Sam Coppieters

Parameter identification for anisotropic plasticity model using digital image correlation: Comparison between uni-axial and bi-axial tensile testing

The basic principle of the described procedure for plastic material identification is the generation of a complex and heterogeneous deformation field, which is measured by digital image correlation (DIC) and compared to Finite Element (FE) simulations. In this paper two tests for the identification of the hardening behaviour and the yield locus of DC06 steel are compared: a uni-axial test on a perforated rectangular specimen and a bi axial tensile test on a cruciform specimen. The work hardening of the material is assumed to be isotropic and the yield locus is modelled by the anisotropic Hill48 criterion. The identification results for the different material parameters, based on both the uni- and the bi-axial test, are discussed and show a significant agreement.

Anisotropic yield surface identification of sheet metal through stereo finite element model updating

Finite element model updating is a powerful technique to inversely identify material behaviour. A profound understanding of plastic anisotropy of sheet metal is crucial in controlling complex sheet forming applications through finite element simulations. In this contribution, a generic stereo finite element model updating approach combining stereo digital image correlation and finite element model updating is described to identify the plastic anisotropy of sheet metal DC06 which is represented by Hill’s 1948 yield criterion. The feasibility of stereo finite element model updating is illustrated by applying the proposed method to an Erichsen bulging test. Additionally, it is found that the unknown friction coefficient between the punch and the sheet can be simultaneously identified. Finally, the reliability of the identified parameters is scrutinized.

Forming Simulation Considering the Differential Work Hardening Behavior of a Cold Rolled Interstitial-Free Steel Sheet

The multiaxial plastic deformation behavior of a cold rolled interstitial-free steel sheet with a thickness of 0.65 mm was measured using a servo-controlled multiaxial tube expansion testing machine for the range of strain from initial yield to fracture. Tubular specimens were fabricated from the sheet sample by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the contours of plastic work in stress space up to an equivalent plastic strain of 0.289 along with the directions of plastic strain rates. The test material exhibited differential hardening (DWH). A material modeling method for reproducing the DWH in a finite element simulation has been developed. Hydraulic bulge forming simulation results based on the DWH model had a closer agreement with the experimental results than those calculated using the isotropic hardening models with selected yield functions.

Modeling of hole-expansion of AA6022-T4 aluminum sheets with anisotropic non-quadratic yield functions

In the hole-expansion of anisotropic AA6022-T4 sheets, the strain around the hole is non-uniformly distributed due to the anisotropy of the material. This was examined by performing experiments with a flat-headed punch and using Digital Image Correlation (DIC). In the experiments, failure always initiated at a unique location, oriented at 45° to the Rolling Direction of the sheet. The use of DIC allowed the probing of the full-strain-field in real-time. Subsequently, the experiments were simulated in DYNAFORM using shell elements and the Yld2000-2D anisotropic non-quadratic yield function, properly calibrated for this material. In addition, the hardening curve of the material was inversely identified at large strains from the tail of the tensile test. The strain evolution is compared between the experiments and the model.

Advances in post-necking flow curve identification of sheet metal through standard tensile testing

The standard tensile test is still the most common material test to identify the hardening behavior of sheet metal. When using standard equipment and well-known analytical formulas, however, the hardening behavior can only be identified up to the point of maximum uniform elongation. Several methods which deal with the problem of extended flow curve identification of sheet metal through a tensile test have been proposed in the past. This paper gives an overview of the four classes of methods to identify post-necking hardening behavior of sheet metal through tensile testing. In addition, identification methods from the first (average values across the neck), second (Bridgeman correction, modified Siebel and Schwaigerer correction) and third class (special case of the VFM) are used to identify the post-necking hardening behavior of DC05. Finally, these results are used to assess the validity of the different methods.

