The implementation of the vegter yield criterion and a physically based hardening rule in finite elements

A new material description for sheet metal forming using Finite Elements has been developed. The description consists of a yield criterion and a hardening rule. In contrast to most former criteria the new criterion is based on multi-axial stress states. The yield criterion is extended with a physically based hardening rule, in which the flow stress depends on the strain and strain rate. A Limiting Dome Height test is used to examine the material description

Material Induced Anisotropic Damage

The anisotropy in damage can be driven by two different phenomena; anisotropic defor-mation state named Load Induced Anisotropic Damage (LIAD) and anisotropic (shape and/or distribution) second phase particles named Material Induced Anisotropic Damage (MIAD). Most anisotropic damage models are based on LIAD. This work puts emphasis on the presence of MIAD in DP600 steel. Scanning Electron Microscopic (SEM) analysis was carried out on undeformed and deformed tensile specimens. The martensite morphology showed anisotropy in size and orientation. Consequently, significant MIAD was observed in the deformed tensile specimens. A through thickness shear failure is observed in the tensile specimen, which is pulled along the rolling direction (RD), whereas a dominant ductile fracture is observed when pulled perpendicular to RD. The Modified Lemaitre’s (ML) anisotropic damage model is improved to account for MIAD in a phenomenological manner. The MIAD parameters are determined from tensile tests carried out in 0o, 45o and 90o to the RD. The formability of DP600 is lower in the RD compared to that in 90o to the RD, due to the phenomenon of MIAD

Validation of Modified Lemaitre's Anisotropic Damage Model with the Cross Die Drawing Test

Dual Phase (DP) steels are widely replacing the traditional forming steels in automotive industry. Advanced damage models are required to accurately predict the formability of DP steels. In this work, Lemaitre’s anisotropic damage model has been slightly modified for sheet metal forming applications and for strain rate dependent materials. The damage evolution law is adapted to take into account the strain rate dependency and negative triaxialities. The damage parameters for pre-production DP600 steel were determined. The modified damage models (isotropic and anisotropic) were validated using the cross die drawing test. The anisotropic damage model predicts the crack direction more accurately

Beta-2 adrenergic effects on the sympathetic nervous system

The origin of the studies, described in this thesis, dates back to 1979. In that year my colleague, R.P. Verhoeven, studied the responsiveness of hyperthyroid patients to beta adrenoceptor activation [1]. In his experiments he made the serendipitous observation that the plasma levels of the sympathetic transmitter, noradrenaline, increased during infusion of the beta adrenoceptor agonist isoprenaline. At the time most of us felt that this could be explained by reflex increase in sympathetic nervous activity, due to the vasodilatation caused by isoprenaline, my promotor, Prof. Schalekamp, took a different view. He suggested that the increase in noradrenaline during infusion of isoprenaline could be mediated by presynaptic beta adrenoceptors, which would serve to facilitate the release of noradrenaline [2,3,4]. He gave the impetus to the studies that followed

Numerical Modeling of Advanced materials

The finite element (FE) method is widely used to numerically simulate forming processes. The accuracy of an FE analysis strongly depends on the extent to which a material model can represent the real material behavior. The use of new materials requires complex material models which are able to describe complex material behavior like strain path sensitivity and phase transformations. Different yield loci and hardening laws are presented in this article, together with experimental results showing this complex behavior. Recommendations on how to further improve the constitutive models are given. In the area of damage and fracture behavior, a non-local damage model is presented, which provides a better prediction of sheet failure than the conventional Forming Limit Diagram

Implementation of an anisotropic damage material model using general second order damage tensor

