O.-G. Lademo

An evaluation of yield criteria and flow rules for aluminium alloys

Abstract The mechanical characteristics of two aluminium alloys, AA7108 and AA6063, have been investigated by means of uniaxial tensile tests. The experiments show that the alloys exhibit significant anisotropy in yield strength, plastic flow and ductility. Based on the experiments, some existing phenomenological models of elastoplasticity were examined. The ingredients of the models are a yield criterion, a flow rule and a strain-hardening rule. Three yield criteria proposed by Hill (1950) , Barlat and Lian (1989) , and Karafillis and Boyce (1993) were evaluated. None of these have been found to give a satisfactory description of the mechanical behaviour.

A numerical study on the influence of the Portevin–Le Chatelier effect on necking in an aluminium alloy

The constitutive relation proposed by McCormick (1988 Acta Metall. 36 3061–7) for materials exhibiting negative steady-state strain-rate sensitivity and the Portevin–Le Chatelier (PLC) effect is incorporated into an elastic–viscoplastic model for metals with plastic anisotropy. The constitutive model is implemented in LS-DYNA for corotational shell elements. Plastic anisotropy is taken into account by use of the yield criterion Yld2000/Yld2003 proposed by Barlat et al (2003 J. Plast. 19 1297–319) and Aretz (2004 Modelling Simul. Mater. Sci. Eng. 12 491–509). The parameters of the constitutive equations are determined for a rolled aluminium alloy (AA5083-H116) exhibiting negative steady-state strain-rate sensitivity and serrated yielding. The parameter identification is based on existing experimental data. A numerical investigation is conducted to determine the influence of the PLC effect on the onset of necking in uniaxial and biaxial tension for different overall strain rates. The numerical simulations show that the PLC effect leads to significant reductions in the strain to necking for both uniaxial and biaxial stress states. Increased surface roughness with plastic deformation is predicted for strain rates giving serrated yielding in uniaxial tension. It is likely that this is an important reason for the reduced critical strains. The characteristics of the deformation bands (orientation, width, velocity and strain rate) are also studied.

Effects of the yield criterion on local deformations in numerical simulation of profile forming

Abstract Three anisotropic plasticity models are implemented in LS-DYNA3D for shell analysis using backward Euler integration algorithms. The models have different yield criteria (Hill, Barlat and Lian, and Karafillis and Boyce) but have all associated flow rules and non-linear isotropic strain hardening. The anisotropic models are applied in finite element analysis of a stretch bending process for aluminium extrusions. In addition, simulations with the von Mises yield criterion and a non-quadratic yield criterion for isotropic materials are carried out. Plastic anisotropy and the shape of the yield surface are found to have significant effect on the local deformations predicted in the simulations.

Aluminium and Magnesium Castings - Experimental Work and Numerical Analyses

The long-term objective of this work is to develop design and modelling tools that allow the structural behaviour of thin-walled cast components to be predicted when subjected to static and dynamic loads such as in crash situations. The approach consists of the following ingredients: casting of generic geometry relevant for automotive design, component and materials testing, constitutive modelling and validation simulations using the finite element method. Here, HPDC components of both AM60 and AlSi7Mg have been subjected to axial crushing and 3-point bending. An existing elastoplastic constitutive equation has been calibrated and evaluated based on material tests. Next, validation analyses of the generic structural components subjected to static loading were performed. The thin-walled cast components are modelled in the explicit FE-code LS-DYNA using shell elements. So far, it has been found that an elastoplastic material model based on the isotropic yield criterion of von Mises reasonably well captures the behaviour of HPDC components subject to both axial loading and 3-point bending. In addition, the ultimate loads in the different load cases are over-predicted analytically with the use of Eurocode 9 (EC9).

Offset Impact Behaviour of Bumper Beam–Longitudinal Systems: Numerical Simulations

Abstract The paper presents the results from numerical simulations of bumper beam–longitudinal systems subjected to 40% offset impact loading. Numerical simulations were carried out with the non-linear finite element code LS-DYNA, searching for an efficient, numerically robust and accurate representation of the observed system behaviour. A comparative study of an industrial-like modelling procedure and another procedure incorporating a user-defined material model has been performed. In the latter procedure, the material model consists of state-of-the-art anisotropic plasticity, an isotropic strain and a strain-rate hardening rule as well as some ductile fracture criteria. Both shell and solid elements were utilized in discretizing the bumper beam–longitudinal set-up. Numerical crash results revealed good agreement with the experiments with respect to overall deformation mode and energy dissipation. The simulations were capable of giving relatively accurate prediction of the collapse mode found in the experimental tests, except for the bumper beam–longitudinal system with AA7003-T1 longitudinals. Sensitivity studies were performed considering both physical and numerical parameters. The physical parameters were strain-rate effects and the heat-affected zone, whereas the numerical parameter considered was adaptive meshing.

Offset impact behaviour of bumper beam—longitudinal systems: experimental investigations

Abstract Repeatability and robustness are the key factors that the automotive producers are demanding in connectionwith the crash performance of bumper beam systems. Bumper beam systems are important structures of an automotive that protect the passengers from front and rear collisions. Assessing the impact performance of only a bumper beam system through full-scale crash tests of a car is not easy. Thus, this paper presents experimental investigations on the bumper beam–longitudinal systems subjected to 40% offset impact. Bumper beam systems that are generally fitted to the automotives include ‘crashboxes’ besides the longitudinals. In the current study, the bumper beam system does not include any ‘crashboxes’. This is also the preferred system for some automotive producers. A longitudinal will offer higher resistance to deformation and thus gives higher energy absorption than if crashboxes are used. Using SIMLabs kicking machine, experiments were performed on rotary stretch–bent aluminium bumper beams, which were connected to two longitudinals at both ends and an experimental database was established. The repeatability and robustness of the bumper beam–longitudinal systems were studied by varying the material and temper condition of the longitudinals. Experimental investigations revealed that the longitudinal members with considerable strain-hardening would change the collapse mode into a global mode and reduce the energy absorption capability, due to the random development of lobes along the length.

Calibration of anisotropic yield criteria using uniaxial tension tests and bending tests

Abstract A method for calibration of anisotropic yield criteria based on yield stresses obtained in tensile and bending tests is developed. It is thus possible to calibrate anisotropic criteria by means of simple tests without using the R -ratios in combination with the associated flow rule. Explicit formulas are derived for the calibration of Hills yield criterion, while a numerical procedure is developed for calibration of the Barlat and Lian yield criterion. Results from tensile and bending tests on the aluminium alloy AA 7108 in temper T76 are presented. Tests in three material directions are carried out in order to characterise the plastic anisotropy of the alloy. In the tensile tests the stress–strain curves and the R -ratios are determined, whereas the bending tests establish the moment–curvature curves.