considering the triaxiality of the stress state to be that corresponding to perfect uniaxiality, over the entire load history, and so ignoring necking induced variations in the triaxiality. The present paper analyses the response of some continuum damage mechanics models which incorporate a correlation between the triaxiality factor and logarithmic plastic strain. These correlation laws, affecting also the true stress–true strain material curve, were derived from experimental data processed using the Bridgman method. Due to the lack of experimental data specifically related to damage evolution, the aim of the study was to evaluate the sensitivity of these models to the variability of the triaxiality parameter during the deformation history.

Approximate Model of the Necking Behaviour and Application to the Void Growth Prediction

Stress distributions at the minimum cross section of axisymmetric specimens undergoing necking are determined using an approximate method recently developed by the authors. This method is briefly presented, then applied to experimental data from tensile tests of ductile metals, available in the literature. The stress triaxiality distributions across the necked section so evaluated are used to predict the void growth according to the model by Rice and Tracey. The resulting damage evolution as well as the above stress and triaxiality distributions are finally compared with results from finite element simulations as well as with experimental damage measurements.

Numerical Verification Of The Bridgman ModelFor Notched And Unnotched Round Specimens

The aim of this work is to evaluate the accuracy of the Bridgman model for stress distributions on the neck of a cylindrical bar, and to compare its results with the corresponding values obtained using finite element analysis. The application of Bridgmans laws to an experimental data set allowed the definition of a constitutive curve, adopted as input for the FE models. Some of the numerical results obtained were compared with the corresponding experimental data, other FEM results were compared to the Bridgman prediction applied to the same experimental data. By this comparative analysis it was possible to evidence that a part of the experimental-numerical disagreement was due to the difficulty of taking damage evolution into account in the FE analysis. For a qualitative evaluation of this aspect, therefore, a new numerical comparison was made between the FEM results and a Bridgman prediction now based on the same FEM results (in terms of instantaneous load, contraction and profile curvature) instead of on experimental data as in the previous case.

Preliminary design and wind tunnel tests of a micro air vehicle for surveillance and sensor-bearing applications

Abstract This work presents the preliminary design of a micro air vehicle (MAV) and the wind tunnel tests of some wing configurations identified, in the design phase, as potentially suitable for the MAV realization. After some considerations about the airfoil adoption and its performance estimation, five possible plan shapes for the wing are defined and the three-dimensional model realization is described. These wings are experimentally tested in a wind tunnel as simple wings and as complete MAV dummies, equipped with vertical fins. The results from the tests allow the selection of the most promising configurations among those tested for the achievement of the desired MAV performance. A final evaluation on the better performing wing is carried out with regard to the placement of the components in its interior and to the calculation of its stability parameters.

Numerical evaluation of a material-independent necking shape factor

The best degree of approximation for the determination of a ductile material hardening curve today is represented by the Bridgman method. This procedure requires measurements of the necking curvature radius other than the load and the neck diameter, and in literature it is usually carried out without dealing with the increasing porosity of the material. The obtained curve is approximate because refers to values of the equivalent stress and strain averaged onto the entire neck section and, when it is evaluated without any void measurement, because of the difference between the apparent and the effective resisting cross section. Furthermore, it has been shown that the response of this method is strongly dependent on the initial specimen shape, showing a certain lack of precision in the triaxiality estimation, In this work a new method for the evaluation of the flow stress-equivalent plastic strain curve has been used, adopting some of the hypotheses flom the Bridgrnan theory, but eliminating the need of curvature radii measurements. “ The relationships found between some variables characteristic of the plastic behavior are almost independent from the materials except for their Consider& strain, and the derived characterization method proposed is also independent of the initial specimen shape.