Vincent T. Meinders

Failure Predictions for DP Steel Cross-die Test Using Anisotropic Damage

The Lemaitres continuum damage model is well known in the field of damage mechanics. The anisotropic damage model given by Lemaitre is relatively simple, applicable to nonproportional loads and uses only four damage parameters. The hypothesis of strain equivalence is used to map the effective stress to the nominal stress. Both the isotropic and anisotropic damage models from Lemaitre are implemented in an in-house implicit finite element code. The damage model is coupled with an elasto-plastic material model using anisotropic plasticity (Hill-48 yield criterion) and strain-rate dependent isotropic hardening. The Lemaitre continuum damage model is based on the small strain assumption; therefore, the model is implemented in an incremental co-rotational framework to make it applicable for large strains. The damage dissipation potential was slightly adapted to incorporate a different damage evolution behavior under compression and tension. A tensile test and a low-cycle fatigue test were used to determine the damage parameters. The damage evolution was modified to incorporate strain rate sensitivity by making two of the damage parameters a function of strain rate. The model is applied to predict failure in a cross-die deep drawing process, which is well known for having a wide variety of strains and strain path changes. The failure predictions obtained from the anisotropic damage models are in good agreement with the experimental results, whereas the predictions obtained from the isotropic damage model are slightly conservative. The anisotropic damage model predicts the crack direction more accurately compared to the predictions based on principal stress directions using the isotropic damage model. The set of damage parameters, determined in a uniaxial condition, gives a good failure prediction under other triaxiality conditions.

Material-induced anisotropic damage in DP600

Plasticity-induced damage development in metals is anisotropic by nature. The anisotropy in damage is driven by two different phenomena: anisotropic deformation state i.e. load-induced anisotropic damage (LIAD) and anisotropic microstructure i.e. material-induced anisotropic damage (MIAD). The contribution of second-phase particles can be anisotropic in terms of shape as well as distribution. Most of the continuum anisotropic damage models mimic the phenomenon of LIAD only. Not much attention has been paid to MIAD. This work shows the existence of MIAD in a (pre-production) grade of dual-phase steel (DP600). The aim is to see the influence of MIAD on post-localization deformation behavior and final failure mode. The deformation in this material is almost isotropic up to localization but the post-localization deformation and final failure mode is different when loaded in 0° and 90° to rolling direction. Tensile specimens were deformed up to final failure. A few specimens were stopped just before the final failure. Scanning electron microscopic analysis was carried out to study martensite morphology and damage in these specimens. The martensite morphology showed anisotropy in shape and orientation in the undeformed specimens. Significant MIAD was observed in the deformed tensile specimens due to the anisotropic martensite morphology. MIAD explains direction-dependent post-localization deformation, final failure mode, and formability of this material. Lemaitres anisotropic damage model is modified to account for MIAD in a phenomenological manner. The MIAD parameters were determined from tensile tests carried out in 0°, 45°, and 90° to the rolling direction.

Identification of plasticity model parameters of the heat-affected zone in resistance spot welded martensitic boron steel

A material model is developed that predicts the plastic behaviour of fully hardened 22MnB5 base material and the heat-affected zone (HAZ) material found around its corresponding resistance spot welds (RSWs). Main focus will be on an accurate representation of strain fields up to high strains, which is required for subsequent calibration of the fracture behaviour of both base material and HAZ. The plastic behaviour of the base material is calibrated using standard tensile tests and notched tensile tests and an inverse FEM optimization algorithm. The plastic behaviour of the HAZ material is characterized using a specially designed tensile specimen with a HAZ in the gage section. The exact location of the HAZ relative to the centre of the RSW is determined using microhardness measurements, which are also used for mapping of the material properties into an FE-model of the specimen. With the parameters of the base material known, and by assuming a linear relation between the hardness and the plasticity model parameters of base material and HAZ, the unknown HAZ parameters are determined using inverse FEM optimization. A coupon specimen with HAZ is used to validate the model at hand.

Boundary and mixed lubrication friction modeling under forming process conditions

A multi-scale friction model for large-scale forming simulations is presented. A framework has been developed for the boundary and mixed lubrication regime, including the effect of surface changes due to normal loading, sliding and straining the underlying bulk material. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. To account for the lubricant effects special hydrodynamic contact elements have been developed. Pressure degrees of freedom are introduced to capture the pressure values which are computed by a finite element discretization of the 2D averaged Reynolds equations. The boundary friction model and the hydrodynamic friction model have been coupled to cover the boundary and mixed lubrication regime. To prove the numerical efficiency of the multi-scale friction model, finite element simulations have been carried out on a top hat section. The computed local friction coefficients show to be dependent on the punch stroke, punch speed and l...

Multi-scale friction modeling for manufacturing processes : the boundary layer regime

This paper presents a multi-scale friction model for largescale forming simulations. A friction framework has been developed including the effect of surface changes due to normal loading and straining the underlying bulk material. A fast and efficient translation from micro to macro modeling, based on stochastic methods, is incorporated to reduce the computational effort. Adhesion and ploughing effects have been accounted for to characterize friction conditions on the micro scale. A discrete model has been adopted which accounts for the formation of contact patches ploughing through the contacting material. To simulate metal forming processes a coupling has been made with an implicit Finite Element code. Simulations on a typical metal formed product shows a distribution of friction values. The modest increase in simulation time, compared to a standard Coulomb-based FE simulation, proves the numerical feasibility of the proposed method.

On the nonlinear anelastic behaviour of AHSS

It has been widely observed that below the yield stress the loading/unloading stress-strain curves of plastically deformed metals are in fact not linear but slightly curved, showing a hysteresis behaviour during unloading/reloading cycles. In addition to the purely elastic strain, extra dislocation based micro-mechanisms are contributing to the reversible strain of the material which results in the nonlinear unloading/reloading behaviour. This extra reversible strain is the so called anelastic strain. As a result, the springback will be larger than that predicted by FEM considering only the recovery of the elastic strain. In this work the physics behind the anelastic behaviour is discussed and experimental results for a dual phase steel are demonstrated. Based on the physics of the phenomenon a model for anelastic behaviour is presented that can fit the experimental results with a good accuracy.

A multi-scale friction model framework for full scale sheet forming simulations

In this paper a numerical framework is proposed which accounts for the most important friction mechanisms. Static flattening and flattening due to bulk strain are accounted for by theoretical models on a microscale. Based on statistical parameters a fast and efficient translation from micro- to macro modeling is included. A general overview of the friction model is presented and the translation from micro to macro modeling is outlined. The development of real area of contact is described by the flattening models and the effect of ploughing and adhesion on the coefficient of friction is described by a micro-scale friction model. A brief theoretical background of these models is given. The flattening models are validated by means of FE simulations on microscale and the feasibility of the advanced macroscopic friction model is proven by a full scale sheet metal forming simulation.

Superplastic forming simulation of RF detector foils

Complex-shaped sheet products, such as R(adio) F(requency) shieldings sheets, used in a subatomic particle detector, can be manufactured by superplastic forming. To predict whether a formed sheet is resistant against gas leakage, FE simulations are used, involving a user-defined material model. This model incorporates an initial flow stress, including strain rate hardening. It also involves strain hardening and softening, the latter because of void formation and growth inside the material. Also, a pressure-dependency is built in; an applied hydrostatic pressure during the forming process postpones void formation. The material model is constructed in pursuance of the results of uniaxial and biaxial experiments.