Notch effects in tensile behavior of AM60 magnesium alloys

The deformation and failure behavior of an AM60 magnesium alloy was investigated using tensile test on circumferentially notched specimens with different notch radii. The strain and stress triaxiality corresponding to the failure point were evaluated using both analytical and finite element analyses. Combining with systematical observations of the fracture surfaces, it is concluded that deformation and failure of AM60 magnesium alloy are notch (constraint) sensitive. The failure mechanisms change from ductile tearing to quasi cleavage with the increase of constraint

Effect of stress-triaxiality on void growth in dynamic fracture of metals: a molecular dynamics study

The effect of stress-triaxiality on growth of a void in a three dimensional single-crystal face-centered-cubic (FCC) lattice has been studied. Molecular dynamics (MD) simulations using an embedded-atom (EAM) potential for copper have been performed at room temperature and using strain controlling with high strain rates ranging from 10^7/sec to 10^10/sec. Strain-rates of these magnitudes can be studied experimentally, e.g. using shock waves induced by laser ablation. Void growth has been simulated in three different conditions, namely uniaxial, biaxial, and triaxial expansion. The response of the system in the three cases have been compared in terms of the void growth rate, the detailed void shape evolution, and the stress-strain behavior including the development of plastic strain. Also macroscopic observables as plastic work and porosity have been computed from the atomistic level. The stress thresholds for void growth are found to be comparable with spall strength values determined by dynamic fracture experiments. The conventional macroscopic assumption that the mean plastic strain results from the growth of the void is validated. The evolution of the system in the uniaxial case is found to exhibit four different regimes: elastic expansion; plastic yielding, when the mean stress is nearly constant, but the stress-triaxiality increases rapidly together with exponential growth of the void; saturation of the stress-triaxiality; and finally the failure.Comment: 35 figures, which are small (and blurry) due to the space limitations; submitted (with original figures) to Physical Review B. Final versio

Experimental characterization and numerical modeling of micromechanical damage under different stress states

The use of HSLA steels for the manufacture of automotive components is interesting from an engineering point of view. This family of steels, while possessing high strength, also has good formability and can be used in forming manufacturing processes. In some forming processes such as blanking, shear strain localization occurs, which causes damage and results in the final fracture of the material. This paper presents an experimental study based on in situ tests to understand and identify the physical mechanisms of ductile damage under two stress states: tension and shear. Different macroscopic tests were performed to calibrate a damage model based on a micromechanical approach. This damage model is based on the Gurson–Tvergaard–Needleman theory and presents recent improvements proposed by Nahshon and Hutchinson and by Nielsen and Tvergaard so as to better predict fracture under a wide range of stress states, especially with low levels of stress triaxiality. These extensions have made the identification of the material parameter more complicated. In this work an identification strategy has been proposed using tests on specimens with different shapes. The identified parameter values are validated and the fracture model show good predictive capability over a wide stress state range

Modeling of ductile damage using numerical analyses on the micro-scale

The presentation deals with a continuum damage model which has been generalized to take into account the effect of stress state on damage criteria as well as on evolution equations of damage strains. It is based on the introduction of damaged and corresponding undamaged configurations. Plastic behavior is modeled by a yield criterion and a flow rule formulated in the effective stress space (undamaged configurations). In a similar way, damage behavior is governed by a damage criterion and a damage rule considering the damaged configurations. Different branches of the damage criterion are considered corresponding to various damage mechanisms depending on stress intensity, stress triaxiality and the Lode parameter. Experiments with carefully designed specimens are performed and the test results are used to identify basic material parameters. However, it is not possible to determine all parameters based on these tension and shear tests. To be able to get more insight in the complex damage behavior under different loading conditions, additional series of micro-mechanical numerical analyses of void containing unit cells have been performed. These finite element calculations on the micro-level cover a wide range of stress triaxialities and Lode parameters in the tension, shear and compression domain. The numerical results are used to show general trends, to develop equations for the stress-statedependent damage criteria, to propose evolution equations of damage strains, and to identify parameters of the continuum model

Analysis of damage and fracture mechanisms in ductile metals under non-proportional loading paths

The paper discusses biaxial experiments and corresponding numerical simulations to analyze the effect of non-proportional loading paths on damage and fracture behavior of ductile metals. Newly developed specimens are taken from thin metal sheets and are tested under different biaxial loading conditions covering a wide range of stress states. In this context, an anisotropic continuum damage model is presented based on yield and damage conditions as well as on evolution laws for plastic and damage strain rates. Different branches of the damage criteria are taken into account corresponding to various damage and failure processes on the micro-level depending on stress triaxiality and Lode parameter. Experiments with biaxially loaded specimens have been performed. Results for proportional and corresponding non-proportional loading histories are discussed. During the experiments strain fields in critical regions of the specimens are analyzed by digital image correlation (DIC) technique while the fracture surfaces are examined by scanning electron microscopy (SEM). Numerical simulations of the experiments have been performed and numerical results are compared with experimental data. In addition, based on the numerical analyses stress distributions in critical parts of specimens are detected. The results demonstrate the efficiency of the new specimen’s geometries covering a wide range of stress states in the shear/tension and shear/compression regime as well as the effect of loading history on damage and fracture behavior in ductile metal sheets

Evaluation of formability and fracture of pure titanium in incremental sheet forming

A forming limit diagram (FLD) is commonly used as a useful means for characterizing the formability of sheet metal forming processes. In this study, the Nakajima test was used to construct the forming limit curve at necking (FLCN) and fracture (FLCF). The results of the FLCF are compared with incremental sheet forming (ISF) to evaluate the ability of the Nakajima test to describe the fracture in ISF. Tests were carried to construct the forming limit diagram at necking and fracture to cover the strain states from uniaxial tension to equi-biaxial tension with different stress triaxialities - from 0.33 for uniaxial tension to 0.67 for equi-biaxial tension. Due to the fact that the Gurson–Tvergaard- Needleman (GTN) model can be used to capture fracture occurrence at high stress triaxiality, and the shear modified GTN model (Nahshon-Hutchinson’s shear mechanism) was developed to predict the fracture at zero stress or even negative stress triaxiality, the original GTN model and shear modified GTN model may be not suitable to predict the fracture in all samples of the Nakajima test as some samples are deformed under moderate stress triaxiality. In this study, the fractures are compared using either the original GTN model, shear modified GTN model or Nielsen-Tvergaard model with regard to stress triaxiality. To validate the ability of these models, and to assess which model is more accurate in predicting the fracture with different stress triaxialities, finite element (FE) simulations of the Nakajima test were compared with an experimental results to evaluate the applicability of the Nakajima test to characterise the fracture from ISF. The experimental and FE results showed that the shear modified GTN model could predict the fracture accurately with samples under uniaxial tension condition due to low stress triaxiality and that the original model is suitable for an equi-biaxial strain state (high stress triaxiality), whereas the stress triaxiality modified GTN model should be considered for samples which have moderate stress triaxiality (from plain strain to biaxial strain). The numerical and experimental FLCF of pure titanium from the Nakajima test showed good agreement with the experimental and numerical results of ISF

A New Model for Void Coalescence by Internal Necking

A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.

Uncoupled material model of ductile fracture with directional plasticity

Proposed paper deals with the application of plastic response with directional distortional hardening (DDH) in uncoupled ductile fracture model and comparison of the results with the same ductile fracture model based on isotropic J2 plasticity. The results of simulations have proven not negligible role of model of plasticity and the response of the model with DDH plasticity is closer to reality then the one of the model with isotropic plasticity