Damage in metals is mainly the process of the initiation and growth of voids. With the growing complexity in materials and forming proc-esses, it becomes inevitable to include anisotropy in damage (tensorial damage variable). Most of the anisotropic damage models define the damage tensor in the principal damage direction, with the assumption that the principal damage direction coincides with that of principal plastic strain direction. This assumption limits the applicability of the model to proportional loads. This research is an effort towards imple-menting an anisotropic damage model for non-proportional loads. The implementation of an anisotropic damage model in an implicit FEA code is presented. The model is based on the hypothesis of strain equivalence. A second order general damage tensor is used as an inter-nal variable to represent the damage at macro scale. Two simulations were carried out to check the implementation of the model; a single element orthogonal load change simulation and a rectangular cup deep drawing simulation. Promising simulation results are obtained at acceptable CPU costs

An analytical solution to solute transport in continuous arterio-venous hemodiafiltration (CAVHD)

In conventional intermittent hemodialysis, the overall mass transfer coefficient (Ko) of a dialyser is mostly calculated at zero ultrafiltration and at relatively high dialysate flow rates. In continuous arterio-venous hemodiafiltration (CAVHD), the dialysate flow rates are low as comparable to the rates of ultrafiltration flows, making the dialysis treatment as slow as possible. Therefore the overall mass transfer coefficient (Kd) of a CAVHD hemofilter has to be calculated in the presence of ultrafiltration. A mathematical model of CAVHD is presented in order to calculate the diffusive mass transfer coefficient (Kd) for a solute when blood, filtrate and dialysate flow rates and solute concentrations are known. The ultrafiltration volume flux (Jv) is assumed to vary linearly along the axial direction of the hemofilter. The calculated mass transfer coefficient Kd shows that at high values of dialysate flow and low values of ultrafiltration, the overall mass transfer coefficient (Kd) of a CAVHD hemofilter equals mass transfer coefficient (Ko) of a dialyser in conventional intermittent hemodialysis. Also, the calculated mass transfer coefficient Kd shows no significant differences when the ultrafiltration volume flux is assumed to be constant along the length of the hemofilter if no backfiltration occurs in the hemofilter

A mathematical model of continuous arterio-venous hemodiafiltration (CAVHD)

Abstract Continuous arterio-venous hemodiafiltration (CAVHD) differs from conventional hemofiltration and dialysis by the interaction of convection and diffusion, the use of very low dialysate flow rates and by the deterioration of membrane conditions during the treatment. In order to study the impact of these phenomena on diffusive transport, we developed a mathematical model of the kinetics of CAVHD solute transport from plasma water to dialysate. The model yields an expression of the diffusive mass transfer coefficient, Kd, as a function of blood, filtrate and dialysate flow rates and solute concentrations, which can be measured in the clinical setting. This paper gives a description of the model derivation. Kd is demonstrated to vary depending on dialysate flow and duration of treatment

Apert syndrome: the Paris and Rotterdam philosophy

Introduction: Apert syndrome is a rare type of syndromic craniosynostosis. Patients have an explicit phenotype with craniofacial dysmorphologies and severe symmetrical syndactyly of the hands and feet. This review includes background information about the syndrome and several aspects of the treatment. Areas covered: The cause of Apert syndrome is found in unique mutations in the Fibroblast Growth Factors Receptor (FGFR) 2 gene in 99%. It results in cranial suture fusion, craniofacial dysmorphologies and severe symmetrical syndactyly of the hands and feet. Patients with Apert syndrome are at risk for mental retardation, mobility impairment and intracranial hypertension (ICHT). This is the result of a complex interaction between (1) abnormal skull growth, (2) ventriculomegaly, (3) venous outflow obstruction and (4) obstructive sleep apnea (OSA). Mental retardation is mainly determined by the FGFR2 mutation and treatment is directed at protecting the intrinsic potential of neurocognition. Expert Opinion: To prevent ICHT, we prefer an occipital expansion in the first year of life. Screening on ICHT and its underlying causes is necessary at least until the age of ten by means of skull circumference measurements, fundoscopy, optical coherence tomography, MRI and polysomnography. Multicentre studies on long-term outcome are required to validate the rationale of different clinical protocols