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A physical-based model for the flow stress of bcc metals is presented. Here, thermally activated and viscous drag regimes are considered. For the thermally activated component of the flow stress, the diffusion-controlled regime at elevated temperature is also taken into account assuming the non-linear dependence of the activation volume on temperature. The model was applied to A508 (16MND5) steel showing the possibility to accurately describe the variation of the flow stress over the entire temperature range (from 0 K to Tm) and over a wide strain-rate range.

The dynamic tensile extrusion (DTE) test offers unique possibility to probe material response under very large plastic strain, high strain rate and temperature to support constitutive modelling development. From the computational point of view, the DTE test is particularly challenging and a number of issues need to be assessed before proceeding with material modelling verification. In this work, an extensive and detailed computational work was carried out in order to provide the guidelines for accurate simulation of DTE test. Two constitutive models, the first phenomenological the latter physically-based, were used to simulated the behavior of fully annealed OFHC copper in dynamic extrusion at different velocities. Material models parameters were calibrated using uniaxial test data at different strain rates and temperatures. The number, size and shape of the ejected fragments at different velocity were used as validation metrics for the selected constitutive models. Results indicate that material behavior under dynamic extrusion can be accurately predicted limiting the influence of numerical parameters not related to the constitutive model under investigation. The physically based modelling allows a more accurate prediction of the material response and the possibility to incorporate microstructure evolution processes, such as dynamic recrystallization, which seems to control the response of OFHC copper in DTE tests at higher velocity.

Abstract.Recently, the continuum damage mechanics model proposed by Bonora (Eng. Fract. Mech. 58, 1997) has been updated to account for stress triaxiality effect on model parameters, (Bonora et al., AIP Conf. Proc. 1195, 2009. This model enhancement allows to predict ductile damage initiation under varying stress states (uniaxial stress, uniaxial strain, and complex load paths) and dynamic loading conditions. In this work, the model has been used to investigate ductile damage developments in Taylor anvil and symmetric Taylor impact (rod-on-rod) configuration. Although the two configurations are equivalent for right scaled impact velocities, experimental evidences show that when ductile damage occurs in rod-on-rod not necessarily also develops in Taylor anvil impact. It has been found that, in the two impact configurations, the stress triaxiality builds up differently with plastic strain leading to different conditions for ductile damage initiation. Taylor impact tests have been designed and performed with the gas-gun facility at the University of Cassino. Damage investigation results obtained on recovered samples have been compared with rod-on-rod data reported in the literature and used to validate the proposed model predictions.

The stress triaxiality effect on the strain required for void nucleation by particle-matrix debonding has been investigated by means of micromechanical modelling. A unit-cell model considering an elastic spherical particle embedded in an elastic-plastic matrix was developed to the purpose. Particle-matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix-particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime. K E Y W O R D Smicromechanics, stress triaxiality, void nucleation, α-iron

Abstract. At equivalent impact velocity, pressure in Taylor and ROR impact experiment is not the same and this reflects in the resulting condition for ductile damage development. In this work, finite element parametric simulation was performed to investigate pressure wave development as a function of material and target work hardening curve. Using the Bonora damage model, the impact velocity necessary for generating ductile damage in high purity copper was assessed. Taylor and ROR experiments were performed at different equivalent impact velocities and metallographic investigation were performed on impacted samples in order to validate damage model predictions. Results seems to indicate that ROR configuration is more appropriate for 2damage model validation while the Taylor anvil is more suitable for strength model assessment.

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