Michael L. Wenner

Mathematical model of a head subjected to an axisymmetric impact

Abstract An improved mathematical model is derived which predicts more closely the response of a head to an axisymmetric impact. The three-dimensional equations of linear viscoelasticity are used to describe the behavior of both the brain and the skull. Responses of both human-sized and small animal heads are calculated for steady-state and for transient loadings. An explicit formula is derived for scaling experimental data on contre-coup damage from laboratory animals to humans. General agreement is found between the predictions of the model and observation, with regard to the location and the nature of possible damage. However, before the model can be used to make satisfactory quantitative predictions of the severity of injury, additional empirical data are required on both damage criteria and the mechanical behavior of the brain and skull.

Strain and strain-rate hardening effects in punch stretching of 5182-0 aluminum at elevated temperatures

The influences of material properties on stretch forming are often studied by testing a wide variety of materials. However, differences in texture, fracture strain, and crystal structure are not taken into account. These material differences are eliminated in the present study by performing tests on a single material (5182-0 aluminum alloy) in which strain hardening, strain-rate hardening, and limit strain vary in a precise manner with temperature and strain-rate. This allows a comparison to be made between experimental results and analytical calculations to separate the contributions of these two types of hardening in distributing strain during elevated temperature forming. Furthermore, the influence of a change in limit strain to overall formability can be assessed since the hardening phenomenon is better understood. The strain distributions developed during forming over a spherical punch are calculated using the finite element method and material properties obtained from tensile tests at 25, 130, and 200°C at varying strain rates. These are compared to experimental strain distributions over the same temperature range. Measurements of limit strains are taken from forming limit diagrams. This research demonstrates that formability is improved at elevated temperatures through increases in both strain-rate hardening and limit strains.

Measurements of anisotropic yielding, bauschinger and transient behavior of automotive dual-phase steel sheets

In order to present better prediction capability in computational analysis, mechanical properties of the dualphase high strength steel have been characterized especially for anisotropy as well as the Bauschinger and transient behavior. As for the anisotropy, the non-quadratic anisotropic yield function Yld2000-2d has been utilized and its material parameters have been obtained using the uni-axial tension tests as well as the hydraulic bulge test. To measure the hardening behavior including the Bauschinger and transient behavior, a newly developed test method has been applied for the uni-axial tension/compression and compression/tension tests, in which solid blocks along the both sides of the sheet specimen prevent buckling. From the tension/compression curves, the equations to describe isotropic and kinematic hardening behavior have been obtained. The modified Chaboche model has been confirmed to well represent the hardening behavior including the Bauschinger and transient behavior.

Effect of hardening laws and yield function types on spring-back simulations of dual-phase steel automotive sheets

In order to simulate the spring-back of DP-steel automotive sheets, the effect of hardening laws and yield function types has been studied based on finite element simulations performed for 2-D draw bending and S-rail tests. Specifically, the performance of the combined isotropic-kinematic hardening based on the modified Chaboche model was compared with those of the pure isotropic hardening and kinematic hardening laws, along with the non-quadratic anisotropic yield (Yld2000-2d) and Mises yield functions. As for the 2-D draw bending test, numerical results were compared with experimental results for verification purposes.

Calculation of Springback and Its Variation in Channel Forming Operations

An approximate plane strain analysis of the springback of sheet metal which has been bent under tension around a small radius is presented. The tensile load can be applied either before the bending takes place (called a «preload») or after bending («postload»). Some of the material and process parameters which govern springback are identified, and it is shown that postloads are always more effective than preloads in reducing spring-back

Overview — Simulation of Sheet Metal Forming

Jet and particle production have been studied in collisions of quasi-real photons collected during the LEP2 program. OPAL and DELPHI report good agreement of NLO perturbative QCD with the measured differential di-jet cross sections, which reach a mean transverse energy of the di-jet system of 25 GeV. L3, on the other hand, finds drastic disagreement of the same calculation with single jet production for transverse jet momenta larger than about 25 GeV. L3 observes similar disagreement between data and NLO QCD in their measurements of charged and neutral particle production at high transverse momenta of the particles. A recent measurement performed by DELPHI of the same quantities does not confirm this observation.After a brief review of the early development of sheet forming simulation, we discuss some of the more recent work. In particular, we consider how it has developed over the years since the NUMISHEET Conferences have been held. The aim here is to give a broad overview of the development of current capabilities in simulating sheet metal forming, not to explore any specific research topic. It is hoped that this short report will help set the stage for the more detailed lectures and papers to follow in this Conference.

