Ductile Failure In Metals Research Articles

A physically based model for dynamic failure in ductile metals

It is well established that high rate failure of structural materials takes place by rate processes occurring at the microscopic level and involving nucleation, growth, and coalescence of voids or cracks. At the submicroscopic level, the mechanism of failure in polycrystalline metals is often dislocation controlled. In the present work, we propose a physically based model describing these processes at high deformation rate such as under the planar impact test. This model combines the mechanical threshold stress (MTS) theory for the evolution of the flow stress and a mechanistic model for failure behavior by cumulative nucleation and growth of voids. This paper describes the approach used to obtain the constitutive equations and the resulting computational modeling for predicting dynamic failure in ductile metals. The model formulation is three-dimensional and is suitable for a general state of stress and strain. Results from the simulations of the planar impact problems for different configurations are presented and compared with the experimental results for OFHC copper and HY-100 steel. This comparison shows the model capabilities in predicting the experimentally measured free surface velocity profile as well as the observed spall pattern respectively in the copper target of a planar impact test and in the steel target of a plate-conic impact. We have also compared the results from this model to those of the phenomenological model of Johnson–Cook.

Identification of the parameters of a non-linear continuum damage mechanics model for ductile failure in metals

Damage modelling in ductile metal has received a lot of attention in the last thirty years. Since 1969, many models have been proposed in the literature even though, in most of the cases, the experimental aspects related to the damage parameter evaluation have often been neglected. In this work the procedure to evaluate the damage parameters, for the continuum damage mechanics based model proposed by Bonora, is presented. Here, the ductile damage is experimentally measured in terms of progressive elastic modulus reduction as a function of strain measured in an hourglass-shaped specimen loaded in tension. The proposed technique, which is based on performing multiple, partial unloading, is reliable and relatively simple to implement. Additional tensile testing, performed with a round notch tensile bar, provides critical data that can be used to identify unequivocally the full damage parameter set and to assure their geometry transferability. The procedure presented here has been applied to identify the damage parameters for 20MnMoNi55 steel.

Brittle failure of ductile metals

Brittle failure of ductile metals

Failure of Ductile Metals in Tension

Abstract The extensive fabrication of aluminum-alloy sheet in the aircraft industries has revealed the lack of fundamental knowledge regarding the phenomena which explain the performance of metals under various stress and strain states. Previous investigations appear to have neglected entirely both the effects of the part geometry and the fact that the strength of the metal and limit of forming are frequently determined by a process of instability, or “necking,” rather than by the process of fracturing. In this paper a theory of necking is developed in order to predict the forming limits for various stress or strain states based upon an analysis of the true stress-strain curve in pure tension. A criterion for necking is advanced, based upon the maximum-load (or pressure) conception of instability, and is applied to different geometrical shapes formed at room temperature. Future publications will present, in support of the criterion advanced, experimental analyses of the forming of different geometrical shapes under various loading conditions and stress and strain states.