Papers by Keyword: Gurson Model

Abstract: The optimization of automotive security components requires good knowledge of the material state after fabrication, particularly with respect to damage that may have been done to the material by the manufacturing process. To achieve this, numerical simulation of the fabrication process is often undertaken. However, classical continuum damage models, like the Gurson [3] model, are not appropriate for the simulation of the blanking by punching operation because the material damage is primarily the result of shear stresses. This work is focused on the use and validation of a modified Gurson type damage model capable of modeling this process, which has recently been proposed by Nahshan [7]. After a brief description of the modification, the implementation and the validation of the modified Gurson model is detailed. A comparison between the original Gurson model and the modified model is presented in order to highlight the importance of the modification for a pure shear stress state and to show that the two models are equivalent for a purely hydrostatic stress state. It is also shown that the results from the modified model are dependent on the finite element mesh size.

Abstract: This paper presents the dynamic growth behavior of the voids in ductile metals under dynamic loading condition. Started from energy conservation law, a dynamic damage model on void growth process is developed, in which inertial effect is taken into account. The proposed model on void growth is introduced into Gurson model through VUMAT subroutine, so the void growth behavior affected by inertial effect can be investigated and compared. Numerical analysis shows that inertial effect decreases the rate of void growth, and with the increasing of the loading rate, the decreasing effect becomes more remarkable. Inertial effect is very sensitive to the initial damage of the material and the distribution of the void density. The larger the initial damage and the sparser the void density, the stronger the inertial effect on the void growth.

Abstract: A constitutive model for ductile porous material is formulated within the thermodynamic framework. A yield function based on the lower-bound solution for a cylindrical void model embedded in a plastic matrix is proposed. The new yield function is compared to the classical Gurson yield function using cell model calculations. The results reveal that the proposed yield function agreed well with the plastic region found from the cell model calculations. In addition to the influence of the void-volume ratio, the elastic part of the free energy is dependent on a scalar damage field which allows the elasticity to be influenced by the void-volume fraction. The degradation is controlled by a scalar valued damage field and enters the formulation via the Helmholtz's free energy. This dependence allows the elastic properties to naturally depend upon the damage accumulation. The numerical treatment of the model is derived and the capability of the model is demonstrated via numerical simulation of the necking of an axi-symmetric bar.

Abstract: A friction stir weld in 6005A-T6 aluminum alloy has been prepared and analyzed by micro-hardness measurements, tensile testing and scanning electron microscopy (SEM). The locations of the various weld zones were determined by micro-hardness indentation measurements. The flow behavior of the various zones of the weld was extracted using micro-tensile specimens cut out parallel to the welding direction. The measured material properties and weld topology were then introduced in a fully coupled micro-mechanical finite element model, accounting for nucleation and growth of voids as well as void shape evolution. The model shows satisfactory preliminary results in predicting the tensile behaviour of the weld and the true strain at fracture.

Abstract: Little work has been published concerning the transferability of Gurson’s ductile damage model parameters in specimens tested at different strain rates and in the rolling direction of a Grade A ship plate steel. In order to investigate the transferability of the damage model parameters of Gurson’s model, tensile specimens with different constraint level and impact Charpy specimens were simulated to investigate the effect of the strain rate on the damage model parameters of Gurson model. The simulations were performed with the finite element program ABAQUS Explicit [1]. ABAQUS Explicit is ideally suited for the solution of complex nonlinear dynamic and quasi–static problems [2], especially those involving impact and other highly discontinuous events. ABAQUS Explicit supports not only stress–displacement analyses but also fully coupled transient dynamic temperature, displacement, acoustic and coupled acoustic–structural analyses. This makes the program very suitable for modelling fracture initiation and propagation. In ABAQUS Explicit, the element deletion technique is provided, so the damaged or dead elements are removed from the analysis once the failure criterion is locally reached. This simulates crack growth through the microstructure. It was found that the variation of the strain rate affects slightly the value of the damage model parameters of Gurson model.

Abstract: Multiple Site Damage (MSD) is a typical problem of aging aircraft structures. On the other hand a similar situation may occur, if damages in the vicinity of the crack tip of a large crack is investigated. In some preceding papers the author has shown the possibility to assess the criticality of a MSD-scenario by comparing certain data compression measures of the crack scenario in an early stage. All of these scenarios were linked to cracks in one row of fasteners of equal distance, and the method was used in conjunction with the Monte-Carlo Simulation. The idea of using such ways to assess multiple crack scenarios is extended now in the paper presented here. The investigations are extended now in the sense that, the scenarios are more complex as they include large cracks and small cracks in the vicinity of the crack, where no fixed distance of the cracks is foreseen. The attempt is made to relate such results to experimental data on residual strength and therefore, to relate them to sophisticated models like the Gurson model.

Abstract: The objective of robust design is to optimize the mean of a given target variable and to minimize the variability that results from uncertainty represented by “noise” factors. A recent strategy for robust design is based on stochastic processes, which has resulted in a new design technique called “stochastic design improvement” (SDI). In this work a home-made procedure is presented, which is based on the SDI technique and which is illustrated with reference to a case study which aims to increase the residual strength of a cracked stiffened aluminum panel.

Abstract: The finite element method was used in order to compute the energy balance and the stress-strain distribution in the Charpy V-notch specimen. Inertial effects were taken into account by a fully dynamic computation. It was shown that inertial oscillations are damped by viscoplasticity ahead the notch and vanish rapidly. 3D modelling is needed since large scale yielding and ductile crack growth occur. The ductile crack front is curved, which is important to account for in order to correctly describe the stress distribution in the specimen. Ductile crack growth in Charpy specimen was predicted by the GTN (Gurson-Tvergaard-Needleman) model. The GTN model allows a good failure prediction with strain rate and temperature independent damage parameters.