A dem displacement-plastic strain formulation of punch stretching

Abstract The differential equation on a manifold (DEM) method is applied to the equilibrium and rate sensitive constitutive equations governing axisymmetric punch stretching. This method is a variant of the mixed finite element method, and finite elements are used to approximate the displacements, pressure, plastic strains and the effective strain. The resulting differential algebraic equation (DAE) is solved using the DASSL software package. Numerical examples are included.bl]

Crash Performance Evaluation of Hydro-formed DP-steel Tubes Considering Welding Heat Effects, Formability and Spring-back

In order to numerically evaluate hydro‐formed DP‐steel tubes on crash performance considering welding heat effects, finite element simulations of crash behavior were performed for hydro‐formed tubes with and without heat treatment effects. Also, finite element simulations were performed for the sequential procedures of bending and hydro‐forming of tubes in order to design process parameters, particularly for the boost condition and axial feeding, considering formability and spring‐back. Effects of the material property including strain‐rate sensitivity on formability as well as spring‐back were also considered. The mechanical properties of the metal active gas (MAG) weld zone and the heat affected zone (HAZ) were obtained utilizing the continuous indentation method in this work.

A Computational Response Surface Study of Curved-Surface-Curved-Edge Aluminum Hemming Using Solid-to-Shell Mapping

Hemming is a manufacturing process of folding a panel onto itself or another sheet. Quality of hemming is characterized by geometry and formability. This paper presents a response surface study of 3D curved-surface-curved-edge hemming of an aluminum alloy, AA6111-T4, using finite element analysis. Solid elements and explicit FE solver are used for simulations of flanging, pre- and final hemming, and shell elements with implicit solver are deployed for springback prediction. A novel procedure called “solid to shell mapping” is developed to bridge the solid elements with the shell elements. Verified to be accurate and efficient, the model is utilized in a Central Composite Design to quantitatively explore the relationships between certain key process variables and the hem dimensional quality and formability. The most significant variables are identified as (i) pre-hemming angle on roll-in/roll-out; (ii) nominal surface curvature on sheet springback; (iii) initial sheet strain and flanging die radius on the maximum hemline surface strain of the produced hem. These results provide insights for process parameter selections in designing and optimizing 3D hems under material formability constraints.Copyright

An In-Situ Tribotest Method Designed for Predicting Wear Life and Frictional Performance During the Aluminum Forming Process

ABSTRACT Quick Plastic Forming (QPF) and Superplastic Forming (SPF) are high temperature forming processes that have been designed to make a variety of aluminum automobile components at both high (QPF) and low (SPF) production volumes. These newly-developed processes operate at high temperatures in excess of 400°C to improve formability, and as a result, special solid lubricants and tool coatings are required to improve wear life and tribological performance under these hot forming conditions. The major aspect of this aluminum forming process is the tribological interaction between the aluminum blank and the forming tool. Tribological characteristics have a significant impact on the surface quality, tool wear life, and durability. The majority of the conventional tribology tests used to measure wear life and frictional performance for conventional sheet metal forming are incapable of monitoring friction behaviour and wear life in-situ. In addition, most of the bench tests used in industry do not capture the deformation state during metal forming, where the forming aluminum material is undergoing sliding and stretching at the same time, leading to transient friction behaviour and fast strain rate changes that are hard to measure in-situ by a bench test. Therefore, an in-situ tribotest is critically needed for measuring friction behaviour and predicting wear life during the quick plastic forming process, including determining the effects of solid lubricants and die coatings, and the basic mechanisms of aluminum blank/tool surface interaction. In this research paper, an in-situ tribotest method has been designed to predict the wear life and frictional performance of aluminum sheets and their lubricants during the quick plastic forming process. This tribotest machine can directly determine wear life and friction characteristics of aluminum sheets during the forming process. In addition, a new probe sensor has been designed and constructed to determine friction force, friction coefficient, and wear depth of aluminum surface contact during the forming process in-situ without any interruption or re-assembly of the metal forming system. It can be observed from in-situ experiments that this developed method and tribotest design provide several technology advantages, including a simple structure, high sensitivity, good stability, and precise measurement of wear life and frictional performance. The correlation between friction coefficient and process parameters for the aluminum forming process has also been studied. This new experimental design and in-situ tribotest method provide valuable tools for evaluation of the tribological characteristics of aluminum sheets, solid lubricants, and tool coatings during the quick plastic forming process.