Identification of post-necking strain hardening behavior of pure titanium sheet

This paper deals with the identification of the post-necking strain hardening behavior of pure titanium sheet. Biaxial tensile tests using a servo-controlled multi-axial tube expansion testing machine revealed that commercial pure titanium sheet exhibits significant differential work hardening (DWH). The latter phenomenon implies that the shapes of the work contours significantly change during plastic deformation which is accurately measured in the first quadrant of the stress space up to an equivalent plastic strain of approximately 0.3. In this paper we focus on the plastic material behavior beyond the point of maximum uniform strain in a quasi-static tensile test. To this purpose, the material is subjected to a post-necking tensile experiment during which the strain field in the diffuse necking zone is measured using a dedicated Digital Image Correlation (DIC) system. The key point in the identification of the post-necking strain hardening is the minimization of the discrepancy between the external work and internal work in the necking zone. In this study, we scrutinize the influence of DWH in the pre-necking regime on the identification of the post-necking strain hardening behavior of pure titanium sheet. Finally, a strain hardening model which enables disentangling pre- and post-necking hardening behavior is presented.

Mechanical Behaviour of Clinched Joints in Configurations

Clinch joining or clinching is a mechanical joining technique for sheet material. In this paper, the mechanical behaviour of multiple clinched joints under mixed-mode loads (peel, shear and pull-out) is investigated using a modified Arcan test. The experimental results are compared with a proposed equivalent model for clinched joints to validate if the model can reproduce the deformation behaviour up to maximum force. The theoretical maximum resistance force of the configurations are then compared to the experimental maximum resistances to investigate the influence of interaction effects on the maximum strength of the configuration. This study is part of a global design strategy for clinched joints in large structures.

Inverse Yield Locus Identification using a biaxial tension apparatus with link mechanism and displacement fields

The main impetus for this work is the reduction of experimental work associated with yield locus identification of sheet metal through homogeneous tests. Inverse identification strategies have been devised to simultaneously extract a number of model parameters from an experiment leading to non-uniform stress and strain fields. Amongst them, the Finite Element Model Updating (FEMU) technique is probably the most intuitive approach for which conclusive proof of concept can be found in literature. A thorough understanding, however, of the key FEMU-ingredients and their impact on the identification of plastic anisotropy of sheet metal is currently missing. This paper scrutinizes via a virtual heterogeneous experiment the impact of the number and spreading of load steps, the boundary conditions and DIC settings on the identification of the yield locus.

The effect of sheet metal anisotropy on the calibration of an equivalent model for clinched connections

Clinching is a mechanical joining technique that involves severe local plastic deformation of two or more sheet metal parts using a punch and die. The local deformation results in a permanent mechanical interlock. It is widely applied as a reliable joining technique in automotive, heating, ventilation, air conditioning (HVAC) and general steel constructions and is still gaining interest. In FEA models of structures containing a large number of clinched joints, it is not computationally feasible to use detailed sub models of the joint. Therefore an equivalent model was proposed by Breda et al. to predict the force-displacement behaviour. This equivalent model was calibrated using a simple shear-lap and pull-out test. During the calibration step, some local effects due to the material properties are captured in the calibration parameters. This paper investigates the impact of the plastic material properties on this calibration method. The effect of strain hardening due to the bending process prior to pull-out testing, potential plastic anisotropy of the base material and their relation to the calibration parameters are investigated. This research has been validated with experimental results on mild deep drawing steel.

Mechanical joining of materials with limited ductility: Analysis of process-induced defects

The paper shows experimental and numerical analyses of the clinching process of 6xxx series aluminum sheets in T6 condition and the self-pierce riveting process of an aluminum die casting. In the experimental investigations the damage behavior of the materials when using different tool parameters is analyzed. The focus of the numerical investigations is the damage prediction by a comparison of different damage criteria. Moreover, strength-and fatigue tests were carried out to investigate the influence of the joining process-induced damages on the strength properties of the joints.The paper shows experimental and numerical analyses of the clinching process of 6xxx series aluminum sheets in T6 condition and the self-pierce riveting process of an aluminum die casting. In the experimental investigations the damage behavior of the materials when using different tool parameters is analyzed. The focus of the numerical investigations is the damage prediction by a comparison of different damage criteria. Moreover, strength-and fatigue tests were carried out to investigate the influence of the joining process-induced damages on the strength properties of the joints.