F. De Bona

Large deflections of microbeams under electrostatic loads

Electrostatic actuated microbeams are frequently encountered in micro electro-mechanical systems (MEMS). The behaviour of these devices is characterized by electromechanical coupling, due to the mutual interaction between the electrostatic field and the deflection of the structure. Besides the non-linearity due to the coupling, geometrical non-linearities due to the microstructures large deflections can sometimes arise. In this work, a new FEM method based on a sequential field-coupling (SFC) approach is proposed, in which electrostatic loads are gradually applied to the deformed shape of the structure. The solutions are compared with those obtained by means of the best-known numerical methods available in the literature. In the case of voltage values that generate large displacements, the proposed method appears more suitable to describe the microbeam behaviour; in particular, the voltage at which instability occurs can be evaluated and post-instability solutions can also be predicted.

Analytical and experimental characterisation of high-precision flexural pivots subjected to lateral loads

Abstract This work addresses the parasitic motion of high-precision rotation mechanisms based on flexural pivots subjected to lateral loads. This case has great importance from the mechanical design point of view, since generally flexural pivots support mechanical elements of considerable weight and their rotation is obtained by loading the pivot with a force instead of a pure couple. From an analytical point of view, the problem is approached by studying the large deflections of an elastic frame. The equilibrium equations are considered and a solution based on the Newton–Raphson method is proposed. This approach is compared with other theoretical approaches. An experimental assessment performed by using laser interferometric techniques is presented. It is shown that the proposed solution allows the influence of lateral loads to be clearly established and proves to be adequate when the most common cases of limited lateral loads and rotations are considered.

Deep X-ray lithography beamline at ELETTRA

Abstract A Deep X-Ray Lithography (DXRL) beamline was just commissioned at ELETTRA in Trieste, the Italian third generation synchrotron light source whose features make it well adapted to the DXRL process. A general description of the beamline is given with an emphasis on the reduction of the effect of secondary emission using a high energy filter based on an horizontal beam-stop. Finally the first fabricated microstructures are presented.

Estimation of Material Parameters in Nonlinear Hardening Plasticity Models and Strain Life Curves for CuAg Alloy

This work investigates the cyclic response and low-cycle fatigue behaviour of a CuAg alloy used in crystallizer for continuous casting lines. Therefore isothermal strain-based fatigue tests are first performed on CuAg specimens at different temperature levels (20 °C, 250 °C, 300 °C). The evolution of stress-strain loops recorded during the cyclic tests is used for the parameter identification of several nonlinear hardening models (nonlinear kinematic, nonlinear isotropic). Cyclic stress-strain data from experiments are compared with results from numerical simulations with the identified material parameters, showing a satisfying agreement. Critical examination of numerical results from different models is also performed. Finally, the strain- life fatigue curves estimated from experimental data are compared with approximate strain-life equations (Universal Slopes Equation, 10% Rule) which are obtained from simple tensile tests. The material parameters determined in this work can conveniently be used as inputs in a elasto- plastic finite element simulations of a crystallizer.

Simplified numerical approach for the thermo-mechanical analysis of steelmaking components under cyclic loading: an anode for electric arc furnace

ABSTRACT The need of a strong improvement of productivity and reliability led the adoption of advanced modeling techniques in the design of steelmaking plants components. In this work a procedure based on a finite element simulation is proposed in order to perform a durability analysis of an anode for electric arc furnace. This component undergoes cyclic thermal loads, which also produce a partial melting of one part, meanwhile the other is maintained at almost constant temperature by a cooling system. A simplified, but effective, procedure is developed to take into account steel melting during the heating phase. Considering the cyclic loading conditions, several material cyclic plasticity models, and their effect on the thermal fatigue behavior, are also systematically investigated. The proposed approach permits the component fatigue life to be assessed by a simple and fast uncoupled thermo-mechanical simulation in steady-state conditions.

Thermal expansion measurement of composites with optical heterodyne interferometry

A dilatometer based on an optical heterodyne laser interferometer for high-precision measurements of thermal expansion coefficient (α) has been developed. The instrument is mainly intended for measuring low-expansion composites at room temperature. Compared with already existing instruments, the proposed system is characterized by a higher thermal expansion resolution (30 nm/m). In the case of low-expansion composites, intervals of uncertainty of α lower than 0.1 μm/mK have been obtained.

Copper Mold for Continuous Casting of Steel: Modelling Strategies to Assess Thermal Distortion and Durability

In this work the durability assessment and the permanent deformation of a copper mold for continuous casting of steel have been investigated using mathematical models based on the Finite Element method. The cyclic plasticity behavior of the material is represented by a combined kinematic-isotropic model experimentally validated. Results from thermo-mechanical analysis are in good agreement with measurements. In particular, creep effects included into the model permit the evolution of bulging near the meniscus area to be correctly predicted. A life estimation is performed considering strain-life and stress-rupture time curves according to a cumulative damage law.

Thermal distortion in copper moulds for continuous casting of steel: numerical study on creep and plasticity effect

ABSTRACT In this work, the thermal distortion of a copper mould for continuous casting of steel is investigated through numerical models based on the finite element method. Special attention is devoted to the accuracy of the adopted material properties: several elasto-plastic models, with or without creep effects, are considered and compared into the analysis. The early formation of a bulge close to the meniscus is correctly simulated and results are in good agreement with experimental data from the literature.

Heat load problems in deep X-ray lithography

Abstract A 3D mathematical model of a complete Deep X-Ray Lithography scanning unit based on the Finite Element Method (FEM) was developed to analyse the replication errors induced by the thermoelastic deformations occurring under irradiation. Different thermal and mechanical constraint conditions were considered in order to evaluate the maximum displacements of the irradiated mask area. The obtained results show that, to evaluate the replication errors, the support ring and its mechanical fittings have to be carefully considered in the model. It is observed that the absolute position error and the blur error and the error induced by the thermal expansion of the mask are both position dependent.

Microfabrication at the Elettra synchrotron radiation facility

The first micro-structures produced at Elettra (Trieste, Italy) with deep x-ray lithography are presented in this work. The most important details concerning LIGA technique and in particular deep x-ray lithography using synchrotron radiation are described. The scanning electron microscopy images of test micro-structures made of polymethylmethacrylate characterized by a height of 200 μm and lateral dimensions in between 10 μm and 200 μm are presented. In all the considered cases good accuracy with high aspect ratios and nanometric roughness is observed.