A. Van Bael

Residual stress determination in cold drawn steel wire by FEM simulation and X-ray diffraction

Abstract This paper presents a study on residual stress in cold drawn wire of low carbon steel by means of finite-element method (FEM) simulation and X-ray diffraction. A thick wire with a diameter of 17.9 mm drawn from an annealed wire with a diameter of 20.1 mm was investigated. First, FEM simulations were performed based on a suitable model describing the boundary conditions and the exact material behavior. Due to the initial texture in the original material, the anisotropy of the material plastic behavior was taken into account on the basis of the texture measurement of the wire. Instead of the isotropic von Mises yield criterion, a texture-based anisotropic yield locus was incorporated into the model to simulate the wire drawing process and calculate residual stresses. Next, X-ray diffraction measurements were carried out at the surface of the wire to obtain the distribution of the lattice spacing versus sin 2 ψ , from which the macroscopic residual stresses at the wire surface were calculated. The comparison between the results from the simulations and the measurements shows that a good agreement has been reached.

Application of Yield Loci Calculated From Texture Data

The concept of the yield locus as a means of representing the plastic anisotropy of a textured material is remembered. It is shown how such yield loci can be used in a very general way, i.e. in full six-dimensional stress space. As an example of how such yield loci can actually be obtained, the series expansion method based on Taylor factors is explained. It is finally shown that these six-dimensional yield loci can be approximated by analytical expressions and under such form brought into finite element calculations of elasto-plastic forming processes.

The Incorporation of Texture-Based Yield Loci Into Elasto-Plastic Finite Element Programs

Elasto-plastic finite elements (FE) methods are nowadays widely used to simulate complex metal forming processes. It is then useful to generate an anisotropic yield criterion from the crystallographic texture and incorporate it into such model. The theory of dual plastic potentials (one in strain rate space and one in stress space) helps to achieve this. There is however a certain danger of losing the convexity of the yield locus during this procedure. Examples of this phenomenon are given and discussed. It is furthermore explained how the yield locus can be used to generate an elasto-plastic modulus for implementation in the FE code. Finally several examples of successful applications of the anisotropic FE code to metal forming problems are given.

Forming Limit Predictions for the Serrated Strain Paths in Single Point Incremental Sheet Forming

The forming limits of sheets subjected to the Single Point Incremental Forming process (SPIF) is generally several times higher than those found in the Forming Limit Curve (FLC). In this paper it is shown that the non‐monotonic, serrated strain paths to which the material is subjected to during the SPIF process, play a role in the high formability, compared to the monotonic loading in the traditional FLC. The deformation history of an aluminium alloy truncated cone formed with the SPIF process is retrieved through a finite element (FE) model, and discussed. Subsequently, the strain paths at three different depths in the sheet are used as input into a Marciniak‐Kuczynski (MK) forming limit model. The usage of different constitutive models in this analysis shows that anisotropic hardening contributes to the delay of the onset of necking in the SPIF process. The large difference in the predicted forming limits that were obtained from the different layers indicates that an interaction between these layers sho...

The design of a biaxial tensile test and its use for the validation of crystallographic yield loci

A biaxial tensile test has been designed for the experimental determination of yield locus points of thin steel sheets. Using texture-based anisotropic finite-element simulations, the geometry of the test sample has been optimized. A detailed accuracy analysis is presented and the range of accuracy of the new specimen is derived. Experimental tests have been carried out on a ultra-low carbon and a bake-hardening steel. Both the yield stresses and the ratios of plastic strains have been compared to the theoretical predictions obtained with the Taylor-Bishop-Hill model using the experimentally-determined crystallographic textures.

Application of a Texture‐Based Plastic Potential in Earing Prediction of an IF Steel

In the present study, an anisotropic yield locus represented by using a texture-based plastic potential in strain-rate space, together with an isotropic hardening law, has been applied in the FE simulations of cup drawing of an IF steel. Only a texture measurement and a tensile test are needed to obtain the material properties required fy a FE simulation. The experimentally observed effects of blankholder force abd frictions of the cup heights are well reproduced in the simulations. The predictions of the earing behavior as well as the average cup heights are also in good agreemebt with the experimental results

Biaxial tests on cruciform specimens for the validation of crystallographic yield loci

Abstract A biaxial tensile test has been designed for the experimental determination of yield locus points of thin steel sheets. Using texture-based anisotropic finite-element (FE) simulations, the geometry of the test sample has been optimised and the accuracy of the conversion procedure has been validated. It was found that the present technique has a high precision for principal stress ratios σ y / σ x between tan(20°) and tan(70°). Experimental tests have been carried out on five steel qualities (low and ultra-low carbon) with thickness between 0.8 and 1.5 mm. Both yield stresses and ratios of plastic strains have been compared to the theoretical predictions obtained with the Taylor–Bishop–Hill (TBH) model using the experimentally determined crystallographic textures. The {110}〈111〉 and {112}〈111〉 slip systems were considered using both full-constraints (FC) and relaxed-constraints (RC) assumptions for the TBH model. These comparisons are clearly in favour of the TBH-FC model.

