Extracting material data for superplastic forming simulations

In subatomic particle physics, unstable particles can be studied with a so-called vertex detector placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Hence, this sheet should be completely leak tight with respect to gases. To produce such a complex thin sheet, superplastic forming can be very attractive if a small number of products is needed. This is a forming process in which a sheet of superplastic material is pressed into a one-sided die by means of gas pressure.\ud In order to develop a material model which can be used in superplastic forming simulations, uniaxial and biaxial experiments are necessary. The uniaxial, tensile, experiments provide information about the one-dimensional material data, such as the stress as a function of equivalent plastic strain and strain rate. These data are extracted from the experiments by using inverse modeling, i.e. simulation of the tensile experiment. To fit these curves into a general material model, three parts in the uniaxial mechanical behavior are considered: initial flow stress, strain hardening and strain softening caused by void growth. Since failure in superplastic materials is preceded by the nucleation and growth of cavities inside the material, the void volume fractions of the tested specimens were also observed.\ud A very important factor in this research is the study of the permeability of the formed sheet with respect to gas. If internal voids start to coalesce, through-thickness channels will start to form, thereby providing a gas leak path. To study the twodimensional behavior, including the gas leakage, bulge experiments were performed. Within these experiments, circular sheets were pressed into a cylindrically shaped die. From these experiments it followed that the plastic straining is dependent on an applied backpressure during the forming stage. This backpressure can postpone cavity nucleation and growth

Superplastic forming simulation of RF detector foils

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

Prediction of Gas Leak Tightness of Superplastically Formed Products

In some applications, in this case an aluminium box in a subatomic particle detector containing highly sensitive detecting devices, it is important that a formed sheet should show no gas leak from one side to the other. In order to prevent a trial-and-error procedure to make this leak tight box, a method is set up to predict if a formed sheet conforms to the maximum leak constraint. The technique of superplastic forming (SPF) is used in order to attain very high plastic strains before failure. Since only a few of these boxes are needed, this makes, this generally slow, process an attractive production method. To predict the gas leak of a superplastically formed aluminium sheet in an accurate way, finite element simulations are used in combination with a user-defined material model. This constitutive model couples the leak rate with the void volume fraction. This void volume fraction is then dependent on both the equivalent plastic strain and the applied hydrostatic pressure during the bulge process (backpressure)

Superplasticiteit bij Cern

Op CERN, het Europees onderzoekscentrum voor subatomaire fysica in Genève, wordt dit jaar een nieuwe deeltjesversneller, de Large Hadron Collider (LHC), in werking gesteld die nieuwe inzichten moet bieden over hoe de kleinste deeltjes der materie zich gedragen. Om hierachter te komen, is op plaatsen waar de versnelde deeltjes met elkaar in botsing komen in deze versneller, een viertal detectoren geplaatst die veel openstaande vragen op dit gebied moeten gaan beantwoorden in de komende tien, vijftien jaar. Bij drie van deze detectoren is het Nationaal Instituut voor Subatomaire Fysica, Nikhef, in Amsterdam betrokken voor wat betreft het ontwerp en de fabricage van onderdelen. Het gaat hierbij niet alleen om de sensoren zelf, maar vooral ook om delen van de draag- en koelstructuur van deze, veelal zeer gevoelige, elektronische apparatuur. Bij een van deze detectoren is Nikhef betrokken bij het ontwerp en de bouw van een zogeheten Vertex Locator. Dit onderdeel heeft als taak om het punt van oorsprong van gemeten deeltjes te bepalen. Het ontwerp en de fabricage van de ombouw van dit detectoronderdeel heeft in de afgelopen jaren voor enkele grote problemen gezorgd, reden voor de start van een onderzoeksproject waarin deze problemen in de toekomst moeten worden vermeden

Design and optimization of vertex detector foils by superplastic forming

The production of one of the parts in a particle detector, called the RF Foil, has been a very intensive process in the past. The design and production process, which had a trial and error character, led eventually to an RF Foil that met the most important requirement: a sufficient leak tightness value. Since these kinds of foils have to be produced in the future, it is desirable to shorten the development stage with a view to cost reduction. This research project investigates how this part can be optimized with respect to the radiation length. An important limiting factor within this optimization process is the leak tightness of the foil. The intended production method this research will investigate is superplastic forming (SPF). On the one hand, the goal is to use finite element calculations to predict the forming behavior. The leak tightness of the formed foil must also be predicted within these calculations. On the other hand, an optimization strategy is necessary to reduce the radiation length of the RF Foil while maintaining the leak tightness

Mechanical experiments on the superplastic material ALNOVI-1, including leak information

In subatomic particle physics, unstable particles can be detected with a so-called vertex detector, placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Therefore, this sheet has to be completely leak tight. However, this can conflict with restrictions concerning maximum sheet thickness of the product. To produce such a complex thin sheet, superplastic forming can be very attractive in cases where a small number of products is needed. In order to predict gas permeability of these formed sheets, many mechanical experiments are necessary, where the gas leak has to be measured. To obtain insight in the mechanical behaviour of the used material, ALNOVI-1, tensile experiments were performed to describe the uniaxial stress–strain behaviour. From these experiments, a high strain rate sensitivity was measured. The flow stress of this material under superplastic conditions was low and the material behaved in an isotropic manner upon large plastic strains. The results of these experiments were used to predict the forming pressure as a function of time in a free bulge experiment, such that a predefined target strain rate will not be exceeded in the material. An extra parameter within these bulging experiments is the application of a hydrostatic pressure during the forming process. Such a pressure postpones the nucleation and growth of internal cavities, which means that higher plastic strains can be reached before failure. Results from these experiments showed that at higher hydrostatic pressures, higher bulges were made. All these bulges were leak tested, showing also that higher hydrostatic pressures lead to a lower void volume fraction at higher hydrostatic pressures, since these bulges were more leak tight at the same bulge height than bulges made without the application of this pressure. This article describes the setup and results of the uniaxial (tensile) and biaxial (bulging) experiments on the superplastic aluminium ALNOVI-1.\ud \u