N-Terminal 10- and 12-kDa POMC Fragments Stimulate Differentiation of Lactotrophs

Medium conditioned by a highly enriched population of gonadotrophs, cultured as reaggregates in the presence of 0.01 nM GnRH, was concentrated, separated on a reversed-phase HPLC column, and tested for activity on lactotroph development in pituitary reaggregate cell cultures of 14-day-old rats. The number of cells expressing prolactin (PRL) mRNA was estimated by image analysis after in situ hybridization of paraffin-embedded sections. The number of these cells entering the mitotic cycle was estimated by autoradiography of [3H]thymidine ([3H]T) incorporation. One HPLC column fraction expanded the section area occupied by PRL mRNA cells without displaying an effect on [3H]T labeling of these cells, indicating that this fraction induces differentiation in the lactotroph lineage. The latter fraction was further purified on a second reversed-phase HPLC column, a gel filtration column, and a final reversed-phase HPLC column. From the last column, four substances were isolated that all selectively induced differentiation of lactotrophs. Each of them had an N-terminal amino acid sequence identical to the N-terminal domain of rat proopiomelanocortin (POMC). As determined by mass spectrometric analysis, the M(r)s were 10,091, 10,289, 12,238, and 12,247 Da, respectively. The C-terminal extension of these compounds is possibly up to Gln74 for the former two compounds and up to Gly95 for the latter two. Authentic purified human POMC(1-76) mimicked the effects of the purified 10- and 12-kDa rat POMC fragments. The present data suggest that certain isoforms of rat POMC(1-74) and human POMC(1-76) can stimulate lactotroph growth through a differentiation-inducing action on progenitor cells.

Forming Limit Predictions for Single-Point Incremental Sheet Metal Forming

A characteristic of incremental sheet metal forming is that much higher deformations can be achieved than conventional forming limits. In this paper it is investigated to which extent the highly non‐monotonic strain paths during such a process may be responsible for this high formability. A Marciniak‐Kuczynski (MK) model is used to predict the onset of necking of a sheet subjected to the strain paths obtained by finite‐element simulations. The predicted forming limits are considerably higher than for monotonic loading, but still lower than the experimental ones. This discrepancy is attributed to the strain gradient over the sheet thickness, which is not taken into account in the currently used MK model.

Mitogenic effects of nerve growth factor on different cell types in reaggregate cell cultures of immature rat pituitary

Treatment of reaggregate pituitary cell cultures of 14-day-old female rats with nerve growth factor (NGF) augmented the number of [3H]thymidine ([3H]T)-incorporating lactotrophs in a dose-dependent manner (0.03-3 nM). At least during short-term treatment NGF increased the total number of cells expressing prolactin (PRL) mRNA and enlarged the cytoplasmic area occupied by PRL mRNA but did not affect the number of cells and the cytoplasmic area containing PRL, suggesting that NGF recruits lactotrophs expressing PRL mRNA but not yet PRL. NGF also stimulated [3H]T incorporation in ACTH cells but not in somatotrophs, thyrotrophs and gonadotrophs. In addition, NGF augmented the total number of [3H]T-incorporating cells to a much higher extent than was expected from its effect on lactotrophs and ACTH cells, suggesting NGF also stimulates [3H]T-incorporation in non-hormone producing cells (progenitors or stem cells?). Around 40% of these [3H]T-incorporating cells in both control and NGF treated cultures showed immunoreactivity for the transcription factor Pit-1 in the nuclei, which is twice the percentage expected (18%) if these [3H]T-incorporating cells were the only known Pit-1 expressing cells in the pituitary i.e. lactotrophs, somatotrophs and thyrotrophs. The present data suggest that NGF has a mitogenic effect on several cell lineages in the pituitary: lactotrophs, corticotrophs and non-hormone-containing cells. The high proportion of mitotic non-hormone containing cells that express Pit-1 is consistent with the proposed role of Pit-1 in cell proliferation in the developing lactosomatotroph lineage.